Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 3 ~1
7653
PRINTING APPARATUS AND METHOD
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to United States Patent
Application Serial Nos. (Our Case Nos. 7581, 7650, 7651,
7652, and 7654) filed on the same date herewith and oommonly
assigned.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to method
and apparatus for providing a copy of an image available in
electronic form and, in particular, to method~s~ and apparatus for
5 providing a hardcopy of an image which has been produced by,
for purposes of illustration and without limitation, medical
imaging equipment such as x--ray equipment, CAT scan
equipment, MR equipment, ultrasound equipment, and the like.
~. Description of the Prior Art
A hardcopy has been defined, for example, in an
article by D. G. Herzog entitled "Hardcopy Output of
Reconstructed Imagery," J. Ima~ing Technoloqy, Vol. 13, No.
5, October, 1987, pp. 167-178, as "an image that is visible to
the human observer, that has a degree of permanence, and can
15 be transported and handled without deterloration of the image.
Hardcopy normally is an image imprinted on transparencies
where the image is viewed by passing light through the medium
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or on opaque material where the image is viewed by reflecting
light off the image." Many attempts have been made by
workers in the field to fabricate apparatus which can make a
hardcopy of an electronically generated or stored image.
It is well known that devices for providing
hardcopies typically receive image information as output from an
image data source such as, for example, a group of sensors, a
computer image i~rocessing system, or storage devices hardcopy
services. Although such may receive image data in either analog
or digital form, the general trend in the art today is to receive
image data in digital ~orm. Further, such devices typically
comprise buffers, memories, look-up tables, and so forth for: ~a~
electronic processing and/or formatting input image data and (b)
modifying ~he apparatus transfer function to compensate for
effects such as, for example, print medium nonlinearities or to
compensate for, or to provide, image contrast enhancement.
~till further, such hard devices typically comprise an image
generator subsystem which includes energy shaping
mechanisms and supporting electronics to convert an energy
source such as, for example, a laser beam or a CRT beam into
focused spots for scanning onto a medium.
There are certain important image quality
parameters which must be taken into account when designing a
hardcopy device. A first, important image quality parameter is
resolution. Most imaging devices have the capability of
recording many thousands of picture elements (pixels~ across
the medium. The ability to distinguish individual pixeis or to
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smooth the image between pixels is determined by the
resolution specification. A second, important image quality
parameter is raster and banding. Raster and banding are
artifacts that usually appear in pixel by pixel recording systems.
Raster is caused by incomplete merging of scan
lines and appears as a regular pattern of density modulation at
the pixel spacing whereas banding is caused by nonuniformity of
pixel placement on the medium and may appear as regular or
random patterns of density variation in across-scan or
along-scan directions. The appearance of banding depends on
the source of placement errors, and since the human visual
system is very sensitive to placement errors, placement errors
on the order of 1% can be discerned. As a result, banding
requirements must be carefully considered due to the cost
implications of providing precise pixel and scan line placement.
A third, important image quality parameter is
geometric fidelity. Geometric fidelity specifications define the
precision with which pixels are located on the medium and relate
to how the medium will ultimately be used.
A fourth, important image quality parameter is
density fidelity. The density fidelity specification defines the
transfer function of the input digital valwe (or analog voltage) to
output density. This specification encompasses the transfer
function of value to density and the transfer function of any
duplicating process utilized. The transferfunction is dependent
on processing variables as well as on the nature of the specific
medium used. The density fidelity specification can be
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separated into Four parts: (a) absolute density repea~ability; (b)
relative-density versus input-signal transfer function; (c) area
modulation vsrsus continuous tone recording; and (d) density
ùniformity. The first of these parts, absolute density
5 repeatability, is the ability of the hardcopy device to consisten-tly
produce the same density values for given input signals. The
second of these parts, relative-density versus input-signal
transfer function, i.e., tone scale, is related to the fact that in
some applica~ions a linear-density versus input-signal transfer
10 function is utilized while in o~hers a deliberate distortion of the
transfer function is utilized to provide contrast adjustment,
compensation, or enhancement in certain parts of the density
range. The shape of the relative-density versus input-signal
transfer function can be adjusted using calibration look-up tables
15 located in a di~ital input signal processing path, and these tabies
can be either fixed, locally adjusted via panel controis, or
remotely loaded via a control interface. Further, if the shape of
the relative-density versus input-signal transfer function is
critical, an operational scenario involving media processor
20 control, periodic transfer function measurement, and periodic
calibration look-up table updating will be required. The third of
these parts, area modulation versus con~inuous tone recording,
will to be described in more detail below. Lastly, the fourth of
these parts, density uniformity, refers to the ability of a
25 hardcopy device to ~enerate a uniform, flat field over the entire
image area.
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A continuous tone recording has an apparent
continuum of gray scale levels such as are observed, for
example, in photographs and in natural scenes. This is
contrasted with an area modulation recording which is typically
5 comprised of geometric patterns of, for example, printed dots
please note that printing with patterns of variable-sized dots
is frequently referred to as halftone recording in the art. In
halftone recording, the printed dot size in a regular array is
varied to provide a range of tones perceived as a gray scale by
10 the human eye.
As is well known to those of ordinary skill in the art,
a continuous gray scale may be approximated in halftone
recording because variations in printed dot size yield, for
example, a varying percentage of light reflection from a printed
15 image and, as a result, create an illusion of a gray scale.
Although halftone recording is basically binary, at first blush,
one would expect a halftone recording image to be like that of a
llne copy.
However, halftone recording is complicated by the
20 presence of spatial frequencies which are not contained in the
original image, which spatial frequencies may result in unwanted
Moiré patterns or other artifacts in the halftone recording image.
As disclosed in the pr;or art, in one halftone
recording method for achieving gray sca3e representations by
25 binary devices, i.e., devices which display or print fixed S;ZQ dots
having no gray scale capa~ility, each halftone cell, herein
denoted as a pixel, is comprised of one or more clusters of
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individual print or display units, herein denoted as pels. The
most common form of halftone pixel is an N by N square pel
matrix of binary, fixed sized pels. The general concept of the
method is to print or display a computed number of pels within a
5 halftone pixel to achieve an average gray scale level which
approximates the averaged density value of a corresponding
portion of the original image. For ~xample, in one such prior art
halftone recording method, pels in a pixel are clustered to
imitate the formation of a single halftone pixel and, in another
10 such prior art halftone recording method, pels are dispersed in a
predetermined manner. Further, in still another such prior art
halftone recording method, referred to as "error diffusion," a
decision to print or not to print a pel is made on the basis of
local scanned density information from the original image as well
15 as on gray scale density errors committed by already processed
neighbors in the recording. In addition to the above, those of
ordinary skill in the art appreciate that while halftone recording
reproduces gray scale levels for a pixel in an averaged sense,
there may be a loss of fine detail resolution in an image if the
20 size of the pixel is too large.
All of the above-mentioned prior art halftone
recording methods disclose the use of binary, fixed size, print or
display dots. In contrast to this, U.S. Patent No. 4,651,287
discloses a halftone recording method in which each picture
25 element to be printed or displayed is programmably adjusted to
have one of a fixed number of gray scale levels. The patent
discioses a halftone recording apparatus which includes: la)
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image data input apparatus such as, for example, a CCD scanner
for scanning an original image and for producing an array of
image input data corresponding to gray scale levels of picture
elements of the original image; ~b~ processing apparatus for
5 receiving the array of image input data and for cornpu~ing an
array of print values wherein each print value corresponds to
one of a fixed number of gray scale levels; and (c) printing
apparatus capable of printing picture elements having a dot size
that corresponds to one of the fixed gray scale levels.
In addition, the patent discloses that a printer which
is capable of printing picture elements wherein each picture
element has a dot size that corresponds to one of a fixed
number of gray scale levels may include apparatus which varies
~he energy necessary for the production of a printed dot.
15 Further, the patent discloses that the energy necessary for the
production of a printed dot is generally prescribed in the forrn of
an electrical signal pulse having a predetermined time duration
and a predetermined voltage levei. Lastly, the patent discloses
that variations of the energy can be affected by changing the
20 following parameters of the electrical signal pulse: the on-time
portion (duty cycle); the voltage level; or the electrical current
f low.
U.S. Patent 4,661,859 discloses an apparatus
which produces a pixel having a variable gray scale. In
25 particular, it discloses a one-dimensional electronic halftone
generating system which is comprised of a source of digital data
representative of pixel gray scale, a counter to store the digital
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data, and pulse producing logic responsive to the counter to
activate a laser modulator in accordance with the digital data
representative of each pixel. More particularly, a six bit data
word is used to represent one of 64 gray scale levels for a pixel,
and the pulse producing logic responds to the data word by
producing a pulse of a predetermined duration or width which
drives the laser for a predetermined time duration to produce a
predetermined gray scale level for the pixel.
Notwithstanding the above prior art halftone
recording methods and apparatus, there still remains a need in
the art for method~s~ and apparatus which can provide a faithful
reproduction of an image rapidly, which method and apparatus
include strong gray scale sensitivity without sacrificing
resolution and which method and apparatus are particularly
suitable for providing a reproduction of an image which is
generated or acquired from medical imaging equipment such as
x-ray equipment, CAT scan equipment, MR equipment,
ultrasound equipment, and the like.
SUMMARY OF THE INVENTION
Embodiments of the present invention satisfy the
above-identified need by providing method(s) and apparatus for
providing a copy of an image and, in particular, for providing a
hardcopy of an image which is generated or acqulred from, for
purposes of illustration and without limitation, medical imaging
equipment such as x-ray equipment, CAT scan equipment, MR
equipment, ultrasound equipment, or the like. In particular,
embodimen~s of the present invention produce an area
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modulated hardcopy of the image, which hardcopy has a large
number of gray levels per area modulation cell (pixel) and a
strong density sensitivity, ~or example, a large number of gray
level steps. This is accomplished by pulse width modulating
5 two different-sized, printing radiation beams.
Specifically, in accordance with a preferred
embodiment of the present invention, the printer comprises:
means for obtaining or measuring as digital input image data
intensity levels of radiation reflected by or transmitted through
10 an image; means for interpolating and/or processing the digital
input image data to provide digital intensity levels which
correspond to areas ~n a medium, which areas are referred to as
area modulation pixels which, in turn, pixels are comprised of
subunits referred to as pels; means for mapping each of the
15 digital intensity levels into a predetermined pattern of pels;
means for pr~viding a drive signal to a source of laser radiation
for activating the source to print the predetermined pattern of
pels on the medium, wherein the source comprises a source of
two different sized printing radiation beams and wherein the pels
20 are formed by pulse width modulating the source of the two
different sized beams .
In a further embodiment of the present invention,
the printer "writes white" to enhance the accuracy of the copy
at high densities where the term "write white" denotes the use
~5 of a medium wherein an unwritten medium has the highes~
density, i.e., all black, and a beam of radiation, for example,
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laser radiation, causes portions of the black to be reduced as
one provides lower densities.
DESCRIPTION OF THE DRAWINGS
The novel features that are considered characteristic
of the present invention are set forth with particularity herein,
both as to their organization and method of operation, together
with other objects and advantages thereof, and will be best
understood from the following description of the illustrated
embodiments when read in connection with the accompanying
drawings wherein:
FIG. 1 shows, in pictorial form, a "paintbrush" of
laser beams used to write an area modulation pixel in an
embodiment of the present invention;
FlGs. 2A-2T show, in pictorial form, pel
configutation patterns for various 90,um x 9~m pixel gray scale
levels in accordance with a preferred embodiment of the present
invention;
FIG. 3 shows a block diagram of an embodiment of
the present invention;
FIG. 4 shows a block diagram of a pixel generator
which is fabricated in accordance with the present invention;
FIG. 5 shows, in pic~oriai form, a oomparison
between an arrangement of 60,um x 60,um pixels and
90,um x 90,um pixels; and
FIG. 6 shows how laser drive data is arranged for a
90,um x 90~m pixel.
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DETAILED_DESCRIPTION
A printer fabricated in accorrlance with the present
invention produces a hardcopy of an image, which image rnay
be any one of a large number of different types of images which
are well known to those of ordinary skill in the art. For example,
the image may be, without limitation, a medical image produced
by equipment such as x-ray equipment, CT scan equipmentJ MR
equipment, ultrasound equiprnent, or the like. In the alternative,
the image may be an image which is stored in, for example,
digital or analog form, on a storage medium such as, for
example, video tape, optical disk, magnetic disk, and so forth.
A hardcopy produced by an embodiment of the
present invention is produced in a medium which is a high
resolution, thermal irnaging medium that forms images in
response to intense radiation such as, for example, laser
radiation.
Suitable medium materials for preparing hardcopy
images using an embodiment of ~he present invention include
the thermal imaging materials disclosed and claimed in
International Patent Application No. PCT/US 87/03249 of M.R.
Etzel (published June ~ 6, 1988, as International Publication No.
W88/04237). A detailed description of a medium material
preferred from the standpo;nt of producing an image having
desired durability is found in the patent application of K.C.
Chang, entitled, "Thermal Imaging Medium", Attorney Docket
No. 7620, filed of even date and assigned to the assignee of the
present patent application.
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A preferred binary thermal imaging medium is a
laminar medium including a pair of sheets, at least one of which
is transparent. The sheets have image forming material
sandwiched between their interior surfaces and, initially,
5 preferentially adhered to one of them. When exposed to pulses
of thermal radiation, the initial preferential adhesion is reversed
so that, when said pair of sheets are separated, unexposed
portions of image forming material adhere to the sheet for which
there is initial preferential adhesion while exposed portions
10 adhere to the sheet for which there is the reversed preferential
adhesion whereby complimentary images can be formed on
respective ones of the sheets. A preferred imaging laminate
medium, actuatable in response to intense image-forming
radiation for production of images in colorant/binder material of
15 the type for uses with the present printer, comprises, in order:
(1) a first sheet-like web material, said web
material being transparent to said image-forming radiation and
having at least a surface zone or layer of polymeric material
heat-activatable upon subjection of said thermal imaging medium
20 to brief and intense radiation;
(2) an optional thermoplastic intermediate layer
having cohesivity in excess of its adhesivity for said surface
zone or layer of heat-activatable polymeric material;
~ 3) a layer of porous or particulate image-forming
25 substance on said thermoplastic intermediate layer, said porous
or particulate image-forming substance having adhesivity for
said thermopiastic intermediate layer in excess of the adhesivity
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of said thermoplastic intermediate layer for said surface zone or
layer of heat-activatable polymeric material; and
(4) a second sheet-like web material covering said
layer of porous or particulate image-forming substance and
5 laminated directly or indirectly to said image-forming substance.
The thermal imaging medium is capable of absorbing
radiation at or near the interface of said surface zone or layer of
heat-activatable polymeric material and the thermoplastic
intermediate layer, at the wavelength of ~he exposing source
10 and of converting absorbed ener~y into thermal energy of
sufficient intensity to heat activate the surface zone or layer
rapidly. The heat-activated surface zone or layer, upon rapid
cooling, attaches the thermoplastic intermediate layer firmly to
the first sheet-like web material.
The thermal image medium is thus adapted to image
formation by imagewise exposure of portions of it to radiation of
sufficient intensity to attach exposed portions of the
thermoplastic intermediate layer and~image-forrning subs~ance
firmly to the first sheet-like web material, and by removal to the
second sheet-like web material, upon separation of the first and
second sheet-lika web materials after imagewise exposure, oF
portions of the image-forming substance and the thermoplastic
intermediate layer, thereby to provide first and second images,
respectively, on the first and second sheet-like web materials.
The optional thermoplastic intermediate layer
provides surface protection and durability for the second image
on the second sheet-like web material.
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Thus, two steps are required to form a hardcopy
with the thermal hardcopy medium. One step comprises
exposing the medium to the proper amount of heat to form a
latent image and the other step comprises processing the latent
5 copy by a peeling process whereby the second sheet carries
with it the unexposed parts of the image forming substance and,
in a preferred embodiment, as will be explained in further de~ail
below, the hardcopy.
Even though the preferred medium is a laminated
10 structure, it will be clear that two unlaminated sheets with
equivalent functions can also be used in practicing the invention.
Lasers are particularly suitable for exposing the
medium because the medium is termed a threshold or binary
typ~ of film. That is, to say, it possesses high contrast and, if
15 exposed beyond a certain threshold value, it will yield maximum
density change, whereas no density at all is obtained below this
threshold.
A hardcopy produced by an embodiment of the
present invention is comprised of a multiplicity of pixels. In
20 particular, in a preferred embodiment of the present invention,
each pixel is about 60,um x 60,um, about 90,um x 90,um, or
some variation of these sizes. Further, the hardcopy is produced
by digital area modulation, also referred to as spatial dithering in
the ar~. Area modulation is a method wherein each pixel is
25 comprised of a predetermined number of pels and a particular
tone, density, or gray scale level for a pixel is produced as a
predetermined pattern of pels. As is well known in the art, area
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modulation provides an illusion of a continuous tone image in a
medium which is capable of producing only black and white pels
since the area modulation tones appear to have different
densities when viewed at an appropriate distance.
The following describes the criteria that are used in
determining pixel size, pel size, and pel configuration patterns
for preferred embodiments of the present invention.
It is well known in the art that, in general, there is a
trade-off between copy resolution and the number of gray scale
levels which are needed to produce a quality copy of an image.
For example, the use of an area modulation pixel comprised of
n x m pels allows reproduction of nm ~ 1 distinct gray scale
levels for a binary medium. Further, using the same pel size, an
increased number of gray scale levels can be obtained by
increasing the size of an area modulation pixel. However, if the
size of a pixel is increased, there is a ioss of resolution in the
hardcopy. On the other hand, if too few gray scale levels are
available for printing, i.e., too few steps in the tone scale can
occur. This is the appearance of a contour in the hardcopy that
was not present in the original image and often occurs when a
reproduction is made of a large, smoothly varying, gray scale
transition .
Thus, in general, at least two measures are
important in assessing the quality of a hardcopy made on a
printer using a binary medium: (1 ) the area modulation
frequenoy, i.e., the number of area modulation pixels per linear
inch, and (2) ~he number of distinguishabie gray scale levels.
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The required number of distinct gray scale levels in a hardcopy
depends on the ability of the unaided eye to distinguish closely
spaced gray scale levels. For example, it has been found that, at
normal reading distance, the human eye can detect a reflectance
5 modulation of about 0.5% at a spatial frequency near 1
cycleslrnm. The inverse of this "just perceptible" rnodulation
has been interpreted as the maximum number of gray scale
levels that the human eye can perceive, i.e., a rule of thumb in
the printing industry is that a "just acceptable" picture should
10 contain about 65 gray scale levels and, for a good quality copy,
100 or more levels is desired but, for medical applications, 200
or more levels are more appropriate. In addition to this, it is also
known that a substantial improvement in copy quality can be
achieved when pels have more than two gray scale levels.
In view of the above, the following criteria were
used in arriving at choices for the size of a pixel and a pel for
preferred embodiments of the present invention: (1 ) a pixel
should be as small as is required to be invisible to the naked
human eye and to produce a high quality copy; (2) for a given
20 pel size, a pixel should be as large as is required to comprise a
large enough number of pels to provide a suitable number of
distinguishable gray scale levels and to provide a suitable
mapping of density levels from the image to the copy (As will be
explained below, although the ratio of the size of a pixel to the
25 size of a pel determines the number of pels which comprise a
pixel and this, in turn, determines the number of gray scaie
levels which can be achieved, this ratio alone does not provide
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the capability for a one-to-one mapping of density from an image
to a copy); and (3) the pel pattern should not contribute to
texturing or contouring in the copy.
In addition to the above, we have developed an
additional criterion which is derived from the fact that a
perceived gray scale level of a pixel is not linearly related to the
ratio of black and white areas therein because the human eye
does not perceive gray scale level as a linear function but as a
logarithmic function of intensity. One implication of this is that
the gray scale level of a pixel whose density is one density unit
from the maximum pixel density is determined by the size of a
pel and, as a result, the jump in density from the highest density
on the gray scale level, i.e., DmaX, to the next highest density on
the gray scale level, i.e., Dr,~aX l, must be small. Las~ly, the
choice of pixel size, pel size, and pel configuration patterns is
made in light of the fact that the number of gray scale levels
which are detectable by the human eye, i.e., the least detectable
contrast, decreases rapidly with spatial frequency. Thus, at the
resolution lirnit of the eye, one need only represent black and
white.
In accordance with the abave-stated criteria, we
have determined that a pixel size of about 60,um x 60,um
provides high resolution copies and solves the problem of pixel
visibil;ty for a copy page of generally available sizes such as, for
example, 8" x 10", 11" x 14", 14" x 17", orthe like. In
addition, due to considerations regarding copy speed, a "print"
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pixel of about 90~m x 90,um is also written in a preferred
embodiment.
Initial attempts to make copies using a l'print" pixel
of about 90,um x 90~m entailed the use of three laser beams,
5 each of which provided a pel having a spot size of about
30,um x 3,um on the medium. However, as was explained
above, such an arrangement can provide only 91 linear
transmission increments and this, it was discovered, provided an
inadequate number of gray scale levels for certain applications.
10 In fact, a far larger nurnber of transmission increments are
needed to provide a more suitable number o~ gray scale levels.
A larger number of transmission increments is provided, in
accordance with the present invention, by pulse width
modulation of the driving signal for the radiation beams, in this
15 embodiment, the driving signal for the laser sources, to produce
variable sized pels.
In accordance with the present invention, a pixel is
"painted" with a predetermined area modulation pattern of pels,
which predetermined area modulation pattern of pels
20 corresponds to a predetermined intensity level in the original
image or to a predetermined intensity level computed by the
printer. In this context, the term "painted" refers to the
exposure of a pixel of heat sensitive medium to beams of laser
radiation. In a preferred embodiment of the present invention, a
25 pixel is chosen to be substantially 60,um x 60,um or
90,um x 90~m in area and a "paintbrush," i.e., the beams of
laser radiation, which is used to "paint" the pixel with pels is
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comprised of four separate beams of laser radiation. As shown
in FIG. 1, each of the first three beams of radiation 200, 210,
and 220 in "paintbrush" 250 provides a spot on the medium
whose smallest footprint thereon is an area which is
substantially equal to 30,um x 3,um. Beams 200--220 are
aligned in an offset configuration so that a stroke of
"paintbrush", labeled as 250 covers one and one-half (1.5)
60,um x 60,um pixels or one 90~m x 90,um pixel. As was
discussed above, the choice for the size of beams 200, 210,
and 220 was determined by the criteria set forth above as well
as factors such as the complexity and expense required to
provide a smaller sized pel, the additional print time required in
producing a hardcopy with a smaller sized pel, and the
complexity, expense and print time involved in utilizing
additional laser beams.
As also shown in FIG. 1, in addition to beams
200--220, "paintbrush" 250 is comprised of a fourth beam of
radiation, beam ~30. Beam 230 provides a spot on the medium
whose smallest footprint thereon is an area which is
substantially equal to 5,Llm x 3,um, and beam 230 is aligned so
that it traverses a line which passes roughly through ~he center
of beam 210.
As described above, in this preferred embodiment,
each of beams 200--230 has a minimum footprin~ width on the
medium, i.e., distance from top to bottom of a footprint, of
substantially 3,um. However, in accordance with the present
invention, the footprint width is variable for each of the four
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beams, i.e., beams 200--230. The footprint width is varied by
allowing a beam to impinge upon the medium for a variable
- amount of tirne as the medium passes under the beam. The
variable amount of time for allowing a beam to impinge upon the
5 medium is provided in the preferred embodiment by pulse width
modulating each laser beam so that the footprint width can vary
from the thickness of the laser beam, i.e., approximately 3.0,~/m
or more, to roughly 60.0,um or 90.0,~m in increments of
.375,um. This method of pulse width modulating the laser beam
10 radiation wiil be referred to below as slicing.
In accordance with the present invention, slicing is
achieved by modulating the writing frequency of a laser drive
signal such that 3 laser is turned on for a minimum writing time
(t) to write, for example, 3,um or for longer times (t + x*dt),
15 where dt is the time to write a slice and x is the number of
desired slices. The use of slicing increases the effective nurnber
of pels in a pixel.
In a particular embodiment of the present invention, the chaice
of slice si e is determined by balancing the need to provide an
20 adequate number of gray scale levels and the complexity
involved in providing very small slices. Very small slices place
great demands on both hardware and rnedium. Hardware need
to become more complex while medium must be capable of
generating small spots. As a result, in the preferred
25 embodiment, we have chosen a slice of about .375,um.
However, it should be clear to those of ordinary skill in the ar~
that the particular choice of the number of slices and the
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minimum and maximum widths for a pel is a matter of design
choice and does not limit the scope of the present invention.
The following describes the advantageous results
which are obtained from the use of laser beams which have
different footprints on the medium, i.e., laser beams 200--220
each have a minimum footprint of about 30,um x 3,um and laser
beam 230 has a minimum footprint of about 5,um x 3,um. If
copies were printed on the above-described medium the
highest gray scale level for the above-described medium
corresponds to a density value, DmaX~ approximately equal to 3Ø
Using 90,um x 90,um pixels and laser beams with a minimum
footprint, i.e., pel size, of about 30,um x 3,um, the next highest
gray scale level in the copies would ccrrespond to a density
value DmaX.1 approximately equal to 2. Another way of
understanding this result is to appreciate that if one were to
produce copies using pels having a minimum footprint of about
30,um x 3,um, one would make the density range between 2 and
3 inaccessible in the copies. This, of course, is unacceptable for
a printer which has to produce copies of images provided by
medical imaging equipment where vital information is recorded
by density variations. Specifically, as stated in Neblette's
Handbook of Photography and Reprography, Seventh Edition,
Edited by John M. Sturge, Van Nostrand and Reinhold Company,
at p. 558-559: "The most important sensitometric difference
between x-ray films and films for general photography is the
contrast. X-ray films are desi~ned to produce high contrast
because the density differences of the subJect are usua!ly low
, . ;-. , . . ,
. . . . , . ~ . . . ..
3 ~
-22-
and increasing these differences in the radiograph adds to its
diagnostic value.
Radiographs ordinarily contain densities ranging
from 0.5 to over 3.0 and are most effectively examined on an
5 illuminator with adjustable light in~ensityUnless applied to a
very limited density range the printing of radiographs on
photographic paper is ineffective because of the narrow range of
densities In the density scale of papers."
As a result, the printer needs to be able to write a
pel having a substantially smaller size than 30,um x 3,um. This
capability is provided, in accordance with the present invention
and as was described above with respect to the preferred
embodiment, by using laser beam 230. Although, in principle,
laser beam 230 could be added to "paintbrush" 250 in any one
of several ways, the placement shown in FIG. 1 provides a
preferred placement wherein laser beam 210 is replaced with
laser beam 230 at predetermined times. In the preferred
embodiment, the minimum size of the small pel is about
5,~m x 3,um and, as a result, DmaX_1 is about 2.7 for a
90,um x 90,um pixel. Since the depth of focus required to
provide a pel of a particular size is inversely proportional to the
square of the pel size, a pel size of about 5,um x 3,um is
reasonable in terms of the complexity and expense involved in
providing a smaller sized pel.
Further, as described above, slicing is also applied
to pels written by the fourth and smallest laser beam and, as a
result, the number of gray scale levels is dramatically increased,
g
-23-
and small increments between gray scale levels are realizabie.
The increase in ~he number of gray scale levels is most
advantageous at high densities because the human eye is most
sensitive to transmittance or reflectance changes which occur at
5 high density. Specifically, the human eye is sensitive to relative
chan~e in luminance as a function of dL/L where dL is the
change in luminance and L is the average luminance. Thus,
when the density is high, i.e., L is small, the sensi~ivity is high
for a given dL whereas if the density is low, i.e., L is large, then
10 the sensitivity is low for a given dL. In accordance with this,
embodiments of the present invention preferably provide small
steps between gray scale levels at the high density end of the
gray scale. Further, in accordance with this, it is also pre~erred
to write the high density part of the gray scale as accurately as
15 possible because the human eye is more sensitive to intensity
differences which occur in that part of the gray scale. In
accordance with a preferred embodiment of the present
invention, this is accomplished, as was described above, by
writing "white" on the medium. As was described above, in the
20 preferred embodiment, tha medium is such that, in an unprinted
or virgin state, the medium is black. The making of a copy
entails the use of radiation from laser beams 2û0--23Q to cause
the copy forming substance on the medium to adhere to ~he
surface of the web. Then, when the cover is peeled, the
25 exposed regions remain on the web and the unexposed regions
remain with cover and form the hardcopy. Since the hardcopy is
written by using laser beams 2û0--230 to denote areas on ~he
- . . ~ . . ... .
2 ~ 3 ~
-24-
?~
ultimate copy wherein black is removed, the formation of the
hardcopy is referred to as a process where one "writes white."
This is advantageous, as can be seen from the above, since
laser beam 230 which produces the small pel is used to provide
5 gray scale levels which corresponds to high density. The
advantage is derived from the fact that the accuracy of the
specification of the high density gray scale levels depends on
the positioning of a single laser beam, namely, laser beam 230
which is responsible for writing the srnall pel. If the medium
10 were written "black" the high dens.ty gray scale levels would be
written by the interaction of several, if not all, of laser beams
200--230 and provide more opportunity for positioning error.
As a result, a printer would have to be more complex and
expensive to achieve a comparable level of accuracy as that
15 achieved by a printer that utilizes a "write white" process. This
is because, as was set forth above, intensity differences are
more readily detected in the high density portion of the gray
scale levels, and medical images typically are darker than picture
photographs. Notwithstanding the above, it should be
20 understood that the present invention is not restricted to "write
white" embodiments and that the present invention also
encompasses "write black" embodiments.
~ n a preferred embodiment of the present invention,
pel configuration patterns for "painting" a gO,um x 90,um print
25 pixel are designed to meet several objectives which are
necessary for repeatable imaging of high ~uality. A first
objective in developing pel configuration patterns for the
., , . . . . ,,, - . - - ~. -
2 '~ 3 ~
-25-
preferred embodiment which "writes white~' is to make as few
changes in an area modulation pixel as is possible for higher
density gray scale levels because the most critical information in
most medical images is in the darker areas of an image. In
addition, a second objective in developing pel configuration
patterns is to minimize the effect of bridging in the medium on
image quality. Bridging is a phenomenon that occurs in the
above-described medium whenever a cover is peeled and closely
spaced exposed material bridges, i.e., pulls unexposed material
between them, from the cover. As one can readily appreciate,
bridging will result in density variations and, hence, iower quality
copies. Bridging can be prevented by utilizing pel configuration
patterns which maintain minimum distances of unexposed
material in the medium between clusters of exposed material.
For example, we have determined that the probability of
bridging, i.e., the probability that two clusters of exposed
material will bridge, is reduced substantially if there is a
minimum unexposed distance between the clusters of about
1 0,um to 1 2,um.
FlGs. 2A-2T show various pel configuration patterns
for various 90,um x 90,um pixel gray scale levels in accordance
with a preferred embodiment of the present invention. These
figures are best understood when they are viewed in conjunction
with TABLE I (see page 57).
The grid in FlGs. 2A-2T represents one
90,um x 90,um pixel which is comprised of 3 columns, each
column being comprised of 30 rows. Pels in the first column are
... .. . . .
~ ` 2 ~
-26-
"painted" by wide laser 3; pels in the second or middle column
are "painted" by wide laser 1 or by narrow laser 4; and pels in
the third or last column are "painted" by wide laser 2. The
coordinates of a particular pel in the grid are designated, as to
5 row, by a number from 0-29 and, as to column, by the laser
number which "paints" that pel. In viewing FlGs. 2A-2T, keep
in mind that we are viewing a "negative" of a "write white"
medium, i.e., the white areas in the figures are unexposed areas
and the black areas are "painted" areas. Thus, the hardcopy will
10 be the reverse of the figures. For example, FIG. 2A shows a
"negative" of a completely unexposed medium and, as a result,
represents a pixel having the darkest gray scale level.
In providing pel configuration patterns for the
preferred embodiment in accordance with the above-described
15 criteria, we have divided the pel configuration patterns into
groups A throuyh J and we have specified certain "painting"
rules for the various groups. The rules are displayed in TABLE I
and are illustrated in FlGs. 2B-2T. In particular, pairs of figures
from FlGs. 2B-2T show the starting pel configuration pat~ern for
20 a group and the last pel configuration pattern in a group,
respectively. Specifically, with reference to TABLE l: the
column headed 'IGROUP'' refers to pel configuration patterns in
the various groups A-J; the column headed "BEGINNING C)F
CLUSTER LOCATION" gives grid coordinates in terms of row
25 and laser for pels in the first pel configuration pattern in a group;
and the column headed "CLUSTER SIZE RANGE iN SLICES"
gives the minimum and maximum number of slices for each of
.
., ' ` ' - , ' ' ~ !
-- ` 2 ~ 3 ~
-27-
the lasers used to produce pel configuration patterns in a 0roup.
TABLE I and FIG. 2B show that the first pel configuration pattern
in group A comprises 6 slices "painted" by laser 4 starting in
row 5. Further, TABLE I and FIG. 2C show that the last pel
5 configuration pattern in group A comprises 200 slices "painted"
by laser 4 starting in row 5. TABLE I and FIG. 2D show that the
first pel configuration pattern in group B comprises 1 10 slices
"painted" by laser 4 starting in row 5 and 12 slices "painted" by
laser 3 starting in row 0. Further, TABLE I and FIG. 2E show
10 that the last pel configuration pattern in group B comprises 200
slices "painted" by laser 4 starting in row 5 and 12 slices
"painted" by laser 3 starting in row 0. TABLE I and FIG. 2F
show that the first pel configuration pattern in group C
comprises 1 10 slices "painted" by laser 4 starting in row 5, 12
15 slices "painted" by laser 3 starting in row 0, and 12 slices
"painted" by laser 2 starting in row 15. Further, TABLE I and
FIG. 2G show that the last pel configuration pattern in group C
comprises 200 slices "painted" by laser 4 starting in row 5, 12
slices "painted" by laser 3 starting in row 0, and 12 slices
20 "painted" by laser 2 startin~ in row 15. The remaining ones of
FlGs. 2B-2T can be similarly understood with reference to
TABLE 1.
Note that groups F-J which correspond to lower
densities do not use small laser 4. However, this is not a
25 drawback since, as was described above, the lo~arithmic human
visual response means that larger transrrlission or reflection
~ - 2 ~ 3 .~it
-28-
differences in regions of low density can still be nearly invisible
to the human eye.
As one can readily appreciate frorn the above, FlGs.
2A-2T and TABLE I provide more pel confi3uration patterns than
5 would be used to provide, for example, 256 gray scale levels.
Thus, in practice, an appropriate subset of the various pel
configuration patterns provided in FlGs. 2A-2T and TABLE I for
use in a specific case depends on the particular requirements of
the specific cass and, an appropriate subset therefor, is selected
10 to approximate the specific tone scale desired. However, one
may consider the followin~ methodology for choosing pel
configuration patterns from among the various possibilities in a
group. First, consider the first pel configuration pattern for a
group and, for each laser, determine the amount of area that can
15 be "painted" to reach the last pel configuration pattern for the
group. Second, pel configuration patterns from that group,
other than the first pel configuration pattern, are first selected as
being those which are obtained by "painting" with the laser that
has the largest area that can be "painted." However, as the
20 selected laser "paints" to provide selected pel configuration
patterns, the amount of area that can be "painted" for that laser
is decreased. Third, when the amount of area that can be
"painted" by the first laser equals the amount of area that can
be "painted" by another laser, pel configuration patterns are
25 then chosen which alternately "paint" these two lasers.
The laser source which is used to provide a beam to
write the small pel may be similar to those used to provide a
3'~
-29-
beam to write the large pels, but with its radiation outpu~
clipped using mirrors of appropriate dimensions. Alternatively,
one could utilize a laser having a smaller emitting region.
FIG. 3 shows a block diagram of inventive printer
10 which produces a hardcspy of image 50 on medium 205. As
shown in FIG. 3, printer 10 comprises: la) Image Scan and
Acquisition Module 100 which acquires image data in electronic
form corresponding to image 50; (b) Image Frame Store 110
which stores the image data provided by Image Scan and
Acquisition Module 100; Ic3 System Controller 115 which: (i)
processes the image data stored in Image Frame Store 110 in a
manner which will be described in detail below, (ii) causes the
processed image data, and other information that will be
described in detail below, to be transferred to other portions of
printer 10, and, in certain embodiments, (iii) receives input
information from a user to provide printing format information
and the like; (d) Pixel Generator 700 which receives image data
from Image Frame Store 110 and control information from
System Controller 115 and, in response thereto, produces
output to Laser Module 750; and (e) Laser Module 750 which
comprises Lasers 195, which lasers produce a hardcopy of
image 50 on medium 205 in response to the output from Pixel`
Generator 70G.
Image Scan and Acquisition Module 100 is
apparatus which is well-known to those of ordinary skill in the
art for scanning image 50, for acquiring image data from image
50 in analog or digital form, and for converting the acquired
- ` 2 ~ 3 '1
-30-
image data into digital form; if necessary. Embodiments o-F
Image Scan and Acquisition Module 100 are well-known to
those of ordinary skill in the art and comprise, for example,
apparatus: (a) for scanning image 50 with radiation output ftom,
for example, a CR~; (b) for measuring the amount of radiation
which is reflected from image 50 and/or which is transmitted by
image 50 with photodetectors in a manner which is also well-
known to those of ordinary skill in the art; and (c) for
converting, for example, output from the photodetectors to
digital image data by sending the output through, for example,
analog-to-digital converters in a manner which is also well-
known to those of ordinary skill in the art. Alternatively, Image
Scan and Acquisition Module 100 may be a CCD scanner. In
the embodiment described below, for purposes of illustration
only and without limitation, it is assumed that the digital image
data output from Image Scan and Acquisition Module 100
comprises eight ~8) bit data, each of which corresponds to a
256 step gray scale. Further, also for purposes of illustration
only and without limitation, each eight-bit image datum
corresponds to the intensity of the radiation which was reflected
from a predetermined area of image 50 or which was
transmitted by a predetermined area of image 50. In addition, it
should be clear to those of ordinary skill in the art that image
data which is output from Image Scan and Acquisition Module
100 and which is applied as input to Image Frame Store 1 10
under the control of System Controller 115 oould just as well
have been read from a storage medium such as, for example, a
- - 2~ 8~
video tape, an optical disk, a magnetic disk, and so forth and, in
such an embodiment, the output from the storage device would
be applied as input to Image Frame Store 110. Alternatively,
the digital image data could also be generated at a remote
5 location and ~ransferred to Image Frame Store 110 over a Local
Area Network (LAN) or through a small computer system
interface (SCSI), and so forth. It should be understood that the
image does not have be stored in any one particular digital or
analog format, and it is well within the spirit of the present
10 invention to accept image information in any type of format.
It should be understood that each image datum
output from Image Scan and Acquisition Module 100 can be
displayed on an area which could be larger than, equal to, or
less than the size of a pixel. For example, the particular choice
15 may be made on the basis of format versus content the
term "format" referring to, for example, the aspect ratio of the
copy and the term "content" referring to the resolution and tone
of the copy. As shown in FIG. 3, in certain embodiments, such
choices may be entered by user input to System Controller 115.
20 However, in the embodiment described below, for purposes of
illustration only and without limitation, an area correspondin~ to
an image datum on image 50 is ordinarily larger than a pixel and
thus of lower spatial resolution. As a resul~, there are more
pixels in the hardcopy produced by inventive printer 10 than
25 there are areas in image 50. Further, for purposes of illustration
only and without limitation, medium 205 is affixed to the outer
surface of a drum (not shown), which drum, as is well-known to
... : - ,.. , ... --- , : , ., . . ~
3 ~
those of ordinary skill in the art, is cylindrical in shape. In a
typical such implementation, as is also well-known to those of
ordinary skill in the art, as the drum and the medium affixed
thereto rotate, radiation output from Lasers 195 in Laser Module
750 impinges upon medium 205 along a line. Still further, a
sufficient number of lines are formed on medium 205 to provide
the hardcopy of image 50 on medium 205 as radiation output
from Lasers 195 of Laser Module 750 is moved in a direction
which is transverse to the direction of a line. Yet still further, a
page of hardcopy output may comprise several images which
are reproduced on, for example, an 8 x 10 inch hardcopy and
the pixel size, pixel aspect ratio, number of active lines per page
in, for example, the 8--ir,ch direction, and the number of active
pixels per page in the 10~inch direction are prograrrimably
variable and embodiments of the present inven~ion are not
limited to any one particular set of such parameters.
Image Frame Store 1 10 is any apparatus which is
well-known to those of ordinary skill in the art which will serve
as temporary storage for image data obtained from image 5û or
from a multiplicity of such images. System Controller 115
composes and formats a "page" which "page" is to be
produced as a hardcopy ima~e on rnedium 205 in Image
Frame Store 110 in a manner which is well-known to those of
ordinary skill in the art. As a result, a `'page" may be comprised
of a single image like image 50 or it may be comprised of a
multiplicity of images like image 50.
3 '1
-33-
System Controller then transfers the foilowing to
pixel generator 700 preferably over a VME Bus 695 as setup
data which is used by Pixel Generator 700 in performing its
function: (a) values for certain programmable parameters of Pixel
5 Generator 700 such as, for example: (i) number of lines per
page; (ii) number of pixels per line in the direction o~ rotation of
the drum; (iii) number of pels per pixel in the direction of the
rotation of the drum; (iv) pixel aspect ratio; and so forth; (b)
look-up table data which is used to generate signals for driving
10 Laser Module 750 in a manner which will be described in de~ail
below; and (c) software for use by a digital signal processor
(DSP) which comprises a portion of Pixel Generator 700. It
should be clear to those of ordinary skill in the art that, in some
embodiments, such data and software can be transferred prior
15 to making each hardcopy image whereas, in other embodiments,
portions of such data and software may be transferred
whenever the relevant data and software or portions thereof
need to change for various portions of the hardcopy.
As shown in FIG. 4, Pixel Generator 700 is
20 comprised of the following components: ~a) VME Interface 119
--VME Interface 119 receives input over VME Bus 695 and
provides an interface between the internal circuitry of Pixel
Generator 700 and VME Bus 695; (b) DSP 120 DSP 120
receives parameter data, software, and image data from System
25 Controller 115 (this data and information is sent frorn System
Controller 1 15 to VME Interface 1 19 over VME Bus 695 and is
relayed by VME Interface 119 to DSP 1 2Q); (c) DSP Memory
- . - . ~ . ,................ . .. ,., . .. . . : ,;: , . - -
.; . ..
. - . .... .
-34-
^ ~ ,
121 DSP Memory 121: li) receives parameter data and
software from System Controller 115 (this data and information
is sent from âystem Controller 1 15 to VME Interface 1 19 over
VME Bus 695, is relayed by VME Interface 119 to DSP 120, and
is finally relayed to DSP 121 by DSP 120) and (ii) transfers
parameter data and sof~ware to DSP 120; (d) INX Memory 130
INX Memory 130: (i) receives image data from System
C:ontroller 115 (this image data is sent from System Controller
1 15 to VME Interface 119 over VME Bus 695, is relayed by
VME Intertace 11~ to DSP 120, and îs finally relayed to INX
125 by DSP 120) and (ii) transfers image data to DSP 120 in
response to commands from DSP 120; (e) Out Buffer 140
Out Buffer 140: ti) receives image data from DSP 120; (ii)
receives addressing information from Pixel Size 163; and ~iii3
transfers image data to LUT Processor 170; (fl Pixel Size 163
--Pixel Size 163: (i) receives parameter data (such as, for
example, number of lines per page, the number of pixels per line
in the direction of drum rotation, and the number of pels per
pixel in the direction of drum rotation) from System Controller
1 15 (this data is sent from System Controller 115 to VME
Interface 119 over VME Bus 695 and is relayed by VME
Interface 119 to Pixel Size 163); and lii) transfers pixel address
information to Out Buffer 140 and pel address information to
LUT Processor 170; (9) LUT Processor 170 which is comprised
of look^up table memories LUT0 and LUT1 ~it should be clear to
those of ordinary skill in the art that LUT Processor 170 is not
restricted to two memories and can be comprised of only one
~.
- h
-35-
memory or even more than two memories), each of which
memories contain look-up tables which provide a mapping o~
intensity level to pel configuration pattern for use in digital area
modulation printing on medium 205--LUT Processor 170~
5 receives mapping data from System Controller 115 (this data is
sent from System Controller 115 to VME Interface 119 over
VME Bus 695 and is relayed by VME Interface 119 to LUT
Processor 170); (ii) intensity level input from Out Buffer 140;
and (iii) pel address information from Pixel Size 163; (h~
Multiplexer and Delay 180 Multiplexer and Delay 180: li)
receives input from LUT Processor t70 which contains laser
drive information in 16 bit words, which 16 bit words are
comprised of four 4-bit values for each of the four lasers which
comprise Lasers 195 and (ii) receives input from DSP 120 which
contains information which is used to determine how to conver~
the mapping information in the two 1~ bit words from LUT0 and
LUT1 of LUT Processor 170 to 16 bits of information
appropriate for specific ones of Lasers 195; (i) Slice 190
Slice 190 (i) receives input from PLL 185; (ii) receives 1~ bit
input from M~ltiplexer and Delay 180; and (iii) transforms the 16
bit input signals into signals for use in driving the lasers of
Lasers 195; (j) PLL 185 PLL 185 is a phase-locked loop
clock which: (i) receives input ~rom Drum Encoder 187 and (ii)
outputs a clock which is synchronized to the rotating drum; and
(k) Drum Encoder 187 which receives a signal when the drum
rotation reaches a predetermined position.
-36-
The following describes the opera~ion of Pixel
Generator 700 in more detail. System Controller 115 obtains
data which corresponds to a portion of an ima~e which has
formatted and stored in Image Frame Store 110. System
5 Controller 115 transfers the eight bit data corresponding to the
portion to Pixel Generator 700 over VME Bus 695 in real time.
The term "real time" means that, for example, data
corresponding to ~he portion such as one or two lines of the
formatted image in Image Frame Store 110 are transferred
10 to and processed by Pixel Generator 700 per drum revolution.
Specifically, for an 8 x 10 inch copy printed using 60,u x 60,u
pixels, the maximum number of eight bit pixels which are
transferred per line in the preferred embodirnent is 4096.
The eight bit pixel data which is transferred from
System Controller 115 to Pixel Generator 700 is transferred over
VME bus 695, through VME Interface 119, and is applied as
input to digital signal processor 120 (DSP 120). DSP 120 then
transfers the data, in turn, to INX Memory 125. INX Memory
125 is apparatus which is well-known to those of ordinary skill
20 in the art for storing digitized image data. For example, INX
Memory 125 may be a random access memory. INX Memory
125 is used as input buffer memory to store image data which is
waiting to be processed by DSP 120. INX Memory 125 may
hold several lines of image data but typically it does not hold an
25 entire"page."
In due course, DSP 120 obtains image data from
INX Memory 125, processes it, and stores the processed data in
- . . : .. :,, ~ , ~
-37-
Out Buffer 140. Embodiments of DSP 120 are well-known to
those of ordinary skill in the art. For example, in a preferred
embodiment of the present invention, DSP 120 is a Motorola
56001 digital signal processor. DSP 120 accesses I~SP Program
5 Memory 121, for example, a RAM memory device, to obtain
so~tware which guides DSP 120 in converting ~he input digitized
image data into a form which is compatible with the output
format required for making the hardcopy, i.e., to convert the
"area-sized" image data into "pixel-sized" "print" data, and/or to
10 enhance the quality of the hardcopy by the process of
"sharpening." For example, for purposes of illustration and
without limitation, in one embodiment of the present invention,
DSP 120 performs a two-dimensional interpolation on the digital
image data by using two one--dimensional interpolation steps:
15 Specifically, DSP 120 performs: (a) a one--dimensional
interpolation step to provide digitized image data for an
"interpolated" line on image 50 which is disposed between two
actual lines acquired by Image Scan and Acquisition Module 100
and tb) a second one--dimensional interpolation step on each of
20 the scan lines, actual or interpola~ed, to produce digitized image
data for "interpolated" data points which are disposed between
the input data points. In particular, such interpolation steps may
comprise, but are not limited to the following interpolation steps
which are well-known to those of ordinary skill in the art:
25 nearest neighbor interpolation; bilinear interpolation; cubic
convolution; and so forth. Further, as was mentioned above,
the digiti~ed image data, including any interpolated digitized
. - .
_' ' : '' ' ' . ' ' :
2 ~ L
-38-
image data, may be sharpened in a manner which is known to
those of ordinary skill in the art. Still further, specific
embodiments of the present invention can apply different
methods of interpolation to different parts of image 50. Yet still
further, as was indicated above, the software which is stored in
DSP Program Memory 121 was transferred thereto from System
Controller 115. It should be noted that: (a) in some
embodiments the software may be loaded prior to printing each
page to provide for the use of different imaging algorithms for
different images; (b) in other embodiments the software may be
loaded prior to printing different portions of an image; or (c) in
still other embodiments the software is loaded once, at the time
the system is powered up.
The output of the image processing provided by
DSP 120, for this embodiment, comprises eight bit numbers
which correspond to gray levels of the processed pixels.
However, it should be understood that the present invention is
not limited to the use of eight bit intensity levels. The image
processing output is stored in Out Buffer 140. The embodimer~t
described herein which entails transferring image data to Pixel
Generator 700, storing it in INX Memory 125, and performing
image processing upon the image data in real time is
advantageous because it reduces memory costs for the inventive
printsr.
In the preferred embodiment, while the drum rotates
through one revolution, image data necessary to create two
output lines on medium 205 is input to Pixel (ienerator 700,
~ ~ 6 ~. 8 ~ .1
-39-
:!~
where an output line is defined to extend in the direction of
rotation. During the next revolution of the drum, two more lines
are transferred while the two lines that were transferred during
the previous revolution are image processed and output to Out
5 Buffer 140. On the third revolution, two more lines are input,
the lines on the previous revolution are processed and stored,
and the lines that were procéssed during the second revolution
are output to be printed on the rotating drum. This continues
until the entire "page" has been printed. However, some images
10 do not require two lines on every rotation for every output line.
In the case of interpolations transferring of input lines to the
Pixel Generator 700 may be less frequent.
As described above, image processing, DSP 120
transfers eight bit digitized output image data to Out Buffer 140
15 for stora~e. Out Buffer 140 is apparatus which is well-known to
those of ordinary skill in the art for storing digitized image data.
For example, in the preferred embodiment of the present
invention, Out Buffer 140 is a dual ported buffer, for example,
dual ported RAM, with read/write capability through one port by
20 DSP 120 at a first rate and with read capability by LUT
Processor 170 through the second port. This enables the data
to be accessed by the remainder of the output path of Pixel
Generator 700 at a rate which is commensurate with the rate at
which the image is to be written and the speed of rotation of the
25 drum. Further, in the preferred embodiment, Out Buffer 140 is
configurable so that one or two lines of pixels may be output
from different sestions thereof, and DSP 120 s~ores up to 4K
2~ 3 ~A
-40-
.~
pixels per line therein. However, Out Buffer 140 is not required
to be a dual ported RAM and may be, for example, a FIF0.
In accordance with the present invention, LiJT
processor 170 receives pixel data from Out Buffer 140 in the
5 form of pixel values and pel address information, referred to
below as row addresses, from Pixel Size 163. LUT Processor
170 uses the input to retrieve pel configuration pattern
information from among a multiplicity of predetermined pel
configuration patterns. The pixel data from Out Buffer 140
the buffer is selected by DSP 120--is transferred to LUT
Processor 170 in response to address information received from
Pixel Size 163.
The manner in which LUT processor 170 converts a
pixel datum, i.e., the digitized output image data for an area
15 modulation pixel, into pel information which is derived from a
multiplicity of predetermined pel configuration patterns will be
explained in further detail below. However, at this point the
structure of LUT Processor 170 is described in further detail.
Specifically, LUT Processor 170 is comprised of look-up table
20 memories LUT0 and LUT1. In the preferred embodiment of the
present invention, each memory contains the same look-up table
data for use in mapping from intensi~y level, i.e., pixel datum, to
pei configuration pattern. LUT0 and LUT1 are comprised, in a
manner which is well-known to those of ordinary skill in the art,
25 from memory storage devices which are well-known to those of
ordinary skill in the art. A pel configuration pattern which
corresponds to each possible intensity level datum is
.. , .: . ~
3 ~
-41 -
predetermined from, for example, the results of psycho-physical
testing. However, the present invention is not limited to the use
of one particular mapping. Specifically, it is within the spirit of
the present invention that, in some embocliments thereof, the
5 tone scale mapping between a particular intensity level and a pel
configuration pattern may be varied by varying the initial
configuration of printer 10 or by storing several sets of
mappings and by receiving manua7 input from a user, as
illustrated in FIG. 3, as to which of the predetermined tone scale
10 mappings is to be used for making a particular copy. For
example, the manual input may be received by means of a user
setting an indicator or depressing a button or by means of a user
providing input to a user interactive sys~em~ The ~one scale may
be varied for use in a particular application for the purpose of,
15 for example, brightness and/or contrast adjustment.
The output from LUT processor 170 is data which is
used to control the behavior of Lasers 195 of Laser Module 750.
Specifically, in a preferred embodiment of the present invention,
LUT Processor 170 provides 16 bit numbers which are
20 comprised of four, hex-coded bits for each of four lasers which
comprise Lasers 195. For purposes of this description, and that
set forth below, we designate lasers 1, 2, and 3 of Lasers 195
as being capable of providing a substantially 30,um x 3,um pel
and laser 4 of Lasers 195 as being capable of providing a
25 substantially 5,um x 3,um pel. The four, hex-coded bits are
encoded so as to effectuate the slice method which has been
described above, which slice method divides up the time during
.. - ,- . 1'' ~ .
2 ~ 3 .'~
-42-
!~
which a laser is activated so as to be able to illuminate medium
205 in areas which comprise fractions of a pel size.
Multiplexer and Delay 180 may be fabricated in a
manner which should be readily understood by those of ordinary
5 skill in the art from commercially available shift registers or from
programmable gate arrays. In particular, Multiplexer and Delay
180 receives the above-described 16 bit numbers output from
LUT Processor 170 as well as information from DSP 120 which
indicates whether a 60,um x 60,um or a 90,um x 90,um pixel is
10 being printed. This information is used, in a manner which is
described in detail below, to select 4 bits per laser. The 4 bits
per laser are used to develop signals which are used, in turn, to
develop further signals that drive lasers 1-4. The signals
corresponding to the four bits for specific ones of lasers 1-4 are
15 also delayed relative to each other by Multiplexer and Delay
180.
The relative delay of the various laser drive signals
is understood as follows. As has been described above, the
preferred embodiment of inventive printer 10 utilizes four lasers
20 in Lasers 195 to provide a "paintbrush" for printing lines of
hardcopy on medium 205. In accordance with that, to prevent
interference between the edges of the beams by, for example,
diffraction and beam irregularities, from causing inadvertent print
errors, which irregularities occur most often at beam edges, the
25 laser beams which make up the "paint brush" are not physically
disposed side-by-side in a line. The beam irregularities resul~
from the fact that the intensity of a focused Gaussian laser
,
~a~L&~
-43-
. ,
bearn gradually decreases from a maximum in the center of the
beam. Thus, since focused laser beams cannot produce a
uniformly intense spot, some areas of the medium may be well
under or well over its e~posure threshold. To avoid problems at
5 the edges, the lasP.rs are spatially offset in the direction of
scanniny. Thus, the firing o~ the lasers must be delayed relative
to each other such that the pels generated by lasers 1, 2, 3, and
4 are aligned with each other when they expose the medium.
As such, Multiplexer and Delay 180 adds or subtracts, as the
10 case may be, predetermined delays in the firing times for the
lasers which generate the "paintbrush" to compensate for their
spatial offset. For example, in the preferred embodiment, lasers
2 and 3 are delayed 64,u relative to laser 1 and laser 4 is delayed
1 28,u relative to laser 1.
Multiplexer and Delay 180 transmits the 4 bit
numbers for each of the four lasers to Slice 190. In the
preferred embodiment, each four bit number is a four bit hex
number from 7 to 15 which determines how many slices of a pet
the laser is to be energized over, a pel having a maximum length
20 of 3,um in the direction of rotation of the drum.
Slice 190 may be fabricated in a manner which
should be readily understood by those of ordinary skill in the art
from commercially available programmable array logic or frorn
programmable gate arrays. In particular, Slice 190 converts the
25 input from Multiplexer and Delay 180 into four digital signals,
one per laser, that are applied as input to laser drivers in Laser
-44-
Module 750, which digital signals are high or low when a laser
is on or off, respectively.
Phase-Locked Loop 185 (PLL 185) receives input
from Drum Encoder 187 which detects rotation of the drum and
5 generates a signal which is input to Slice 190 so that the output
from Slice 190 is synchronized to the rotating drum. In the
preferred embodiment, one tick of the slice clock corresponds to
.375,um at 2150 rpm or any other suitable speed.
In response to the digital signals ou~put from Slice
190, laser drivers in Laser Module 750 produce high current
drive signals which are applied to drive Lasers 195. In response
to the drive signals, Lasers 195 output timed bèams of radiation
which impinge upon medium 205 and produce therein a copy of
image 50. It will, of course, be clear to those of ordinary skill in
15 the art that further lines are printed upon medium 205 as the
radiation output from Lasers 195 is moved across medium 205
in a direction transverse to the direction of a line when the
optical head (not shown) in Laser Module 750, which holds
Lasers 195, is moved in the transverse direction. An example of
20 a suitable opticai head is shown for example in U.S. Patent
Application (Our Case No. 7584) entitled "Printer Optical Head"
filed on the same date herewith and commonly assigned.
Further, the lasers are only driven when their beams would
impinge on medium 205 and they are not driven when their
25 beams would impinge, for example, on drum clamps. In
addition, it should be clear to those of ordinary skill in the art
that inventive printer 10 further comprises apparatus which are
.
-45 -
well-known in the art but which have been omitted for ease of
understanding the present invention. For example, inventive
printer 10 includes, without limitation, the Following types of
modules: drum drivers, synchronizing means for drum
5 positioning, laser autofocus apparatus, medium transport, and
the like.
We will now describe the manner in which data
stored in Out Buffer 140 is applied as input to LUT processor
170 to generate laser drive signals. The eigh$ bit processed
10 data in Out Buffer 140 are output as the upper address of LUT0
and LUT1. The address of the eight bit data in Out Buffer 140 is
determined by a signal transferred thereto from Pixel Size 163
and is the address of the printer side of the dual ported RAM of
Out Buffer 140. This address signal is updated at a pixel rate.
15 For example, for a 60,um x 60,um pixel, the address is updated
every 60,um, whereas, for a 60,um x 80~m pixel, the address is
updated every 80,um. The lower part of the address of LUT0
and LUT1, i.e., the row address, is generated in response to an
output signal from Pixel Size 163 which is applied as input to
20 LUT Processor 170. The row address counter counts from 0 to
29 at a pel rate and rolls over at a rate corresponding to 3,um
pels.
In a particular embodiment, the pixel and pel rates
can be determined from the following information: the length of
25 the page, for example, 10 inches; the size of the pixel, for
example, 60,um x 80,um, 90,um x 90,um, and so forth; the size of
the pel; and the rotation speed of the drum. For example, the
~ .... . , . ................ . , .:
, . . - . . .
2 ~ ~ ~ & ~ . ~
-46-
,
pel rate is equal to ~slice clock)/8 and, in an embodiment where
the drum rotation speed is 2400 rpm and a pel is 3~m, the pel
ra~e is 30MHz/8. Further, the pixel rate is the ~pel rate)/(number
of pels in a pixel~. Lastly, for a 60,um x 60,um pixel, there are
20 pels/pixel and, for a 90~m x 90,um pixel, there are 30
pels/pixel.
We now turn to describe, in detail, the manner in
which data is retrieved from LUT Processor 170 with reference
to FlGs. 5 and 6. FIG. 5 helps to show how data stored in LUT0
and Ll~lT1 is retrieved to supply pel information which is used to
drive Lasers 195 in Laser Module 750. Specifically, FIG. 5 helps
to show how data is retrieved to supply pel information for a
60,um x 60,um pixel and for a 90,um x 90,um pixel in accordance
with our discovery that the mapping for a 90,um x 90,um pixel
may also be used to provide a 6~m x 60,um pixel and other
pixel sizes as well.
In particular, first consider the case of a
90,um x 90,um pixel. As was described above with respect to
the preferred embodiment of tha present invention, a paintbrush
for Lasers 195, as shown above arrow 2000 in FIG. 5, is
comprised of laser 3, lasers 1 and 4, and laser 2. The footprint
of each of lasers 1, 2, and 3 is 30,um and the footprint of laser 4
is 5um along the direction indicated by arrow 2000. Thus, as
lasers 1-4 are excited and impinge upon medium 205 along the
path between lines 1003 and 1004, they "paint" with a
brushstroke which is 90,um across. Further, as shown in FIG. 5,
the distance between arrows 2000 and 2002 are 90~m. Thus,
. . .
-~7-
?~
there are 30 pels in the 90,um x 90,um pixel whose borders are
lines 1003 and 1004, and the lines indicated by arrows 2000
and 2002.
The data which are stored in LUT0 and LUT1 are
5 identical and these data correspond to the 90,um x 90,um pixel
just described. As a result, for a 90,L/rn x 90~m pixel, one only
nP~eds to retrieve data which is stored in LUT0. FIG. 6 shows a
ma~rix of data corresponding to a 90,um x 90,um pixel. The
rows 0--29 correspond to pels for lasers 1--4 and each row,
i.e., rows 0--29, contains a 16 bit number which has four bit,
hex coded values for each of lasers 1--4.
In order to retrieve this data, one needs to present
LUT Processor 170 with two pieces of information, i.e., the
intensity level of the pixel --in the preferred embodiment this
is an eight bit number between 0 and 255 and a pel number
in this embodiment a pel number is a row address between
O and 29 which corresponds to the pels which are painted as
the laser beams impinge upon medium 205 between arrows
2000 and 2002. In response to this information, LUT Processor
170 retrieves a 16 bit number from LUT0 where bits 0--3 are
used for laser 2; bits 4--7 are used for laser 1; bits 8--11 are
used for laser 3; and bits 12--15 are used for laser 4. Of
course, those of ordinary skill in the art understand that this
choice of bits is arbitrary and may be changed in other
embodiments. For example, this choice of bits may be changed
in software or in cabling.
~ . , ' ' ! . ' ' ! , ~ .
-48-
The inputs to LUT Processor 170 which correspond
to the intensity levels of the pixels and the row addresses of the
pels are obtained from Out Buffer 140 and Pixel Size 163,
respectively. Pixel Size 163 has three registers which contain
the following information, respectively: the number of pels/pixel;
the number of pixels/line; and the numbsr of lines/page. As
such, Pixel Size 163 transmits a number to Out Buffer 140
which corresponds to the location of the pixels in a line to be
printed. Out Buffer 140 uses this number to address the pixels
which are stored therein and which correspond to a line. Out
Buffer 140 retrieves the value in its memory which corresponds
to the intensity level of the pixel and applies it as input to LUT
Processor 170. At the same time, Pixel Size 163 applies the
value of a row counter which cycles between 0 and 29 as input
to LUT Processor 170.
As one can readily apprecia~e, as Out Buffer 140
cycles through the pixels stored in its memory and, for each
such pixel, as Pixel Size 163 cycles through 0-29, a line of data
is retrieved for use in firing Lasers 195 in Laser Module 750.
We now turn to the case of a 60,um x 60~m pixel.
This case is complicated by two facts. First, in order to take
advantage of all four lasers, a 6~m x 60,um pixel requires the
simultaneous printing of one and one-half such pixels. Second,
due to the real time constraints on the system, there is not
enough time available to retrieve the necessary data from a
single look-up table memory.
: . ~ ~ . . . ",
-49-
With reference to FIG. 5, LUT Processor 170
retrieves the necessary laser drive data as follows. First,
consider the region denoted by A, between lines 1003 and 1005
and arrows 2000 and ~001 to be pixel 1; the region denoted by
A2 between lines 1005 and 1006 and arrows 2000 and 2001 to
be pixel 2; and the region denoted by A3 between lines 1006
and 1007 and arrows 2000 and 2û01 to be pixel 3. The pixels
in the line of pixel 1 are painted with laser 3 and lasers 1 and 4
using data obtained from LUT0; the pixels in the line of pixel 2
are painted with laser 2 and laser 3 using data obtained from
LUT1; and the pixels in the iine of pixel 3 are painted with lasers
1 and 4 and laser 2 using data obtained from LUT0. As one can
readily appreciate, the lines of pixels across a page, i.e., the
direction transverse to the direction in which lines are painted,
obtained data to drive the lasers alternatively from LUT0 and
LUT1 in a variety of sequences.
In addition to the above, since a "paintbrush"
utilizes laser 3, lasers 1 and/or 4, and laser 2, the paintbrush
covers one and one-half of a 60,um x ~O,um pixel simul~aneously
The data to accomplish this task is retrieved as follows. (1 ) The
data for laser 3 and lasers 1 and 4 for the pixel between lines
1003 and 1005 and arrows 2000 and 2001 are obtained from
LUT0 by providing intensity level A~ and row addresses 0--19 to
LUT Processor 170. For each 16 bit number retrieved
therefrom: bits 8--11 are for laser 3; bits ~1 7 are for laser 1;
and bits 12--15 are for laser 4. (2) The data for laser 2 for
one-half of the pixel between lines 1005 and 1004 and arrows
I. .
3 l~
-50-
2000 and 2001 are obtained from LUT1 by providing intensity
level A2 and row addresses 0--19 to LUT Processor 170. For
each 16 bit number retrieved therefrom: bits 0--3 are used for
laser 2. (3) The data for laser 3 and lasers 1 and 4 for the pixel
between lines 1003 and 1005 and arrows 2001 and 2003 are
obtained from LVT0 by providing intensity level B1 and row
addresses 20--29 to LUT Processor 170 for the portion
between arrows 2001 and 2002 and by providing intensity level
B1 and row addresses 0--9 to LUT Processor 170 for the portion
between arrows 2002 and 2003. For each 16 bit number
retrieved therefrom: bits 8--11 are for laser 3; bits 4--7 are for
laser 1; and bits 12--15 are for laser 4. (4) The data for laser 2
for one-half of the pixel between lines 1005 and 1004 and
arrows 2001 and 2003 are obtained from LUT1 by providing
intensity level B2 and row addresses 20--29 to LUT Processor
170 for the portion between arrows 2001 and 2002 and by
providing intensity level B2 and row addresses 0--9 to LUT
Processor 170 for the portion between arrows 2002 and 2003.
For each 16 bit number retrieved therefrom: bits 0--3 are used
for laser 2. (5) The data for laser 3 and lasers 1 and 4 for the
pixel between lines 1003 and 1005 and arrows 2003 and 2004
are obtained from LUT0 by providing intensity level C1 and row
addresses 10--29 to LUT Processor 170. For each 16 bit
number retrieved therefrom: bits 8--11 are for laser 3; bits 4--7
are for laser 1; and bits 12--15 are for laser 4. (6) The data for
laser 2 for one-half of the pixel between lines l 005 and 1004
and arrows 2003 and 2004 are obtained from LUT1 by
- . . ~ . ~ . i, , ~
:. . ,. ............... , -
, " ~ . ~. , :
2 ~
-51 -
. ~
providing intensity level C2 and row addresses 10--29 to LUT
Processor 170. For each 16 bit number retrieved therefrom: bits
0--3 are used for laser 2.
We will now describe the manner in which the laser
drive data for the second half of the line of pixel 2 and the line
of pixel 3 are obtained. (1 ) The data for laser 3 for one-half of
the pixel between lines 1004 and 1006 and arrows 2000 and
2001 are obtained from LUT1 by providing intensity level A2
and row addresses 0--19 to LUT Processor 170. For each 16
bit number retrieved therefrom: bits 8 11 are used for laser 3.
(2) The data for lasers 1 and 4 and laser 2 for the pixel between
lines 1006 and 1007 and arrows 2000 and 2001 are obtained
from LUT0 by providing intensity level A3and row addresses
0 `19 to LUT Processor 170. For each 16 bit nurnber retrieved
therefrom: bits 4--7 are for laser 1; bits 12--15 are for laser 4;
and bits 0--3 are for laser 2. t3) The data for laser 3 for
one-half of the pixel between lines 1004 and 1006 and arrows
2001 and 2003 are obtained from LUT1 by providing intensity
level B2 and row addresses 2~29 to LUT Processor 170 for the
portion between arrows 2001 and 2002 and by providing
intensity level B2 and row addresses 0--9 to LUT Processor 170
for the portion between arrows 2002 and 2003. For each 16
bit number retrieved therefrom: bits 8--11 are used for laser 3.
(4) The data for lasers 1 and 4 and laser 2 for the pixel between
lines 1006 and 1007 and arrows 2001 and 2003 are obtained
from LUT0 by providing intensity level B3 and row addresses
20--29 to LUT Processor 170 for the por~ion between arrows
-5~-
, ~
2001 and 2002 and by providing intensity levei B3 and row
addresses 0--9 to LUT Processor 170 for the portion between
arrows 2002 and 2003. For each 16 bit number retrieved
therefrom: bits 4--7 are for laser 1; bits 12--15 are for laser 4;
and bits 0--3 are for laser 2. 15) The data for laser 3 for
one-half of the pixel between lines 1004 and 1006 and arrows
2003 and 2004 are obtained from LUT1 by providing intensity
level C2 and row addresses 10--29 to LUT Processor 170. For
each 16 bit number retrieved therefrom: bits 8--11 are used for
laser 3. (6) The data for lasers 1 and 4 and laser 2 for the pixel
between lines 1006 and 1007 and arrows 2003 and 2004 are
obtained from LUT0 by providing intensity level C3 and row
addresses 10--29 to LUT Processor 170. For each 16 bit
number retrieved therefrom: bits ~1 7 are for laser 1; bits
12--15 are for laser 4; and bits 0--3 are for laser 2.
As above, the inputs to LUT Processor 170 which
correspond to intensity levels and row addresses are obtained
from Out Buffer 140 and Pixel Size 163, respectively. However,
in this case, instead of sequencing through a single line of pixel
intensity level data, Out Buffer 140 sequences through two lines
at the same time. As was indicated above, this enables LUT
Processor 170 to apply the intensity level from one line to LUT0
while the intensity level from the other line is being applied to
LUT1. Specifically, as was shown above, The intensity level
from pixels in the line of pixel 1 are applied to LUT0 and the
intensity level from pixels in the line of pixel 2 are applied to
LUT1. Then, after the line of pixel 1 and the first one-haif of the
, , .,: "
- . . ~ . , , , ; ,
. - , ~; .,
. . - .. ~. . ,.~ . . . .
$
-53-
.~
line of pixel 2 have been printed, the intensity level from pixels
in the line of pixel 2 are applied to LUT1, and the intensity level
from pixels in the line of pixel 3 are applied to LUT0 to print the
second half of the line of pixel 2 and the line of pixel 3. This
alternating technique continues until all of the lines on the page
are printed.
In addition to the above, it should be understood
that embodiments of the present invention also apply to
situations which utilize pixel replication and magnification. For
example, using repeat factors for lines and/or for pixels, an
image may be magnified in either direction in integer increments
with the smallest size being such that one pixel is mapped into a
single output pixel as has been described in regard to the
preferred embodirnent set forth above. In addition, as a special
case, shading characters are realized when the replication
factors are such that each input pixel produces an integral
number of output pixels. In this case, the in~ensity level is
represented by a whole matrix and never by a fraction of a
matrix. Further in addition, the aspect ratio of the pixels may be
adjusted by using non-equal pixel and line replications to correct
for non-square input pixels, output pixels, and/or both. Such
various embodiments may be provided by appropriately
programming DSP 120 in a manner which should be clear to
those of ordinary skill in the art.
It should be noted that, in the preferred
embodiment, the pixel to pel configuration pattern mapping was
a particular type of mapping. tlowever, i~ should be noted, that
- ., , ....... . - ~ , - .
- ~ . . . . . . ` . .. . . .
3 ~
-54-
the present invention is not limited to the use of the mappirig of
the preferred embodiment. In general, the presen~ invention
applies to embodiments wherein the pixel to pel configuration
pattern mapping is a whole host of different mapping functions
such as, for example and without limitation, area modulation
imaging produced by clustered threshold arrays, dispersed dot
ordered dither mapping, rectangular or hexagonal array
structures, non-monotonic pel configuration patterns wherein
pels that are used in a lower gray scale level do not have to be
used in higher gray scale levels, and so forth.
Embodiments of the present invention which utilize
such variations in pixel to pel configuration pattern mappings
may be fabricated by fabricating LUT Processor 170, in a
manner which should be clear to those of ordinary skill in the
art, to retrieve the appropriate data from matrices which
comprise such mapping data. For exarnple, in an embodiment of
the printer wherein DSP 120 provides pixel intensity levels that
are buffered in Out Buffer 140 so as to print multipie lines in a
single pass of a multiple writing element print head comprised of
Lasers 195, the lines in Out Buffer 140 may be double buffered
so that, while one group of lines is being printed, the next group
of lines can be read therein.
For example, in such an embodiment, Pixel
Generator 700 i~ initialized with: the number of lines; pixel
intensity levels to be printed per line; and the number of pels in
a pixel. Further, space is allocated for buffers in Out Buffer 140;
pointers to the current printing and loading buffers in Out Buffer
,. ... . . . . - . ... . ...
- .- ~ . - . .
, . .. ,, . . . . ~, .. , .. .
- ~ g~ J 3
-55-
.
140 are initialized; and corresponding flags for these two
conditions are set.
The first step in printing is to load a line into the
buffer. There is a signal, PGactive, that indicates the position of
5 the rotating drurn. PGactive indicates when the lasers are
active, i.e., printing a line, and when the lasers are over a clamp,
i.e., the lasers are off. As the drum rotates through one
revolution, DSP 120 fills a buffer in Out Buffer 140, beginning
when the iassrs are over the clamp, wi~h 1 or 2 lines of eight bit
10 pixels, depending on whether a 60,um or a 90,um pixel is being
printed. Durin~ the same revolution, 1 or 2 lines of eight bit
pixels that where written to Out Buffer 140 from DSP 120
during the previous revolution are output from Out Buffer 140 to
LUT 170 to be printed on a page. During the next revolution,
15 DSP 120 fills the buffers that were previously used for printing
and Out Buffer 140 outputs from the buffers that were filled by
DSP 120 during the previous revo!ution.
In such embodiments, printing a pixel requires the
retrieval from a memory such as LUT Processor 170 of the pixel
20 to pel mapping. For example, the inputs for the mapping are
intensity l~vel, column pointer, and row pointer for the pel at a
particular column and row of the matrix corresponding to the
intensity level. The manner in which such mapping matrices
may be stored and retrieved from storage is well known to those
25 of ordinary skill in the art.
Other embodiments of the invention, including
additions, subtractions, deletions and other modffica~ions of the
2 ~
-56-
preferred disclosed embodiments of the invention will be obvious
to those skilled in the art and are within the scope of the
following olairns.
h ~ J
TABLE I
GROUP BEGINNING OF CLUSTER SIZE RANGE
CLUSTER LOCATION IN SLICES
Row LaserMin. Max.
A 5 4 6 200
B 5 4 110 200
0 3 12 12
C 5 4 110 200
2 12 12
0 3 12 12
D 0 3 12 88
4 200 200
2 12 88
E 0 3 40 88
3 0 88
4 200 200
2 40 88
0 2 0 88
F 0 3 64 96
0 1 24 96
0 2 64 96
1~ 3 64 96
1 24 96
2 64 96
(i O 3 192 216
0 1 192 216
0 2 192 216
H 0 3 240 240
0 1 192 216
0 2 192 216
0 3 240 240
0 1 192 216
0 2 240 ~40
J 0 3 ~40 240
0 1 240 240
0 2 240 240
