Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The printing process commonly used in industries
which raquire reproducing graphic material, the newspaper
and book publishing industries, for example, deposits a
uniform density of ink on paper whenever it is desired to
print all or a portion of an image and deposits no ink when
the a~sence of an image is desired.
The all or nothing process poses no problem when
alphabetical or other aLphanumeric characters are printed.
However, when pictures such as photographs are printed, the
problem o~ reproducing the continuous tones (i.e. light
gradations) arises. This problem has been solved by trans-
forming the continuous tones in the original image into a
halftone image which comprises a large numher o~ ink dots
of various sizes. This is referred to as "screening" and is
performed by projecting the image through a ~ine mesh screen
onto a photographic medium. When the largest dots and the
spaces on the paper between the dots are made small compared
with the visual acuity of the human eye, the dots and the
spaces between the dots fuse visually in the screened image,
the eye believing it is seeing continuous tones.
However, in an automated system in which electronic
image reproduction forms at least part o~ the process of
converting a continuous ori.ginal image into a haLftone image,
the necessiky ~or switching from electronic to photogxaphic
techni~ues in order to produce hal~tone is a ~actor which
adds tothe cost and complexity of the process~ An electronic
photocomposition system which obviates this problem is dis-
closed in U. S. Patent No. 3,465,199. The system disclosed
therein translates the tonal in~ormation on an original
transparency into a corresponding image on the face of a
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cathode ray tube. The halftone images are recorded on film
and thereafter may be processed into a printing plate by
well known techn:iques. Another system which eliminates
the aforementioned photographic technique is disclosed in
U.S. Patent No. 3,646,262 which also discloses means to vary
the size or shape of the halftone dots formed on a photo-
sensitive member. The aforementioned systems are primarily
concerned with reproducing, as halftone, a black and white
original. Color reproduction reguires the reproduction of
many different colors and shades. The multitude of colors
is produced in conventional printing processes by the three
subtractive primary colors, cyan, magenta and yellow. For
high-quality reproduction a fourth ink, black, is also
utilized. For large-volume reproduction of an original
color pattern, there is prepared a set of halftone printing
plates, with each carrying a halftone image of one color
component of the original pattern. The original pattern is
reproduced by overprinting with each printing plate so that
the three printing inks visually combine to produce the
_ J correct colors.
The printing plates needed for color printing may
be derived by scanning the original pattern in an electronic
color scanner machine as set forth in U.S. Patent ~o. 3,622,690.
The color scanner typically scans the oriyinal pa-ttarn with light
and measures the tones or color in the pattern by filtering
the scanned signal with red, blue, and green color ~ilters.
The amplitudes of the filtered signals indicate the color
content of the original pattern. Since the color printing
inks are not spectrally perfect and hence do not correspond
exactly to the three subtractive colors, the filtered signals
are corrected for these deficiencies by means of color correction
circuits in the color scannerO The color corrected signals
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are utili~ed to modulate the light emitted from a laser to
produce continuous tone color separations of the original
pattern~ The continuous tone color separations are then
screened photographically and further processed to prepare
the halftone printing plates. Alternately, screened colo
separations are directly provided without requiring a
separate photographic screening step.
Other halftone techniques utilize variations of
character generation schemes whereby various elements of a
two-dimensional matrix are turned on or off to create various
dot patterns and characteristics~ Alternate techniques
deflect a CRT beam or laser beam in such a manner as to draw
dots of various shapes and characteristics~ The dots are
then repeated spatially to generate a halftone grid.
Prior art systems may incorporate electronic schemes
which generate a horizontal or vertical line halftone, the
scheme utilizing a pulse width modulation technique. In
particular, a reference signal, which may be triangular, sine,
cosine, waveform, depending upon the desired amplitude to
pulse width conversion characteristics, is applied to a
voltage comparator which compares the reference signal with
a signal representing the tonal values of a scanned oriyinal.
The comparator output may be coupled, for example, to a
cathode ray -tube to control spot size. The aEorementioned
Patent No. 3,~65,199 is an example of such a system. ~.S.
Patent No. 3,916,096 discloses a technique for constructing
a two--dimension halftone by using an electronic line screen-
ing technique. In particular, a single reference signal is
amplified in separate, parallel channels. The amplified
outputs are compared with a video signal in separate com-
parators, the screened video output being switched between
comparator outputs thereby providing two different dot line
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widths. The system described in this patent provides, in
essence, a line halftone and not a con~inuously varying
two-dimensional spot. Although the screened video output
pattern may be recorded on a reproduction device, limiked
control of the shape of the dots generated and the angular
relationship of the generated dots in relation to the
scanning direction is provided.
The line hal~tone techniques set forth hereinabove
for converting continuous tone originals into halftones do
not provide the reproduction details required in many
applications. Further, it would be desirable to adapt
electronic halftone techniques to directly reproduce, or
copy, a black and white or color original document either
locally or at a remote location. Although black and white,
and recently, color copiers, are commercially available, the
techniques utilized therein provide reproductions which
although satisfactory for most purposes, are limited in some
respects. In particular, reproduction of continuous tone,
black and white and color originals have not provided the
~0 details required in certain applications.
It would be desirable, therefore, if two-dimensional
electronic halftone techniques can be provided for black and
whit:e and color copying processes which allow the shape and
characteristics of the halfkone dots to be easily controlled,
provides for electronic screen simulakion and angular rotation
thereof to reduce Moire' pattern effects, is economical and
reliable and which provides a reproduction or copy whose
konal characteristics are a substantial replica of that in
the original.
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SUMMARY OF THE PRESE~T INVENTION
The present invention in one aspect provides method
and apparatus for scanning a document original, either black
and white or color, and reproducing a corresponding halftone
reproduction thereof either locally or at a remote location.
A halftone signal is generated by pulse width modulating~ or
comparing the scanned or video signal with a periodic signal
having two frequencies and phases to create a two-dimensional,
continuously va~ying dot pattern output which is a function
of the frequency and phase of the two combined modulating
signals. The halftone reproduction generated has variable dot
configurations that are controllable to enable both rotation
of the dot pattern (screen angle~ and geometric modifications
of the dot pattern. If the document original is in color,
light of three different colors is caused to scan the document,
each resultant video signal being processed in a manner as
set forth hereinabove. In a preferred embodiment, the
diferent screen angles are utilized and each color comprises
the reproduction.
It is an object of an aspect of the present inven-
tion to provide method and apparatus for scanning either a
black and white or color original document and reproducing a
corresponding black and white or color halftone image either
locally or at a remote location.
It is an object of an aspect of the present inven-
tion to provide an electronic halftone generator which
generates a halftone dot matrix which corresponds to a con-
tinuous tone original, the dots varying in size and shape in
accordance with a predetermined periodic function.
It is an object of an aspect of the present inven-
tion to provide an electronic halfton~ generator which
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utilizes a screening function which is periodic in time with
dual frequencies and phases, the screening function allowing
the characteristics of the ~wo-mentional dot grid which com-
prises the halftGne pattern to be varied and the screen angle
thereof to be rotated, the latter to avoid Moire pattern
problems inherent in using multiple screens.
It is an object of an aspect of the present inven-
tion to provide a two-dimensional grid of halftone dots
wherein the dot characteristics can be varied by varying a
screening function and wherein the halftone grid can be
rotated relative to the input or output scanning direction.
It is an object of an aspect of the present inven-
tion to provide an electronic halftone generator for repro-
duction and~or display purposes which is operative in real
time and requires no data storage.
It is an object of an aspect of the present inven-
tion to provide method and apparatus for displaying an
electrical signal representing an image or video infoxmation
as a predetermined halftone grid pattern on a di~play device.
It is an object of an aspect of the present inven-
tion to provide method and apparatus for scanning a document
original, either black and white or color, and reproducing
a corresponding halftone reproduction thereof either locally
or at a remote location. A halftone signal is generated b~
pulse width modulating, or comparing, the scanned or video
signal with a periodic signal having two frequencies and
phases to create a dot pattern output which is a function of
the frequency and phase of the two combined modulating signals.
The halftone reproduction generated has variable dot configu-
rations that are controllable to enable both rotation ofthe dot pattern (screen angle) and geometric modifications
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of the dot pattern. If the document oriyinal is in color,
light of three different colors is caused to scan the docu-
ment, each resultant video signal being processed in a
manner as set forth hereinabove. In a preferred embodiment,
the different screen angles are utilized for each color that
comprises the reproduction.
In accordance with ano~her aspect of this inven
tion there is provided apparatus for converting an electrical
an-alog input signal representing an image into a coxrespond-
ing output signal in the form of a dot pattern correspondingto said image comprising~ means fox generating a time-vary-
ing electrical function which is a function of first and
second signals of ~irst and second frequencies, respectively,
said first and second frequencies being separately adjustable,
means for accepting said analog signal as a series of succes-
sive san lines during the generation of each dot which will
form said dot pattern~ said analog signal being produced by
scanning said image in first and second directions, means
coupled to said generating means and said accepting means for
comparing said successive scans with said function and gene-
rating a comparison signal when said function differs from
said successive scans, rneans responsive to said difference
signal for providing an output signal, and means for gene-
rating said dot pattern by scanning said output signal in
directions corresponding to said first and second directions,
the direction of alignment of the dots relative to said
first direction of scan being determined by the selection
of said first and second frequencies.
In accordance with another aspect of this invention
there is provided a method for converting an electrical analog
input signal representing an image into a corresponding output
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signal in the form of a dot pattern corresponding to said
image comprising the steps of: generating a time-varying
electrical function which is a function of first and second
signals of first and second frequencies J respectively, said
first and second frequencies being separately adjustable,
accepting said analog signal as a series of successive
scans during the generation of each dot which will form said
dot pattern, said analog signal being produced by scanning
said image in first and second directions, comparing said
successive scans with said function and generating a com-
parison signal when said function differs from said succes-
sive scans, and providing an output signal, said dot pattern
being generated by scanning said output signal in directions
corresponding to said first and second directions, the direc-
: 15 tion of alignment of the dots relative to said first direction
of scan being determined by the selection of said first and
second frequencies.
DESCRIPTION OF THE DRAWING
For a better understanding of the invention as well
as other objec-ts and further features thereof, reference is
made to the following description which is to be read in con-
junction with the accompanying drawings wherein:
Figure 1 illustrates a pulse width modulator utilized
in the prior art;
Figure 2 illustrates the application of pulse width
modulation teclmiques to line halftone generation;
Figure 3 is a line halftone time phase diagram;
Figure 4 is a simplified diagram illustrating the
basic concept of the present invention;
Figure 5 illustrates characteristic dot shapes for
a particular screening function;
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Figure 6 illustrates alternate implementations of
the electronic halftone generator of the present invention;
Figure 7 is a slmplified schematic of the electronic
halftone generator of the present invention;
Figures 8-10 show a schematic diagram of one embodi-
ment of the present invention for a fixed screen angle;
Figure ll(a) ~hows a dot matrix for a 45 standard
screen and Figure ll(b) shows a dot matrix for a 45 ellip-
tical screen; and
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Figure 12 is a bloc~ diagram illustrating a half-
tone color reproduction system which utilizes the electronic
halftone generator of the present invention.
DESCRIPTION OF THE P~EFER~ED EMBODIMENT
In order to illustrate the novel features of the
present invention, a brief description of prior art electronic
halftone dot generation techniques will be briefly set forth.
- The majority of approaches use variations of
character generation schemes whereby various elements O:e a
two-dimensional matrix are turned on or off to create various
dot patterns and characteristics. The patterns are then
repeated to construct the halftone dot matrix. Other tech-
niques deflect a CRT beam or laser beam in such a manner as
to draw dots of various shapes and characteristics. The
dots are then repeated spatially to generate the halftone
dot matri~ (grid).
The technique which is similar in some respects to
the technique of the present invention is the application of
pulse width modulation techniques to generate a horizontal
or vertical line halftone. The pulse width modulation (PWM)
technique isillustrated schematically in Figure 1. The
reference signal is a periodic function of time with frequency
fc The reference waveform can be, for example, triangular,
sine or cosine, in shape depending upon the desired amplitude
to pulse width conversion characteristics. The fre~uency
(clock) fc is generally a factor of -two or more greater than
the high frequency cutoff of the video signal to satisfy
information and sampling theory criteria. The dynamic range
(D.R.) of a system using a dot matrix halftone ~dynamic range
being defined as the ratio between maximum reflectivity [or
brightness3 to minimum reflectivity [or brightness] in the
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output excluding the complete absence of dots or lines) is
given by the ratio of the clock period Tp ( 1 ) to the
minimum pulse duration (t ) that can be toleraCted and/or
produced at the output,
D.R. = p
t f t
p c p
The application of PWM to line halftone
is schematically
illustrated in Figure 2 and is basically the same. The
reference frequency is
f = f~ v
c
where f~ is the line halftone spatial frequency and v is
the scan velocity in the appropriate direction (X or Y).
The reference signal phase ~ is adjusted to obtain proper
alignment of the halftone line pattern. In general, the
reference frequency is less than the high frequency cutoff
of the video (particularly for line halftones oriented such
that the lines are parallel to the X (high velocity) scan
direction). The line halftone time phase diagram using a
_0 triangular reference waveform is shown in Figure 3. A light
output i9 produced when the video input is greater than the
for positive printing or displayO
reference signal/ This technique enables high frequency and
high contrast video to be retained as shown at A in Figure 3.
rrhe dynamic range is given by the ratio of the line spacing
(~ ) to the minimum reproducible line wid-th (d), i.e.,
D.R. ~
d ~d
The limited dynamic range and the general limita-tions of
vertical or horizontal line halftone orientation
is characteristic of the application of PWM
prior art to halftone generation. As will be set forth hexe-
inafter, the present invention increases the dynamic range of
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halftone systems while providing additional advantages such
as screen rotation and dot shape selection using electronic
techniques.
Figure 4 illustra.es the basic concept of the
present invention. F(v) is a function of the input video
and S(fl, f2~ 01~ 02~ t) is the screening function which,
as will be set forth in detail hereinafter~ provides for
screen rotation and dot shape selection. The screening
- function is periodic in time with dual frequencies and
phases. It is the dual frequency naiure of the screening
function which provides a significant improvement irl
capabilities over the line halftone technique described
hereinabove. The normalized output of comparator 8 is
defined as
VO = 1, F ~ S; V = 0, F~ S
for direct output and
V = 1, F C S; V = 0, F~ S
o o
for complimentary output. The direct output or complimentary
output is the halftone signal to be used for reproduction
and/or display with an output of 1 defined as white and an
output of 0 defined as bLack. In the preferred embodiment
the input and output scan techniques and devices are of a
rectilinear X-Y nature.
~owever, this does not preclude the use of alter-
nate s~anniny techniques, such as circular, spiral, etc. in
the present invention. In X~Y scanning applications the
frequencies are defirled as
1 x Vxfd; f2 - fy = v f
where v and v are the scan velocities in inches/sec in the
~ and Y directions respectively and fd is the spatlal dot
frequency desired in dots per unit length, i~e. dots/inch.
.
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The phase terms are defined by
0 =0, 0 =0
l x 2 Y
and provide proper synchronization of the halftone screen
with the scanning devices. The phase texms can be dropped
from further discussion since the phase (absolute) defines
starting point (where scanning spots start from edge of the
reproducing medium) determined, for example, by a scan start
signal, without the loss of generality as long as the
relative phase is maintained.
The condition for which the comparator switches
states is given by
S(f ,f ,t~ = F(v)
and represents the locus of points defining the halftone
dot shape. In particular, at the scanning point where S=F,
the output VO changes from white to black or black to white.
A saries of scan lines, typically 7 or 8, builds up the actual
dot. l'he scxeening function S determines characteristic dot
shapes for uniform grey value inputs (F constant) whereas the
video function F determines the grey value displayed and/or
~0 reproduced for various grey value inputs. The folLowing are
examples of halftone patterns produced for various screening
functions:
(l) A screening function which is the linear s~m
of two triangular waves with fre~uencies fx and f , i.e.,
S(f ,f ,t) = T (f t) -~ T (f t)
x y l ~ 2 y
will genera-te a halftone parallel to the X and Y scan directions
having dots which are diamond shape for constant grey value
input (a video signal of uniform in-tensity during the active
portion of the scanning system and represents a uniform
density or reflectivity or transmissivity of the scanned
original)> The tone reproductlon curve (density in/density
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out) will have a yamma = 2 for
F(v) = cv + d
and a gamma = 1 for
F(v) = ~ + d
where c and d are arbitrary constants to match input and
output white and black and characterize the electronic signal
representative of the scanned original document~
(2) A screening function given by
S(fX,fy,t) = Tl (f t) + T2 ~f t)
will produce a halftone grid with characteristic dot shapes
which are circles. The circles will merge for certain values
of F(v) and the tone reproduction curve (TRC) is in general
non-linear. However, an appropriate choice of F(v) can
linearize the TRC.
From these examples it can be seen that appropriate
choice of the screening unc~ion, S, and video function, F,
can provide a wide variety of dot shape characteristics and
TRC's.
The most useful screening function is a combination
of sines and/or cosines waveforms as the dot shape character-
istics will match those presently achieved in the photo-
lithographic industry using optical contact halftone screens.
The screening function
a cos 27rfX t ~ b cos 2Tr f t ~ E'(v) (1)
~5 will produce -the dot shapes shown in Figure 5 for various
values of K = ~1 ~ and a - b. For a ~ b the dot shape will
expand or compress in one direction, without changing the
dot frequency in either direction, thereby duplicating the
characteristics of "elliptical" screens in the photolitho
o yraphic industry. "Elliptical" screens have the effect of
reducing visual contouriny effects when the poinb of adja~ent
dots just touch. The constant "a" can represent the voltage
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or curren-t gain in the electronics and the constant "b"
: can represent the D.C. offset voltaye or current. The
magnitude of these constants are determined at the input
to the comparator by the peak-to-peaX voltage and D.C.
offset of the halftone reference signal and the halftone
dot size desired in the highlight and shadow regions in
the reproduction and/or display of the original document.
rrhe above screen function produces a halftone
grid defined as a 0 screen since the grid is parallel to
1~ the X or Y scanning directions. Screen angles other than
0 can be produced with the screening function
S(f ,f ,t) = a cos 27r (f cos9 + f sinO~t +
x y x y
b cos 2 ~ (f sinO - f cosO)t (la)
where ~ is the desired screen angle. For a = b~a standard
screen (for example, highlight and shadow data will be
circular in shape) is produced and for a ~ b an "elliptical"
screen is produced where the chaining direction, i.e., direction
in which the adjacent dots first touch at midpoint grey, is
either parallel or perpendicular to the screening direc~ion
O. If ~ is -+ 45 the above screening function simplifies to
S = a cos ~r~ ~ (f + f ) t + b cos ~2 R(f - f )t =
x y x y
2a cos ~ llf t cos ~ 7Tf t + (b-a~ siny~l~fxt sin ~ ~ f t (2)
for elliptical screens.
This simplif.ies ~urther for a standard screen (a~b)
to
S = 2a cos ~ ~f t cos ~ ~f t (3)
x Y
and differs from 0 screens by a reduction in frequency o e ~2,
a factor of 2 increase in gain, and multiplying the references
instead of adding~ It should be noted that screen rotation
can be achieved with e~uation (2) as a starting point instead
of equation (1), thereby reducing the frequency range required
of the references (f cosO, f sin~, f cos3, f sin~) for certain
x x y y
ranges of ~.
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In general, the video function F(v) is selected to
be a monotonically increasing or decreasing function of the
input video. A monGtonically increasing f~lction is defined
as a function of a variable which increases as the variable
increases and decreases as the variable decreases without
discontinuities within the variable range i.e. for f = f(x),
~- ~ O and continuous for xl' x ~ x where x and x define
the rangeof monotonicity. A monotonically decreasing function
is defined as a function of a variable which decreases as the
' 10 variable increases without discontinuities over the variable
range, i.e., f = f(x), dd/l~ and continuous for x _ x ~x2
where x and x dafina the range of monotonicity. For example,
the function f = x is a monotonically increasing function of
x for all x greater than zero and is a monotonically decreasing
function of x for all x less than zero; the function f = x is
a monotonically increasing function of x for all x. If g(v)
; is defined as a monotonically increasing function of video,
then F(v) can be represented as
c(y(v)+d) ~r ( (c)~ d)-
0 where c and d are the constants as set forth hereinabove. The
threshold conditions can then take the forms Case
S(f ,f ,t) = c(g(v)~d)
x y
' S - c(g(v)~d) = O II
cJ(v)-~d' - c III
g(v)+d IV
S ~ y(v)-~'d~~~ = ~ V
(y(v)+d)s = c VI
The implementa-tion of these conditions are shown in simplified
form in Figure 6~ l'he outputs will be positive or negative
depending upon the Case. The complimentary outputs can be
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used or obtained if desired. The gain and offset need not
be applied to g(v) but can instead be applied to S(f ,f ,t)
if desired without loss of generality. An electronic gain
adjustment sets the constant "c" and the addition or subtraction
of a D.C. offset voltage or current sets the constant "d". As
15(a)
~Z1734
set forth hereinabove~ the actual settings are determined by
the desired halftone characteristics in the reproduction and/
or display (highlight and shadow dot sizes). Case's I, II,
IV and V are sometimes referred to as additive screening.
Case III is divisional screening and Case VI is multipli-
cative screening. Case VI is analogous to photolithographic
techniques where g(v) is the negative or positive to be hal~-
toned~ S is the halftone screen, and c and d are analogous
to bump and flash exposures ~'bump and flash" are terms utilized
to define the procedures used in the photolithography industry
to adjust the halftone characteristics, i.e. highlight and
shadow dot sizes). A11 the aforementioned cases are e~uivalent
as ~ar as dot shape characteristics are concerned and dif~er
only in TRC corrections required in g(v) to obtain the desired
result.
For purposes of illustrating the present invention,
the Case I electronic implementation will be described herein-
below. The simplified schematic diagram of the halftone
generation for Case I is shown in Figure 7 and incorporates
~0 both a 0 reference angle (implementation of e~uation (1))
and 45 refexence angle (implementation of equation (2)) for
screen angle rotation. The functions cos fit, cos f~t, sin flt
and sin f2t may be generated by phase locked oscillators,
digital synthesizers or an array of PROMS approximately pro-
grammed to generate the desired functions. Analoy multipl~rs
10 and 12 yenerate 45 referenced waveforms. Multiplier 10
generates a standard screen by multiplying cos f t and cos f2t
and multiplier 12 provides ellipticity correction for the 45
screen. Ganged switches Sl, S and S determine the ellipticity
chaining direction whereas ganged potentiometers 14 and 16 and
variable resistors 18 and 20, respectively, adjust the amplituda
of the screen functions referred to 0 for the amount of
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ellipticity. Resis-tor l~ is trimmed to equal the resistance
of potentiometer 14 as determined by the setting of its adjust-
able tap 22 and resistor 20 is trimmed to equal the resistance
of potentiometer 16 as determined by the setting of its
adjustable tap 24. Adjustable tap 26 of potentiometer 28
adjusts the amount of dot ellipticity for screen functions
referred to 45. Switch S4 selects 0 re:~erence or 45
reference for appropriate screen angles. Summing amplifier
30 assembles the screen function and gain and offset device
32 ad~usts the video function for appropriate maximum and
minimum densities in the reproduced and/or displayed output,
the adjusted video function being compared with the screening
function in comparator 34v Adjustable taps 22 and 24 adjust
the constants a and b in equations (l) and (la) in such a way
that a + b equals a constant such that the voltage ~alues of
white and black (0% and 100% relative dot area) as defined by
the screening function are independent of the setting of taps
22 and 24. Tap 26 adjusts (b-a) in Equation (2), with "a"
predetermined prior to the halftone generator, in such a way
that voltage values of white and black (0% and 100~/o relative
dot area) as defined by the screening function are independent
of the setting of tap 26. The chaining direction switches Sl
and S determine the condi.tions b ~ a or b~ a which eskablishes
the direction of "chaining", i.e., the direction in which the
halftone dots change size most rapidly with changes in video
signal. The switch S3 selects (b-a)~'0 or (b-a)~ 0, i.e.,
b >a or b _a to establish the chaining direction.
The combination of mu1tiplier 12, inverter 13, switch
S3 and tap 26 of potentiometer 28 determine~ (b-a) sin flt
sin f2t which is the second term in Equation (2). In particular
multipliPr 12 generates the product of sin f t and sin f t,
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the inverter changes the sign of the product, l.e~, plus to
minus or vice versa, S3 selects -the appropriate sign for
the chaining direction desired, and 26 and 28 determine the
magnitude of (b-a). S4 in the position as shown in Figure
7 selects the 0 = 0 case and all rotation angles referred
to 0, i.e. ~ = 23 to 45. When S is placed in the alternate
position, the ~ - 45 and screen angles referred to 45, i.e.,
~ = 0 to 22 are selected. By having the 23 to 45 angles
réferred to 0 and the 0 to 22 angles referred to 45, the
inherent problems of generating a sine function of a small
angle, a very small value, and multiplying it with other
values to obtain fl and f2 can be obviated. The actual screen
angles are de-termined by proper generation of the appropriate
reference frequencies f and f such that
f = f
x
~ = 0
f = f
2 y ~
Referring to Equation (la)
f = f cos ~ + f sin 0
~0 1 x y ~,
\ ~ = 23 to 45
f = f sin 0 - f cos 0
2 x Y
and referring to Equation (2)
f = x ~
1 ~
~ 0 = 45
25~ ~ )
Equation (3) gives:
f = 1 [f cos (45~ f sin(45 - ~)] ~
< ~ = 0to22
30f2 = 1 [f sin (45 - 0) - f cos (45 - Qj
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It should be noted that for the elliptical case,
interchanginy fl and f and the left side of the above equations
has the effect of interchanging the chaining direction, i.e.,
the switch positions of S , S , and S in Figure 7 are inter-
changed for a given chaining direction. The rotation angles
that can be achieved are not limited to the above values but
can be any angles. The above angle ranges ~ = 0 to 22 and
9 = 23 to 45 were chosen to ease the dynamic range require-
ments on fl and f frequencies. The rotation angles are not
limited to integer values as non-integer rotation angles can
be impLemented, e.g., 0 = 22.333 O~
Figures 8, 9 and 10 show a particular implementation
of Case II shown in Figure 6 for a screen angle of 45. In
this case, fl = fx and f = ~ . Assuming a desired spatiaL 4
dot frequency of 100 dots per inch and a scan velocity/inches
per second in the horizontal direction, fx = 2.8 MHz and f
1.98 MHzo For a scan velocity of 5.32 inches per second in
the vertical direction and a dot frequency of 100 dots per
inch, fy = 532 Hz and f2 = 376 Hz.
Typical dot frequencies are in the range from about
65 dots/inch to about 150 dots/inch (horizontal and vertical),
typical values cf fx are in the range of about lMHz to about
6 MHz and typical values of fy are in the range of about 250
Hz to about 8 KHæ (preferred ratio of fx to fy is 10 ).
For a standard screen, the screening function is
given by e~uation (3) hereinabove.
The cos fLt signal 50 is applied to terminal 52 of
balanced modulator 54 via function synthesizer 49, the modulator
functioning as a four quadrant multiplier. The cos f t signal
is applied to terminal 58 of modulator 54 via function synthesizer
57. The output 60 appearing at terminal 62, the screen function
desired, is a suppressed carrier double sideband signal which
--19--
73~L
is coupled to a compara-tor for comparison with the input
electrical signal. The output of the comparator may be
applied to a modulator which provides an information containing
optical signal which is coupled to an appropriate reproduction
-l9(a)-
734
device. The device which synthesi~es the f and f functions
i9 synchronized by a start of scan signal tin the x-direction)
from the reproduction device to initiate the waveform
generation at the same time the scanning device starts to
scan each line, The function synthesizer is also responsive
to the nuniber of times an original is being scanned such that,
in a color reproduction mode the screen angle can be varied
for each scan of the original. For reproductions of a blac~
and white original, the screen angle is maintained constant,
preferably a-t 45.
The function synthesizer, in the preferred mode,
generates sine/cosine waveforms of variable frequency determined
by the values of the screen angles as applied to the equations
set forth hereinabove. A programmable waveform generator, such
as the XR-205 ~onolithic Waveform Generator, manufact~lred by
Exar Integrated Systems, Inc., Sunnyvale, California, is typical
of a precision function synthesizer which can provide a variable
frequency signal output which is dependent upon a controllable
input~ Alternately, two separate wave generators can be
~ provided to generate the required two separate frequencies,
the frequencies desired being entered into the waveform
(frequency) generators by, for example, external switches.
For color scanning, a sequence selector can be provided to
automatically select an appropri.ate output frequency from the
wavefo~m generator in accordance with the color being scanned
(actually the selection is dependent on whether the original
is bei.ng scanned the first, second or third time as will be
explained hereinafter). Alternately, three pairs of waveform
generators (six total) could be provided for three different
screen rotations in the color scanning mode, a switch.;driven
off the reproduction davice) being provided to allow for the
proper pair selectionO
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73~
In order to modify the circuit of Figure 8 to produce
a non-standard screen (elliptical dots)g the block diagram of
Figure 9 is utilized. The signals 50 and 56 are applied to
balancsd modulators 72 and 74, to the latter via 90 degree
phase shi~ters 76 and 78, via ~unction synthesizers 49 and 57.
The outputs 81 and 83 from the modulators 72 and 74, respectively,
are summed in summer 80 to produce the screening ~unction 85 as
set ~orth in ec~uation (2) hereinabove. The values for constants
"a" and "b" can be electronically controlled in the modulators
or circuitry provided with the comparator as shown in Figure
10. The degree of ellipticity depends on the ratio o~ the
output signals from each modulator (or the di~erence in peak-
to peak amplitudes of the output of each modulator) 72 and 74.
The output signal 85 from summer 80 is the elliptical, 45
screening function desired.
Figure 10 illustrates the comparator schematic circuit.
The electrical analog input signal, such as a video signal, is
applied to input terminal 90, the gain thereo~ being controlled
by potentiometer 92. The input signal is applied to the base
~0 of NPN transistor 94 via resistor 96. The screening ~unction
is applied to terminal 98 and to the base of transistor 94 via
resistor 100. Potentiometer 102 and the 5 volt source applies
offset currents to the transistor base via resistors 104 and
112 respectively~ The currents appearing at the transistor base
~5 are summed and converted to a voltage by a summer amplifier
circuit comprising transistor 94, load resistor 108 ancl ~eedback
resistor 110 connected as shown. The summed voltage signal at
the collector o~ transistor 94 is coupled to the non-inverting
input o~ comparator 114. A threshold circuit, comprising a
resistive divider circuit (resistors 116 and 118), the ~ ltage
source and capacitor 120 (acting as a ~ilter) provides a threshold
signal to the inverting lead d comparator 114. When the signal
on the non-inverting input is greater than the signal on the
-21-
1734
inverting input, comparator ll~ gene~ates a variable width
positive pulse (sliced video) at terminal 122 which varies
from 0 volts to +3 volts in amplitude. The signal at
terminal 122 is then appli.ed to a modulator, as shown in
Figure 12, the modulator controlling the on-off times of
the laser beam to provide the desired halftone reproductions~
Figure ll(a~ illustrates the generation of a portion
of a 45 standard screen at the 50 percent gray point
(corresponding to the dot K = .5 in Figure 5), the dots in
the standard screen being symmetrical to adjacent dots both
in the screening direction and orthogonal thereto. For
purposes of illustration, the pattern generated is initiated
at start point 140 which is synchronized with the scanning
reproduction device utilized as described hereinafter with
reference to Figure 12. Reference arrow 14~ indicates the
direction of the x-scan and reference arrow 143 indicates
the direction of the y-scan.
The dots 144, 148, 150, 154, 160 and 164 constructed
for the 50 percent gray point illustrated are assumed for
illustrative purposes, to comprise our scan lines, or rows,
each. These dots are formed within diamond shaped halftone
cells 142, 1~6, 152, 156, 158 and 162, respectively, each
halftone cell being typically 10 mils on each side (for a
100 dot/inch screen frequency). The diamond shaped cell
area (the outline of which is shown in the Figure) corres-
ponds to the dot shape (K = 0.5) of Figure 5. It should be
noted that for a screen pattern which represents full black,
the dots shown would substantially Eill its associated hal~- .
tone cell and adjacent cells would be printed black (or in
color for the color scanning mode). The ~5 screen function
causes the grid matrix illustrated to be reproduced by an
emitting beam such as a laser, coupled to a reproduction
-22-
~Zl~
device such tha~ a dot in cells 144, 148, 150, 154, 16n,
164 ... is produced whereas no dot is constructed in
adjacent cells 142, 146, 152~ 156, 158, 162 ~
~eferring more particularly to the construction of dot 144
for illustrative purposes, the firs~ write, or laser scan,
produces dot portion a, the second scan produces dot portion
b, the third scan produces dot portion c and the fourth scan
produces dot portion d (the dot portions generally overlap).
The pattern shown corresponds to midtone gray as those cells
corresponding to the dots actually constructed ~ould be
printed as completely black (or in color for the color
copying mode). As can be seen in the Figure (and Figure
ll(b) described hereinbelow), the direction of the dot
portions (a, b, c and d) in each scanline is the same,
i.e., in the dlrection of scan indicated by arrow 1~1. The
alignment of the entire dots, which comprise all the dot
portions (four in the example illustrated), is variable and
controlled by the screening function utilized.
Although not shown, other dot configurations could
also be constructed. For example, a circular highlight dot
could be constructed by appropriate selection of the screen-
ing function (K = .1, Figure 5).
Figure ll(b) illustrates the generation of a
portion of a 45 non-standard (elliptical) screen grid using
the circuit configuration of Figures 9 and 10. The dots
generated in this grid by definition, are non-symmetrical
to adjacent dots both in the screening direction and ortho-
gonal thereto, For grays corresponding to highlight and
shadow areas, the dots constructed actually look like
ellipses, hence the name for the non-standard screen. In
the mid~tone range, the cell which defines the maximum dot
area is not entirely printed as black (or in color for the
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73~a
color copying mode). For increased levels of gray, the
dots shown in Figure ll(b) will continue to increase in
area until adjacent dots merge.
The scanning operation is initiated at point 170,
the x-direction of scan being indicated by reference arrow
171 and the y-scan direction being indicated by reference
arrow 173. The dots 174, 178, 180, 184, 190 and 194
generated are formed within a plurality of respective half-
tone cells 172, 176, 182, 186, 188 ancl 192 of a size, for
example, as set forth hereinabove with reference to ~igure
ll(a). As can be seen, the dot matrix forms a 45 angle to
the x-direction of scan. Referring to halftone cell 172
for illustrative purposes, the dot 174 generated comprises
portions a, bl c, and d which may or may not overlap. The
portions a, b, c, d of dot 174 may be considered to form
the outlîne of an ellipse, the chaining direction of which
is parallel to the screening direction. The dots in half-
tone cells 172, 176, 182, 186~ 188 and 192 are similarly
constructed to form the mid-tone pattern illustrated. In
this situation, only two corners of a constructed dot are
touching adjacent dots. For example, the corners of dot 184
touch the corners of dots 178 and 190 but no portion of dot
184 is touching dots 174 and 194. Other non-standard screens
can be provided to provide alternate dot configurations using
the teachings of the present invention.
Referring now to Figure 12, a block diagram
illustrating a color copying system in which the present
invention may be utilized is shown. It should be noted that
the present invention may be utilized in a black and white
copying system wherein a laser having a single output wave-
length~may be utilized to scan an original and print a
reproduction thereof on a laser sensitive medium.
-24-
~f~7~
In this case, the screening angle is maintained constant
for each line scan. An original document 210 i9 positioned
on a rotatlng member 212 which, in the embodiment shown, is
drum shaped. The document 210 is secured to the drum 212
by suitable means and the drum is caused to rotate in the
direction of arrow 214. Original docu:ment 210 may be black
and white or in color. The discussion set forth hereinafter
will be directed to the scanning of a color document 210.
Since the concept of the present invention is directed to scan-
ning the original document 210 and reproducing a copy either
-24a-
739~
locally or at a remote location, document 210 is scanned to
generate appropriate e~Etrical (video) signals which represent
the tonal (color) information on doc~ment 210. In particular,
read lasers 216, 218 and 220 are provided, laser 216 comprising
a helium-neon laser for generating red light, laser 218
comprising a helium-cadmium metal vapor laser for generating
blue light and laser 220 comprising an argon-ion laser for
generating green light. It should be noted that a properly
excited helium-cadmium laser can provide light having wave-
lengths corresponding to both blue and green and therefore
a single laser can be utilized in place of lasers 218 and
220. The light beam 222 from laser 216 is directed to a ~ully
reflecting mirror 224 which directs the beam to mirror 226
which is transmissive thereto. Mirror 226 also reflects beam
228 generated by laser 218 so that resulting beam 230 comprises
both red and blue light. The beam of light 232 generated by
laser 220 is directed to mirror 234 which transmits beam 230
and reflects beam 232. The resuLting beam 236 from mirror
234 combines the red, blue and green wavelengths generated by
~0 lasers 216, 218 and 220, respectively, and is incident on
mirror 238. seam 236, which is essentially white light, is
directed via mirror 238 into input scanner 240 which may
comprise a rotating multifaceted polygon. The scanning light
~rom scanner 2~0 is directed to the document 210 via cylindricaL
Lens 242 which has its plane o~ no power in the direction o~
scan. The light reflected ~rom docl~ent 40 is collected by
light pipe 244 which in turn dlrects the collected Light to
a detec-tor 246 which comprises sections 246a, 246b and 246c
which is responsive to the red, blue and green light, respectively,
reflected Erom color document 210. The detected output is
coupled to a color correction computer 248 for appropriate
-2s-
73~
processing. Color correction computers are well known in
the prior art (see U. S. Patent No. 3,622,690, for example)
and correct for the deficiencies in the developer powder
(toner) and provides consecutively a plurality of electronic
color separation signals therefrom corresponding to the colors
yellow, magenta and cyan. As will be explained hereina~ter
with respect to the printer utilized, original document 210
is scanned three times to provide video signals corresponding
to-the three primary colors, color correction computer 24
thereafter being operated in a corresponding sequence to
provide color corrections for the yellow, magenta and cyan
developer powder. The color correction generated by color
correction computer 248 i9 applied to the halftone electronic
generator 250 o~ the present invention via lead 252. A start
of scan detector 254 is provided adjacent document 210 to
provide the required synchronizing signal to the electronic
generator 250 via lead 256. The function synthesizers, utilized
for generating sine/cosine waveforms, are gated by the start
of scan signal to insure the same phase (i~e. phase equal to
zero degrees in the x-scan direction) for each scan line. A
shaft encoder 260 generates a pulse for each ravolutlon of the
drum, the pulse train generated thereby being coupled to counter
262 which counts three pulses and is reset thereafter. For
color reproduction, the document 210 is scanned three t:Lmes,
once each to scan Eor the red, green and blue colors which
comprise the document in~ormation. It should he noted that a
fourth color, such as black, can be scanned and reproduced with
an additional scan and screening ~unction. Since it is desired
to provide a different screen angle for each scan (a single
screen angle can be used for black and white reproductions)
the first pulse detected causes counter 262 to generate a
-26-
734
signal on lead 264 which is coupled to electronic halftone
ganerator 250. The function synthesizer therein generates
an appropriate
-26(a)-
~L~Z~3~
screen function having a first screen angle (relative to
the x-direction of scan) therefrom. The second pulse
detected, corresponding to the second document scan, causes
counter 262 to generate a signal on lead 264 which causes
the function synthesiæer to generate a screen function
having a second screen angle, different from the first
screen direction. The third pulse detected by counter 262,
corresponding to the third document scan, causes the function
synthesizer to generate a screen function having a third
screen angle, different from the first and second screen
directions. For example, the first screen angle may be 0,
the second screen angle 2~ and the third screen angle 45.
n this manner, an accurate color halftone reproduction of
the original document 210 is produced by the reproduction
device. In particular, the halftone signal is applied to
an electro-optic modulator 266 via lead 268 to produce half-
tone separations of the document 210. The output beam from
write laser 270 is also applied to modulator 266 via mirror
272. As will be explained in more detail hereinafter with
reference to reproduction device, or printer 274, it is preferred
that write laser 270 generate red light such as that generated
by a helium-neon laser. The modulator 266 modulates laser
beam 276 in accordance with the amplitude of the electronic
signals derived from halftone generator 250. In general,
when these signals are high, more light i8 passed by modulator
266 then when the signals are low. Consequently,the light
-transmitted through the modulator 266 ls a function of the
amplitude of the electronic signals on lead 268 and hence is
a function o~ the density (color) of the tones on document
2100 The light transmitted through the modulator 266 is
applied -to an output scanner 278 which is similar to input
3fl~
scanner 240 and is in synchronism therewith. The scanning
light from output scanner 278 is focused by cylindrical lens
280 onto a photoconductive medium 280 formed on xerographic
drum 284. The printer, generally labeled 274, in the pre-
ferred mode comprises the system disclosed in U. S. Patent
No. 3,854,449 modified to incorporate laser 270, modulator
266 and output scanner 278.
As set forth herein-
above, electronic halftone generator 250 provides the
necessary signals to produce the required halftone dot
matrix on the output copy. The particular circuitry utilized
allows various shaped dots to be generated at variahle screen-
ing angles to the scanning direction. With reference to
Figure 12~ the scanning, or x direction, is in the direction
perpendicular to the plane of the figure.
In oFeration, the reading lasers 216, 218, and 220
are turned on and the monochromatic light beams therefrom are
merged into a single scanning beam 236 which is focused onto
i.nput scanner 240. The rotation of scanner 240 causes the
scanning beam, focused b~ lens 242 into a ~ine spot, to scan
the document 210. A number of scan lines are produced as
drum 212 rotates in the direction of arrow 214. Each scan
line produces varying amplitudes light signals due to the
color con-tent of the document 210 which liyht signaLs are
transmitted and collected by light pipe 214 and detected hy
detectors 246a-246c. Detector 246a extracts the red liyht
in the transmitted light beam and converts it to a varying
electronic si.gnal. The blue light in the scannin.g beam is
extracted by detector 246b and the corresponding electronic
signal is generated and detector 246c detects the green light
-28-
3~
in the scanning beam and converts it into an electronic signal.
The color component signals from the detectors 246a-246c are
applied to the color correction computer 248 ko produce color
corrected ma~enta, cyan and yellow output signals. These
varying signals are applied to the modulator 266 via electronic
generator 250 Laser beam 276 derived from laser 270 also being
applied thereto. Modulator 266 passes or inhibits the laser
beam light in accordance with the amplitude of the electronic
signal derived from the halftone generator 250. The output
from modulator 266 is focused onto the xerographic drum 284
to expose photoconductor 282, the image being developed as
set forth in UO S 7 Patent No. 3,854,44g. Since the modulated
light is a replica of the corresponding color component in the
original pattern formed on document 210, the hard copy output
produced from the image on khe drum 284 produces a hal~tone
replica of the color tones in document 210.
Since the developing process in printer 274 requires
three scanning cycles, document 210 is scanned three times
by input scanner 240, output scanner 278 similarly scanning
0 drum 284 three times in synchronism with the scanning of
document 210.
Although the present invention has been described
with reference to analog device implementations, the devices
could be implernented digitally.
The invention described hereinabove has great
versatility and many advantages over the prior art in producing
electronic halftones for reproduction and/or display purposes.
It provides a two dimensional grid of halftone dots where the
dot characteristics can be altered through appropriate choice
of screening function. The dot area is modulated by the video
function thereb~ providing a greater dynamic range than line
halftone techni~ues are capable of. With appropriate choice
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.73~
of screening frequencies and phases the halftone grid can
be rotated relative to the input and/or output scan directions
and with use of appropriate phase and/or frequency locking
techniques the halftone grid pattern can be compensated for
scan irregularities. Coordinate transformations techniques
may be provided to allow the present invention ~which
produces rectilinear halftone grids) to be produced with
spiral, circular, etcO input/out scan devices. Paralleling
many of these circuits with appropriate time delay of the
same screening function will allow the use of a different
number of output channels than input channels for application
to multiple channel scanning devices. The video function is
not band limited by the screening function and output re-
solution for high contrast is not appreciably degraded. The
output is binary and need not have any greater bandwidth than
the input video function, thereby providing data compression
for video format signals. The aforementioned advantages are
accomplished in real time without any storage requirements.
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73~
As set forth hereinabove, the dynamic range of
the line halftone technique is
D.R~ = 1
f~w
where f~ is the line hal~tone frequency in lines per unit
length and w is the minimum reproducible line width. The
invantion described herein provides a dynamic range given
by 2
D.R. = K(
f d
d s
where f is the dot frequency in dots per unit length, d
is the diameter of the smallest reproducible dot, and K is
a geometric area factor depending upon dot shape, K being
4/Tr for a circular dot. In general, the output scanner
determines w or d and for w = d , f~ = f , the dynamic range
of the electronic halftone generator is the square of the
dynamic range for line halftone, e.g., a line halftone
capability of 8:1 will increase to 64:1 for dot halftone
with the same output scan limitations. This represents a
considerable improvement in performance.
The dual frequency and phase nature of the screening
function allows for coordinate manipulation using ~requency
and/or phase modulation techniques~ For example, proper
choice of frequency a~d phasa can produce a rotated halftone
yrid relativ~ to an X-~ ~canner. With appropria-te frequency
and/or phase modulation, coordinate trans~ormation and stabili-
zation can be achieved. For example, a rectilinear halftone
grid can be produced with a spiraL scan output by setting
1 Vrfd cos wt
f = v f sin wt
2 r c
where v is radial scan veLocity and w is the angular scan
velocity.
73~
If the scan veloc~ities have ir~egularities due to
mechanical constraints or other causes which can be sensed
then frequency and phase lock techniques can be applied to
f and f2 to stabiliza the halftone grid pattern.
In devices having input/output scanners of a
multiple channel nature, the electronic halftone generator
can be incorporated in each channel. The screening function
is applied to each channel with an appropriate time delay
between channels thereby registering the reference halftone
grid.
Partial dots refers to the capability of modulating
dot area within one halftone grid ce~ in conjunction with a
rapidly changing density input in the video signal. The
screening function allows a dot to change from black to ~hite
the
for example, in the middle of constructinc~ dot. The fact
that the electronic halftone generator technique does not
fundamentally limit the video function bandwidth guarantees
paxtial dot capability.
The present technique inherently provides a data
- compression feature. In particular, the video function has
a bandwidth of ~ f and an analog dynamic range of D. If the
video functions were con-verted to digital format, the bit
rate, B, would he at least
B = 2 Q f loct2 ~
The output of the electronic halftone ~enerator is
hinary in nature with a minimum bandwidth of A f and represents
a blt rate of
B = 2 ~ f
This would be a data compression factor of log2D
except for the fact that the output pulses are duration
modulated. However, dynamic range considerations yield a
net compression factor, Cd of
d --2
-32-
~ IILZ~739~
for digitally converted halftone output. Further data
compression may be ob-tained through conventional techniques.
In special applications where clocked bits are not required
the full compression ~actor
Cd = log2D
can be realized.
In summary, the electronic hal~tone generator of
the present invention converts electronic signals in video
format to binary output halftone signalsO Input devices
providing video format signals can be TV scanners, laser
scanners, ~lying spot scanners, video tape, computer
constructed imagery, facsimile, etc. The output of the
generator, being electronic in nature, can be used with
output devices incorporating any number of marking and/or
~15 display technologies. Output scanners can be, for example,
CRT, lasers, flying spot, LED and electronic matrix. The
various applicable marking technologies may be photographic,
xerographic, ink jet, electrophoresis, magnetographic, etc.
The electronic halftone generator has application
~O to display purposes where additional dynamic range is needed
and/or non-linearities in output brightness are difficult to
compensate such, for example, as required in LED display
systems, solid state matri~ displays and specialized multi-
channel C~T's. The binary nature of khe generator output
~5 does not require linear transfer ~unctions for displays or
marking technologies. All that is required of output devices
is that th~y produce black or white (or appropriate colors
in multi-color applications) displays.
The data compression aspet of hal~tones generation
allows the device output to be combined with more conventional
data compression techniques, providing reduced storage re-
-33-
73~
quirements, reduced transmission bandwidths or run lengths
and reduced transmitter power.
While the invention has been described with
reference to its preferred embodiment, it will be understood
by those skilled in the art that various changes may be made
and equivalents may be substituted for elements thereof
without departing from the true ~pirit and scope of the
invention. In addition, many modifications may be made to
adapt a particular situation or material to the teaching of
the invention without departing from its essential teachings.
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