Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1: ~ 77/339
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B~CKCROUND OF T~IE INVE~JTION
The printiny techniques commonly used in the
graphic arts inclustry, i.e., newspapers, books, etc , uti-
lize an all-or-nothinc3 inking procedure; i e., ink is
ei-ther deposited at each point on the printing surface or
it is not~ While this technique poses no problems when it
is desired to print -text, the printing of pictures, such
as photographs, introduces the problem of printing the con-
tinuous tones in the gray scale ranging from ~lack to white.
Th.e problem is generally solved by transforming the continu-
ous tones of the original image into halftones. Halftone
images are typically produced by a large number of inked dots
whose size or spacing relative to one another tricks the eye.
into perceiving~shades of gray rather than the'individual dotsO
This technique works best when the largest dots and the spacing
.. between the dots is small compared with the visual accuity of
the eye. Early halftone generating systems used either vari-
able sized dots with uniform spacing or uniform sized dots
. wi'~h variable spacing. ''
~0 Electronic phototypesettincJ systems developed over
the years have greatly increased the speed of type composition~
Many such systems incorpora-te a halftone generati~g capability '.
and in addition to storing typeface characters in a font memory
also store halftone dot characters representative of discrete .
levels of the gray scale. In order to produce a halftone umage, these
systems scan the original image, sample the gray scale value
at discrete intervals and convert these values -to multi~it
binary numbers which are then used to access the stored dot
character representative of -that value from the dot
character font memory. U.S. Patent ~o. 3,~06,64L to Croo~s
discloses a system of this type.
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Th~ data processing capabilities of such elec-
tronic phototypesetting systems have made possible a drastic
re~uction in the time necessary to prepare plates for
printing material comprised of both text and illustrations.
However, -the resolu-tion of halftone images produced hy such
sampled data sys-tems generally falls short of that ob~ainable
when non-digital techniques are employed.
SUMMARY OF THE INVENTION
The present invention is directed to a system for
extracting digital i.nformation from original images storing
that information and/or transmitting the informa-tion to a
remote location, and thèn utilizing the information to pro-
duce a halftone replica of the original image. The invention
is par-ticularly useful in a photocomposing system in which
.
digital information representing a iarge.number of pictures
is stored in a mass memoxy such.as magnetic tape or disk
and used together with othe.r infoxmation to compose pages of
tex-t and pictures.
A photocomposing system in accordance with the
invention is capable of producing halftone replicas o~ ~igher
apparent resolution for a given mesh value than prev.iously
known systems utilizing a pure sampled data approach.
Alternatively, a photocomposing sys-tem in accordance with
the invention can produce halftone replicas of given resolu-
tion with lower data storage requirements than previously-
known systems.
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8~3
In accordance with the present invention, the
imac3e to be reproduced is scanned by methods well-known
in the art to obtain digital representations of the tonal
values of incremental areas thereof. However, unlike
pri~r sys-tems, the image is scanned at a yreater number
of horizontal and vertical coordinate points than is de-
sired in the halftone replica. For example, by doubling
the number of horizontal and vertical sampling points,
four samples rather than one are taken for each discrete
area or dot cell interrogated by the scanner. Ihe four
samples are then compared with one another to determine
the rate of change of image density within the area or
dot cell. If a threshold value is not e~ceeded, the cell
is deemed a low detail cell. If, on the other hand, the
threshold value is exceeded, the cell is deemed a high
detail cell.
A photocomposing embodiment of the invention is
comprised o~ an image read subassemb1y w~ich extracts tonal
aensity data from the original image and an image record
subassembly which responds to the density data to produce
the halftone replica. In a preferred embodiment, the image
read subassembly determines whether a cell is of high or
low detail. In the case of a low detail cell, the four
individual samples are averaged to develop a single density
value for the cell which is then placed in a memory, e.g.
magnetic tape. In the case of a high detail. cell, all four
of the cell samples are recorded on the magnetic tape.
77/~39
The image record subassembly subse~uently
responds -to the information on the magnetic tape to
select a single dot character from a fon-t memory which
best represen-ts the original four individual samples.
In the case of a low detail cell, thc stored single
density value is employed to select the dot charac-ter
of appropriate size from the font memory. In the case
o~ a high de-tail cell, the four samples are processed
to select the dot character of appropriate size and
shape from the font memory. In addition, the four
samples are used to migrate, i.e., alter the position
in the cell at which the dot character is placed. The
position can be either the "centerof gravityl' of the
four, in terms of average density, or some other func- -
tion relating the desired displacement from cell center
(~X, ~Y) to the magnitude and posi-tion'of the'four
samples. Experimenta-tion has shown that good results
-
are obtained by calcuiating ~X and ~Y as follows:
QX = KlL (DE Dw)
2 ( N S)
where L equals the distance from the center of the cell
to the sample locations; DN, D~, DE, DW equal the
image densities of four samples oriented at 90 `
to one another within the cell (e.g., north, south,
east, west) and Kl and K2 are cons-tants.
~14~S~
(e.g., north, south, east, west) and Kl and I~2 are constants.
Thus, in accordance with one broad aspect of the invention,
there is provided a photocomposing system for producing a halftone replica
of an original image comprising in combination: means for scanning said
original image to produce a digital representation of the tonal density
of each of a plurality of points of said image; comparison means for
comparing digital representations of adjacent points within an area form-
ing a cell for generating a "flag" signal when the differences in tonal
density between said points exceeds a certain threshold; arithmetic means
responsive to the digital representations of points within a cell for
produclng a single digital weighted value for that cell; dot character
storage means storing a plurality of different halftone dot characters;
means responsive to each digital weighted value for selecting one of said
halftone dot characters from said dot character storage means; output
means responsive to each halftone dot character selected for producing a
visible representation of that dot character positioned at substantially
the center of a cell area; and means responsive to the generation of a
"flag" signal associated with a cell for altering the horizontal and
vertical positions of the visible representation produced in the area of
that cell.
In accordance with another broad aspect of the invention there
is provided a photocomposing system for producing a halftone replica of an
original image comprising in combination: means for scanning said original
image to produce a digital representation of the tonal density of each of
a plurality of points of said image; comparison means for comparing digi-
tal representations of adjacent points within an area forming a cell for
generating a "flag" signal when the differences in tonal density between
said points exceeds a certain threshold, each of said cells containing
four points successively displaced by 90 from one another, the tonal
densities of said four points being respectively represented by DN, DE,
Ds~ Dw; arithmetic means responsive to the digital representations of
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2~
points within a cell for producing a single digital weighted value for
that cell, said digital value being dependent upon where is proport-
ional to the sum of DN -~ D~ + DS + DW; dot character storage means
storing a plurality of different halftone dot characters of different
size and shape;
According to another broad aspect of the invention there is
provided a method for producing a halftone replica of an original image
comprising the steps of: scanning said original image to derive a digital
representation of the tonal density at each of a plurality of points;
comparing digital representations of tonal density of adjacent points
defining a cell area to ascertain whether the difference therebetween
exceeds threshold so as to identify a high detail cell or is less than
said threshold so as to identify a low detail cell; selecting for each
cell area a particular one of a plurality of stored dot characters in
accordance with the values of the digital representations of pOilltS with-
in that cell area; and displaying each of said selected dot characters
on an output medium at a certain position within the cell area oE low
detail cells and at a variable position within the cell area of high
detail cells which position is dependent upon the digital representationS
of points within that cell area.
According to another broad aspect of the invention there is
provided a photocomposing system for producing a halftone replica of an
original image comprising in combination: means for scanning said
original image to produce a digital representation of the tonal density of
each of a plurality of points of said image; comparison means for com-
paring digital representations of adjacent points within an area forming
a cell to identify a cell as being of either high or low detail; arith-
metic means responsive to ~he digital representations of points within a
cell for producing a single digital weighted value for that cell; dot
character storage means storing a plurality of different halftone dot
characters; means responsive to each digital weighted value for selecting
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one of said halftone dot characters from said dot character storage means;
output means responsive to each halftone dot character selected for pro-
ducing a visible representation of that dot character positioned at sub-
stantially the center of a cell area; and means responsive to said
comparison means determining a cell is of high detail for altering the
horizontal and vertical positions of the visible representation produced
in the area of that cell.
In accordance with another broad aspect of the invention there
is provided a photocomposing system for producing a halftone replica of
an original image comprising in combination: means for scanning said
original image to produce a digital representation of the tonal density
of each of a plurality of points of said image, comparison means for
comparing digital representations of ad~acent points within an area form-
ing a cell to identify a cell as being of either high or low detail; a
digital memory; arithmetic means responsive to the identification of a
low detail cell for processing the digital representations of the points
therein to produce a single digital weighted value for that cell; means
for storing said single digital weighted value in said digital memory;
means responsive to the identification of a high detail cell for storing
the digital representations of each of the points therein in said digital
memory; dot character storage means storing a plurality of different half-
tone dot characters; means for accessing said digital memory; means
responsive to the accessing of a digital weighted value associated with a
low detail cell for selecting one of said halftone dot characters from
said character storage means; means responsive to the accessing of digital
representations of points within a high detail cell for selecting one of
said halftone dot characters from said dot character storage means; output
means responsive to each halftone dot character selected for producing a
visible represen~ation of that dot character positioned at substantially
the center of a cell area; and means responsive to the accessed digital
representations associated with a high detail cell for altering the hori-
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Zt55~
zontal and/or vertical position of the visible representation produced in
the area of that cell.
In accordance with another broad aspect of the invention there
is provided, in a photocomposing system for producing a halftone replica
of an original image, an image read subassembly comprising: means for
scanning an original image to produce a digital representation of the
tonal density of each of a plurality of points of said image; comparison
means for comparing digital representations of adjacent points within a
cell area of said image to identify a cell as being of either high or low
detail; a digital memory; means responsive to the identification of a low
detail cell for processing the digital representations of the points
therein to produce a si.ngle digital weighted value for that cell; means
for storing said single digital weighted value in said digital memory;
means responsive to the identification of a high detail cell for storing
the digital representations of each of the points therein in said digital
memory.
According to another broad aspect of the invention there is
provided, in a photocomposing system for producing a halftone replica of
an original image in response to data stored in a digital memory represent-
ing a low detail cell in the form of a single digital weighted value and
a high detail cell in the form of multiple digital representations of
adjacent points within a cell area, an image record subassembly comprising:
dot character storage means storing a plurality of different halftone dot
characters; means for accessing said digital memory; means responsive to
the accessing of a digital weighted value representative of a low detail
cell for selecting one of said halftone dot characters from said dot
character storage means; means responsive to the accessing of multiple
digital representations representative of a high detail cell for selecting
one of said halftone dot characters from said dot character storage means;
output means responsive to each halftone dot character selected for pro-
ducing a visible representation of that dot character positioned at
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~L~42~5~
substantially the center of a cell area; and means responsive to the
accessing oE digital representations associated with a high detail cell
for altering the horizontal and/or vertical position of the visible
representation produced in the area of that cell.
The novel features of the invention are set forth with parti-
cularity in the appended claims. The invention will best be understood
from the following description when read in conjunction with the accompany-
ing drawings.
''BRIEF DESCRIPTION OF'THE'DRAWINGS
Figure 1 is a block diagram of the image read subassembly
in accordance with the invention;
Figure 2a schematically illustrates the division or an
original image into a matrix of dot cells;
Figure 2b schematically illustrates a single dot cell
area showing sampling points therein;
Figures 2c~2f schematically illustrate exemplary halftone dot
characters, varying in size, shape and position which are formed to
represent the indicated dot cells in the original image; and
Figure 3 is a block diagram of the image record subassembly
in accordance with the invention for recording a halftone replica on film.
DESCRIPTION'OF A'DET~ILED E~BODIMENT
Halftone replicas of original images are formed by the juxta-
pOSition of discrete dot characters which may seemingly be of arbi~rary
shape, such as small dots or line segments. Multiple dot characters when
juxtaposed together can be configured to give an impression of continuous
gray scale tones comprising a halftone image.
Digital memories have been used to store the shapes of such
discrete dot characters. More particularly, a font memory is typically
provided in a halftone recording system for storing the shapes of various
dot characters generally
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selec-ted so as to cover the entire gray scale from white to
black in selected s-teps~ Such dot characters have been
utilized in prior art systems, and examples are disclosed
in various United States patents, such as 3,806/641 to Crooks,
and 3,922,484 to Keller. The present invention similarly
makes use of do-t characters of various sizes and shapes which
are stored in and retrieved from a dot character font memory.
As will be more fully explained hereinafter, the
dot chaxacters are preerably arranged in the font memory
in order of increasing or decreasing image density. This
arrangement offers the simplest access to the.dot characters
because the apparatus used to scan the original image con-
verts tonal density to a multibit binary number propoxtional
to that density. Thus, the multibit binary number can be
used as a direct address to gain access to the dot character
font memory in order to call out the appropriate dot character.
Any halftone generating system.must begin by ex- ;
-tracting information from the original image to be reproduced.
Simi].arly to prior ar-t systems, the presen-t invention extracts
the required tonal density information from the original image
by a scanning process. Prior to considering.the scanning
apparatus depicted in Figure 1, attention is initially direc-ted
to Figure 2a which illustrates a pattern comprised o imaginary
lines running at 45~ to the vertical and intersectin~ one
25 another -to form a matrix of cells 12. The cells 12 may be
considered as being arranged in rows and columns such that any
particular cell Ealls within a particular row and column. For
example, cell 14 is in column 5 and row 3. Although the cell
matrix or mesh depic-ted in Fi~ure 2a is formed by lines e~- -
tending a-t 45 to -the ~ertical, it should be unders-tood that
other cell matrices can also be employed, as is readily kno~
' 77/33~
, . ~
~14Z~58
in t~lC' art. The si~nificance of the cell ~natri~. depicted
in Figure 2a is that -the original image to be ultimately
reproduced by a halftone replica is first scanned to
determine it5 tonal density within each of the discrete
cell areas. More particularly, as will be discussed in
greater detail hereinafter, the image read apparatus of
Figure 1 scans an original image which may be borne on a
transparency 16 by successively sampling the cells in
accoxdance with the pattern represented by the matrix de-
pieted in Figure 2a. Subsequently, the image reeord sub-
assembly depicted in Figure 3 produces a half-tone repliea
o~ the original image by recording an appropriate dot
character in each cell.
It should be apparent that as the number of dot
eells (Figure 2a) into which the original image is divided
is increased (increasing the mesh value as it is ealled is
graphie arts), the resolution of the halftone replica to be
produced, will also be increased. In typical pxior art sys- ;
tems, in order to increase thç resolution of the halftone
replica, it is necessary to commensurately increase the
, amount of data which must be stored and processed. That
is~ in order to double the resolution of the halftone
.
replica in aecordance with prior art systems, the amount
of data which must be stored and processed is quadrupled
since the number of points seanned in the original image
must be doubled in both the horizontal and vextical dimen-
sions. Moreover, to do so ma~ result in a halftone replica
which is not readily printable.
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In accordance with a significant feature of the
present invention, each dot cell 12 of the original imaye
is sampled at mul-tiple points and the resultiny multiple
samples are processed to yield a single dot character which
more accurately represents the density of the original
image within that cell area than prior art sys-tems.
More particularly, attention is directed to Fig- -
ure 2b which represents a single cell 12. In accordance
with the present invention, the cell is sampled at multiple
points. In accordance with the exemplary embodiment dis-
closed herein, it will be assumed that each do-t cell 12 is
sampled at four points displaced from one another by substan-
tially 90. The four points will be respectively referred
to herein as north, east, south, and west. The terms DN,
DE, Ds, and D~l will be used hereinafter to respectively repre-
-~ent the tonal densities of the north, east, south and wesk
points of a dot cell area and additionall~ these terms may
sometimes be used to designate the particular multlbit binary
number representative o~ that tonal density. Whereas prior
art systems typically sample each do-t cell at ~ single point~
a system in accordance with the present invention samples each
dot cell at multiple points, e.g. the four points illustrated
in Figure 2b. As will be seen hereafter in discussing the
apparatus o Figures 1 and 3, the data representing -the tonal
densities of those four points of the original image in each
cell is examined to determine whether the dot cell comprises
a high or low detail portion of the image. The de-termination
of whether a cell comprises a high or low detail portion ~f
the image is based upon comparing the four density values
to de-termine the ra-te o~ change oE image density
.~ 77/779
2~5~3
withi.n the cell. This deterltlirlation can be made by, for
example, calculatin~ the ra-tios or differences between the
density values. If the rate of change exceeds a certain
threshold r then -the data corresponding to that cell is tagged
with a high de-tail "flay" and the four density values are
recorded and p.reserved for use in developiny the hal~tone
replica. On the other hand, if a cell is determined to be
a low detail cell, then the four density values are processed
to derive a single value which, in the simplest embodiment,
comprises the arithmetic average of the four density values.
The data derived during the image read process is in most
applications of the invention recorded on a memory such as a
magnetic tape which is later used by the image record sub
assembly o Figure 3 to produce the halftone replica. As will
be seen hereinafter, in producing the halftone replica, the
single density value associated with low detail cells is used
to access a dot character of appropriate size from the dot
character memory. The four density values associated with a
high detail cell are processed in the image record subassembly
. 20 to derive an address to access a dot character o appropri.ate
size and shape from the dot character memory. Additionally,
the four density values associated with a high detail cell are
used to determine whether the dot character to be displayed in
a cell in the halftone replica should be migrated within the
.cell. That is, depending upon the relative magnitudes of the
four density values within a cell, the do-t character displayed
within that cell can be displaced horizontally (aX) and ver-
tically (~Y) rom the center of the cell.
~ 7J33g
1~4Z8~
The halftone image generating system in accordance
with the inven-tion essentially operates in t~Jo modes; namely
(1) an input mode during which the image read apparatus of
Figure 1 scans an original ima~e and extracts density data
therefrom and (23 an output mode during which the image re-
cord appara-tus of Figure 3 responds to the density data to
produce a halftone replica. The input mode will be considered.
first and reference will initially be made to Figure 1 which
illustxates a pho-tographic transparency 16, bearing the ori
ginal image to be reproduced. The transparency 16 is placed
in front of a scanner 22 which preferably comprises a cathode
ray tube 24 capable of producing a spot of light 26 ~or scanning .
the transparency 16. The scanning spot 26 is deflected in a
manner readily understood in the art under the control of
horizontal and vertical deflection coils 28 and 30. The coils
28 and 30 are respectively driven by drive circuits 32 and 34,
each of which includes a digital to analog converter and amp-
lifier circuits. The scanning spot 26 is focused by a lens
` 36 onto the txansparency 16. The light penetrating through
the transparency 16 :is focused by lens 38 onto a photodetector
40 such as a photomultiplier tube.which develops an analog
signal related to the magnitude of the ligh-t passing through
the transparency 16 which in turn is determined by the density
. of -the image carried by the transparency. I-t should be rec-
ogni~ed that although it has been ass~ned the original image
is carried on a transparency 16, other types of scanners can
be used for scanning opaque film or the li.ke in which the re-
flec-ted ligh-t from the original image causes a photodetector.
to develop the image signal. Prior to discussing the processing
oE the image signal produced by photodetector 40, the manner
77f33~
~L4~8~
in whic}l the scanning spot 26 is controlled to derive the
image signal will be explained.
It will be recalled from Figure 2a that the ori-
ginal image is to be scanned by successively examining the
density of ~he image in each of a multiplicity of discrete
cells 12. In order to cause the spot 26 to move from cell
to cell, appropriate deflection signals must be applied to
coil ~8 and 30. Moreover, within each eell 12, the spot
26 must be suceessively deflec-ted to the north, east, south
and west points depicted in Figure 2b. One manner of
sequentially generating the appropriate deflection signals
for application to the coils 28 and 30 is to utilize a read-
only memory 44 which, in sequential locations, stores digital
in-formation representing the deflection signals required to ..
move the spot 26 successively to each eell and within each
eell to eaeh point. Thus, the locatio.ns of the read onl~
memory 44 ean be sequentially accessed in response to an
address counter 46. The address counter 46 is enabled by
application of a start signal 48 and is then con~rollea by -:
a timing and control cireuit 50. That is, the timing and
control circuit 50 will periodically supply a clock pulse
to increment the address counter 46. In response to each
new eount developed by the eounter 46, a different location
o~ the read only memory 44 will be aceessed to supply digi-
tal deflection signals to the drive circuits 32 and 34 to .cause the spot 26 to sequentially scan the cells depicted
in Fiyure 2a and the points depicted in E`igure 2b.
The ou-tpu-t of the photodetector 40 is applied to
a sample and hold circui-t 54 which is also under the control
of the timing and control circuit 50. Thus, each time the
~.2
~2~5~ 77/339
spot 26 is positione~d at a new po:int, the sample and hold circuit
54 samples the output of -the photodetector 40 to store an analog
signal which is then applied to an analog to digital converter 56.
The ou-tput of -the analog to digital conver-ter 5~ is in turn coupled
to s-torage registers 58 capable of storing a-t least four multi-bit
numbers, DN, DE, Ds, Dw, respectively representative of the tonal
density of the north, east, south and west points of a cell 12.
The converter 56 and registers 58 are similarly controlled by the
timin~ and control circuit 50 which enables -the registers 58 to
accumulate the digital density values of the points o a single cell
An ari-thme-tic/comparison logic unit 60 is provided to
determine, based upon the values DN, DE, Ds,-and Dw for a cell~
whether the portion of the original image within that cell is com-
prised of high detail or low detail. This determination is made
b~ comparing the magnitudes of the values DN, DE, Ds, and DW to
determine the rate of change of density within the cell area.
This determination o~ rate of change of density within a cell
area can be performed in a variety of manners such as determinLng
the ratio or differences between points within a cell. In accord-
20 ; ance with one preferred technique, the digital density samples DN,DE, Ds, DW are summed to develop a signal ~. The cell is deemed
a high detail cell if (DN - 4) or (D~ - 4) or (DS ~ -~) or (D~
is ~ T where ~ equals K (DN ~ DE -~ DS ~ DW) and T defines a thresh-
old constant. Other criteria can be used to determine whether a
cell comprises a high detail cell. For example only, the difference
between the highest and lowes-t valued samples can be compared with
some threshold; i.e. D maximum - D minimum > T.
From the foreyoing, it should be apparent that a cell
will bc deemed a high de-tail cell if the densi-ty value of any
of the four points within tlle cell :is greatc^~r tl~an the averacJe
of the fo~lr poin-ts by some threshoLd determille~ by the
13
77/339
~L~4Z~
va:Lue T. It will o~ course he readily recoyni~ed that the
ari-thmetic capability -to make the determination in accord~
ance with -the foregoiny equations is ra-ther simple and that
accordingly the arithmetic/comparison logic unit 60 can be
readily implemented either by special purpose loyic circuitry
or by a properly programrned computer. In any event, if the
logic unit 60 determines that the cell is a high detail cell,
it generates a high detail "flag" signal on output llne 62
f~llowed by the digital density values of the cell DN, DE, Ds,
Dw. On the other hand, in the event the logic unit 60 deter- !
mines that the eell-is a low-detail cell, then unit 60 merely
outputs a single digital value ~ which, as has been pointed
out, is related to the sum of the density values DN, DE, Ds,
- DW for that cell.
The outpu-t of the logic unit 60 is applied to a
memory write circuit 66 which then records the data in memory
6g which preerably comprises a magnetic ~ape or magnetic
disk drive.
: In a typical embodiment of the invention, the single
digital value ~ representing the density of a cell lS repre-
sented by eight bits, meaning that 256 density or gray scale
levels for that cell can be represented. Inasmuch as in accord-
ance with the preferred embodiment, three of those eight-bit
eombinations are used as special flags, the eight bits repre-
senting ~ define 253 different density or gray scale levels.The three reserved eight-bit combinations can be used as ~ollows:
code 0 indicates no dot in this cell, code 255 indicates the
end of the current row or column of cells, and code 254 indicates
a high detail dot cell has been encountered. Thus, the code 254
comprises a high cletail "~lag" which identifies that the data
]."
77/339
~L~L4~
fol:LowirlcJ represents the digit:al densi-ty values of the four
sample points wi-t}~ a cell. In a preEerred embodiment, each
o-f -the digital density values DN, DE, Ds, DW is represents
by six bits. Thus, a high de-tail cell is represented by 32
bits in the memory 68 while a low detail cell is represented
by only eight bits. However, since hiyh de-tail cells will
occur very infrequently, i.e., only at a sharp edge or transi-
tion in the original image, the amount of additional data re-
quired to be stored in memory 68 to represent the original
image is not much greater than in prior art systems in which
only a single point was sampled in each cell. For example,
if ten percent of the dot cells in a given image are "high
detail", then the to-tal data is increased by only thirty pe~cent
over the same image data without the image enhancement
yielded by the present invention.
Prior to proceeding~to an explana~ion of the ~mage
record subassembly of Figure 3 for executing the output mode,
atten-tion is dlrected to Figures 2c-2f which illustrate
typical halEtone dot characters to be printed on the repliGa
in response to particular original image cells. Consider
initially the original image high detail cell of Figure 2c
in which the north and south points are considerably more
dense than the east and west points. In order to best repre-
sent this original image cell by a single halftone dot
character, a dot character having a shape elongated in the
vertical direction as shown is used. Similarly, in Figure 2d
a high detail original image cell is represen-ted in which
the east and west poin-ts have a density considerably greater
than the north and sou-th points The halftone dot character
3~ utili~ed -to bes-t represent this original image cell comprises
~4Z~S~ 77/339
a dot eloncJated in the horizontal clirection. Figure 2e
illustrates an original ima~e cell in which the southwest
and north points are of considerably greater density than
the east point. The halftone dot character used to repre-
sent this original cell configuration comprises a largedot displaced along the horizontal axis of the replica
cell. Figure 2f illustrates an original image do-t cell
- in which the west and north points are of considerably
greater density than the east and south points~ A halftone
dot character, smaller in size than the dot character o~
Figure 2e, is shown displaced both vertically and horizontally
from the center of the replica cell.
Thus, as should now be apparent from Figures 2c-2f,
the size, shape, and position of the halftone-dot character
within a cell of the replica can be selected to best represent
a cell of the original image. As will be seen hereinafter,
a character font memory is preferably used to store the size
and shape of each dot charac-ter. This character font memory
is similar to that disclosed in the aforecited Crooks patent
3,806,641. Although the character font me~oxy ~heoretically
can be of infinite size, thus able to store any desired number
of different dot character shapes, it will be assumed herein
that the character font memory to be discussed in connection
with Figure 3 stores characters of three differen-t shapes, as
represented in Figures 2c, 2d, and 2e, with each shape being
stored in 253 differen-t sizes. As will be seen hereinafterr
the particular dot character to be accessed from the font
memory to represent a cell in the halftone replica is deter-
mined in part by the previously-mentioned value ~ associated
with the cell. Th~t is, the size of the dot chaxactex used
16
- 77/339
S~
in the rcp~ica to represellt a cell is determined by the
density of the correspondincJ cell in the original image
which of course is dependent upon the surn of -the density
val~es DN, DE, Ds, DW for that cell. ~s will be seen
hereinafter, the shape of -the dot charac-ter -to be accessed
from the fon-t memory to represent a cell in the replica is
determined based upon a comparison of the density values
representative of the four points within a cell. Similarly,
the displacement from the cen-ter of the replica c211 is
determined based upon a comparison of the density ~alues
of the four points withln a cell.
P~ttention is now directed to the image record sub-
assembly of Figure 3 which executes the previously-mentioned
outpu-t mode to create a halftone replica which is recorded
on film or some other photosensitive medium 100. The image
record subassembly deveiops the halftone replica"ba'sed'upon ''
the data read from memory 68 which, as previously noted, may
comprise a magnetic tape or disk. The data stored in memory
68 is read by memory read circuit 102. In the case of a
low detail cell, the memory read circuit 102 will read the
value ~ ~hich will then be transferred through enabled gate
104 to addressing circuit 106. The value ~ can be used as
an address to access an address from primary memory 108 which
stores the starting address of a related dot character stored
in font memory 110. Font memory llO stores the information
re~uired to form and display each differen-t do-t character.
It will be recalled that i-t has been assumed that the pre-
ferred embodiment is cclpable of storing dot characters of
three different shapes, each shape bein~ a~ailable in 253
dif-crent sizes. The inEormation necessary to form each of
77/'~9
2~358
these dot characters (3 X 253) is contained in the font
memoLy 110. The information necessary to form a dot
character may be contained within a block of addresses
within the font memo:ry 110 and the primary address memory
108 con-tains -the starting address ~or each block of ad-
dresses wi-thin memory 110. Thus, in the case of a low
detail cell in which the value ~ is directly applied to
the addressing circuit 106, a starting address will be
read out of memory 108 into memory control circuit 112.
This starting address then is utilized to read the infor-
mation from the character font rnçmory 110 required to form
that charac-ter is a display device such as a cathode ray
tube 114~ In a preferred embodiment of the invention, the
character will be formed on the face of the cathode ray tube
11~ by scanning the spot 116 in a regular pattexn and by
blanking and unblanking the.spot based upon the information
read from the.character font memory. The output o~ the
character font memory 110 is applied to blanking circuit . ; . -
118 which modula-tes the intensity of the spot 116. The spot .:
.20 116 is caused to scan within a particular cell area by sig-
nals provided by a cell scanning circuit 120 which supplies
analog signals to horizontal and vertical mixer circuits 122
and 12~. The outputs of the mixer circuits 122 and 124 are
applied to horizontal and vertical deflectiQn amplifiers 126
and 128 whose outputs in turn are connected to deflection
coils 130 and 132. A lens 134 focuses the spot 116 on the film
110 which is driven past the face of the cathode ray tube by
a motor 140.
The spot 116 is not only scanned within each cell
area in response to si~nals ~enerated by cell scanning ci.rcuit
18
' 77/339
120, bu~ is also of course moved from cell -to cell corres-
pondincJ to the matrix depicted in Figure 2~. In order to
do this, a scan format read only memory 144, similar to the
scan format memory 44 discussed in connec-tion with~Figure 1
is provided~ The scan Eormat read only memory 144 is driven
by an address counter 1~6 which is enabled by a start signal
148. The address counter is under the control of a timing
and control cireuit 150. The timing and control circui-t 150
similarly controls the other elements of the subassembly of
Figure 3 in a manner well-known in the art to of course
synchronize the positioning oE the spot 116 within a par-
ticular cell with the character information being read from
the memory 110 to cont,rol the intensity of the spot within
the eell, More particularly, in response to a start- signal
148, the address counter 146 is enabled and is inerèmented
to cause~'the read-only memory 144 to generate X ana Y positioning
signals which are applied to horizontal and vertieal digital
to analog converters 152 and 154. The outputs of the eonverters
' 15~ and 154 are applied to mixer' circuits 122 and 124 and
therethrough to deflection amplifiers 126 and 128 Thus, the
X and ~ defleetion signals supplied by the read-only memory
144 determine the cell position of the spot 116~ The sean
si~nals provided by the cireuit 120 cause the spot 116 to scan
within the cell. As previously pointed out, in the case o a-
low de-tail cell read from the memory 68, the value ~ ean be
direetly used as an address by addressing circuit 106 to aeeess
an address from the primary address memory 108 to thus cause
the font memory to supply the appropriate information to the
blanking circuit 118 to draw do-t charac-ters o various size on
the face oE the cathode ray tube 114~
~l~Z~3S~3
l:n tlle case o:~ a high detail dot cell read from
memory ~8, a flag detect circuit 160 will respond to the
hig}l detail flag and dis~ble gate 10~ via inverter 162.
Further, flag detect circuit 160 will enable registers 164
causiny the digital density values DN, DE, Ds, DW following
the high detail 1ag to be read in-to reyisters 164. Further,
the 1ag detect circuit 160 will enable a.rithmetic comparator
logic unit 166 which will then operate upon the stored
density values DN, D~, Ds, DW to develop size, shape, and
displacemen-t information. More particularly, logic unit
166 will develop dot character size information by calculating
the value ~ in accordance with ~ = K (D~ -~ DE + DS ~ D~).
.The value ~ developed by the logic unit 166 is applied vi~
output line 170 to the input of gate 172 which is enabled by
the output of the flag de-tect circuit 160. The output of
.
gate 172 is applied to the input of addressing circuit 106.
In addi-tion to developing -the size inormation ~, .-
the logic unit 166 examines the density values DN, DE, DSt DW
~o deter~ine whether a shape different from a cixcle should
be used. That is, in accordance with a preerred embodiment,
the logic unit 166 will examine the indi~idual density values
to determine whether the dot character should have a shape as
represented in Figure 2c or 2d, for example. In an embodiment
of the invention, this determination is made by examining the
following relationships: DN ~ DS > T - 2 or DE -~ DW > T 2
The first of the two cases represented by the oregoing rela-
tionships is represen-ted in Figure 2c. If the relationship
is satisi.ed, that is, if DN approxima-tely equals D5 and i
DN ~ DS is substantia:Lly greater than DE -~ DW~ tllen it is
desircd -to select the verticall.y oriented half~one dot characters
?()
~ 77/339
3~
shown in Fi(~ure 2c. If, on the other hand, the second rela-tion-
s~ip is satis:Eied, that is, if DE approxima-tely e~uals DW and
DE ~ DW is much greater than DN * DS~ it is desired that a
ha]ftone dot character as represented in Figure 2d be created
Since i-t has been assurned that the system is capable
of producing three different character shapes, it is sufficient
or the shape to be represented by two bits. After the logic
unit 166 determines based UpOII the examination of the values
D~, DE, Ds, DW as to which dot character shape should be pre-
sented, it supplies those two bits on output line 174 to the
input o gate 176.- Gate 176 is enabled by the flag detect
circuit 160. The ou-tput of gate 176 is applied to addressing
circuit 106. Thus, it should now be apparent that when a high
detail cell is detected by flag de-tect circuit 160, the logic
unit 166 operates upon the succeeding density values DN, DE,
Ds, DW to determine the shape and size of the dot charac~er to
be drawn by the .cathode ray tube 114. This determination re-.
: sults in the generation o bits on outpu-t lines 170 and 17~
which are then used as an address by addressing circui-t 106
to ~ccess the information from ont memory 110 to draw the dot
character o~ selected size and shape on the cathode ray tube.
In addition to determining the slze and shape of the
dot character to be presented by the cathode ray tube 114, the
logic unit 166 determines whether the dot character should be
- 25 displaced within the cell. This situation is represented in
Figures 2e and 2f wherein the dot characters are shown as being
displ.aced horizontally (~X) and vertically (~Y) within the
cell in the replica. In accordance with a preferred embodiment,
the dot character is mi.grated or displaed wit~hin a cell, in
accordance with the follow:in~ formulations:
A~ = K3(DN ~ DS) and ~Y = K~tDE ~ DW)
~ ere L represents the dis;tarlce from the center of a cell
-to the center of one of said points and K3, K4 represent a
constant.
The logic unit 166 provides digital representations
of ~X and ~Y on output lines 180 and 182 respectively. These
lines are coupled to the less significan-t bi-t input stages of
the digi-tal to analog converters 152 and 154 to thus effectively
reposi.tion the center of that particular cell to cause any
character to be displayed to be offset or migrated from the
predetermined cell position.
In accordance with one further aspect of the invention,
an operator controlled -translation table lookup means 190 may
be optionally provided. The table lookup means 190, when pro-
vlded, is coupled to the memory control circuit 112 for modifying,
under operater controlj the addresses provided by the primary
address memory 108 to access the font memory 110. The purpose
of the translation table lookup means 190 is to enable an
operator to enhance shadow or highlight effects in the halftone
replica produced on the film 100 and to essentially linearize
the system f~om original image to the apparen-t effect ~f th~
produced half-tone replica. More particularly, lt will be re-
called that the dot characters are stored in font. memory (or
at least the starting addresses stored in memory 108) in sequence
of increasing or decreasing density so that the value of ~ can
be directly used as an address by the addressing circuit 106.
This means that ideally, each value of will be ~uantized into
one of 253 levels, in accordance with a preferred embodiment,
and that one of 253 levels will be used to access a corresponding
one of 253 differently-sized do-t cha.racters. In order to
~ 2~ 77/339
hi~hl:ight certain effects or enhance certain types of imaye
subjec-t mat-ter, it is n.ot always desired -that the relation-
ship between the value and the dot character size be the
same. That is, under some ci.rcumstances, it
able to effectively superimpose a desired non-linear
equalization curve upon the values of ~. For example, it
ma~ be desired that a ~ value of 28 indeed access a dot
character of size 28 but that a value o:E 32 access some dot
character of size different than size 32. The translation
table lookup means 190 enables an operator to effectively
introduce such an equalization curve. The translation table
lookup means operates by responding to a primary address read
out of memory 108 to provide a different address for accessing
the font memory 110. From the foregoing, it should now be
apparent that a photocomposing system has been disclosed
herein having a halftone generating capabilit~ in which con-
siderably enhanced halftone rep:Licas can be developed requiring
only a relatively small increase in data storage. The objec-
tives of the invention are achieved by sampling the original
image at a resolution grea-ter than that desired for the half-
tone replica. Thusl in accordance with the p.referred embodi-
- ment, each dot cell is sampled at four points.rather than one
point. Based upon these four samples, the image read sub-
assembly determines whether the cell is a high or low detail
cell. If the cell is a low detàil cell t -then the four samples
are combined and the cell data is processed essentially as in
prior art sys-tems. If on the other hand the ce:Ll i5 a high
detail cell, then the four individual points are recorded in
memory and the image record subassembly u-tilizes those four
sample va:Lues to select a partic~llar size and shape of haltone
23
77/3~
,
character to be displayed, as ~/ell ~s de-texm;.nin~ the amount
of mic~ration or displacement of the half-tone character ~Jithin
the cell. It should be understood that although a specific
embodiment of -the invention has been disclosed herein, varia-
tions within the scope o the invention ~7ill readily occur tothose skilled in the art. For example only, although it has
been assumed that each cell is sampled at four individual
points, the invention is not restric-ted to any par-ticular
number of points and indeed a yreater number of sample poin-ts
within a cell can be utilized Further, although exemplary
criteria for determining whether a cell is a high or low
detail cell have been set forth, different criteria can be
utilized. Thus, for example, ratios of density values within
a cell, rather than differences, can be utilized to determine
whether a cell is high detail. Similarly, different but re-
lated criteria can be utilized to de-termine the shape of the
halftone character to be selected by the logic unit 166.
Further, although a special-purpose hardwired embodiment of
the invention has been disclosed herein, i~ should also be
recognized that an equivalent embodiment o~ the invention
can be readily implemented by a programmed general purpose
computer.
Although particular embodiments of the invention
have been described and illustrated herein, it is recognized
that modifications and variations may readily occur -to those
ski.lled in the art, and consequently, it is intended that
the claims be in-terpreted to cover such modifications and
equivalents.
24
