CA1142858A - Method and apparatus for forming high resolution halftone images - Google Patents
Method and apparatus for forming high resolution halftone imagesInfo
- Publication number
- CA1142858A CA1142858A CA000331790A CA331790A CA1142858A CA 1142858 A CA1142858 A CA 1142858A CA 000331790 A CA000331790 A CA 000331790A CA 331790 A CA331790 A CA 331790A CA 1142858 A CA1142858 A CA 1142858A
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- Prior art keywords
- cell
- points
- digital
- area
- dot
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
- H04N1/41—Bandwidth or redundancy reduction
- H04N1/4105—Bandwidth or redundancy reduction for halftone screened pictures
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41B—MACHINES OR ACCESSORIES FOR MAKING, SETTING, OR DISTRIBUTING TYPE; TYPE; PHOTOGRAPHIC OR PHOTOELECTRIC COMPOSING DEVICES
- B41B19/00—Photoelectronic composing machines
- B41B19/01—Photoelectronic composing machines having electron-beam tubes producing an image of at least one character which is photographed
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
- B41B—MACHINES OR ACCESSORIES FOR MAKING, SETTING, OR DISTRIBUTING TYPE; TYPE; PHOTOGRAPHIC OR PHOTOELECTRIC COMPOSING DEVICES
- B41B27/00—Control, indicating, or safety devices or systems for composing machines of various kinds or types
- B41B27/28—Control, indicating, or safety devices for individual operations or machine elements
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
- H04N1/40—Picture signal circuits
- H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
- H04N1/4055—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern
- H04N1/4058—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern with details for producing a halftone screen at an oblique angle
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- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Image Processing (AREA)
- Facsimile Image Signal Circuits (AREA)
Abstract
METHOD AND APPARATUS FOR FORMING
HIGH RESOLUTION HALFTONE IMAGES
ABSTRACT
Halftone images are formed from dots selected from a dot character font memory storing information representing halftone dots of different sizes and shapes corresponding to different gray scale tone values or image densities. An orig-inal image is first scanned to sample its tone values at minute intervals exceeding the desired dot resolution of the reproduction. These sample values are digitized by an A/D
converter which converts them to multibit binary numbers.
Adjacent samples are compared to determine the average image density in a particular area, as well as the rapidity of rate of change of image density within that area. If the rate of change of image density in a given area is below a selected threshold value, the area is designated as low detail and the multibit binary number representing the result of the average density calculation over the area is used to select a dot character from the font memory for reproducing that area of the image. If, however, the rate of change of image density exceeds the selected threshold value, indicating an area of relatively high detail, the area samples are used to select special dot shapes and positions for use in reproducing that sampled area. This technique achieves a higher resolution halftone image for a given amount of stored data than prior techniques, as well as achieving a higher "apparent" resolu-tion to the observer.
HIGH RESOLUTION HALFTONE IMAGES
ABSTRACT
Halftone images are formed from dots selected from a dot character font memory storing information representing halftone dots of different sizes and shapes corresponding to different gray scale tone values or image densities. An orig-inal image is first scanned to sample its tone values at minute intervals exceeding the desired dot resolution of the reproduction. These sample values are digitized by an A/D
converter which converts them to multibit binary numbers.
Adjacent samples are compared to determine the average image density in a particular area, as well as the rapidity of rate of change of image density within that area. If the rate of change of image density in a given area is below a selected threshold value, the area is designated as low detail and the multibit binary number representing the result of the average density calculation over the area is used to select a dot character from the font memory for reproducing that area of the image. If, however, the rate of change of image density exceeds the selected threshold value, indicating an area of relatively high detail, the area samples are used to select special dot shapes and positions for use in reproducing that sampled area. This technique achieves a higher resolution halftone image for a given amount of stored data than prior techniques, as well as achieving a higher "apparent" resolu-tion to the observer.
Description
1: ~ 77/339 42~35~
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.
77/33g 85~
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.
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)
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.
77/33g 85~
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.
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 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 -6a-285~
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--6b-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 -6c-~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 -6d-~42~5~ 77/339 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.
~4Z~ 77/339 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 ~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 ]."
~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
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 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 -6a-285~
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--6b-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 -6c-~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 -6d-~42~5~ 77/339 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.
~4Z~ 77/339 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 ~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 ]."
~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 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 ' 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 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.
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 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 ' 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 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.
Claims (16)
WHAT IS CLAIMED IS:
1. A photocomposing system for producing a half-tone 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 representa-tions 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;
arithmetic means responsive to the digital repre-sentations 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 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 pro-duced in the area of that cell.
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 representa-tions 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;
arithmetic means responsive to the digital repre-sentations 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 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 pro-duced in the area of that cell.
2. The photocomposing system of Claim 1 wherein said arithmetic means includes means for arithmetically averaging the digital representations of points within a cell to produce said single digital weighted value.
3. The photocomposing system of Claim 1 wherein said dot character storage means stores dot characters of different size and shape.
4. The photocomposing system of Claim 1 wherein each of said cells contains four points successively dis-placed by 90° from one another and wherein the tonal densities of said four points are respectively represented by DN, DE, DS, DW and wherein said comparison means generates said "flag" signal if the tonal density of any one of said four points differs by more than a threshold value from the tonal density of any other one of said points.
5. The photocomposing system of Claim 1 wherein each of said cells contains four points successively displaced by 90° from one another and wherein the tonal densities of said four points are respectively represented by DN, DE, DS, DW and wherein said arithmetic means for producing said weighted value includes means responsive to DN + DS >> ? or DE + DW ? where .SIGMA. = K (DN + DE + DW).
6. The photocomposing system of Claim 1 wherein each of said cells contains four points successively displaced by 90° from one another and wherein the tonal densities of said four points are respectively represented by DN, DE, DS, DW and wherein said means for altering the horizontal and vertical positions include means for altering the horizontal position .DELTA.X proportional to L (DN - DS) and the vertical po-sition .DELTA.Y proportional to L (DE - DN) where L represents the distance from the center of a cell to the center of one of said points.
7. A photocomposing system for producing a half-tone 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 representa-tions 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 repre-sentations of points within a cell for producing a single digital weighted value for that cell, said digital value being dependent upon .SIGMA. where .SIGMA. is proportional to the sum of DN + DE + DS + DW;
dot character storage means storing a plurality of different halftone dot characters of different size and shape;
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.
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 representa-tions 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 repre-sentations of points within a cell for producing a single digital weighted value for that cell, said digital value being dependent upon .SIGMA. where .SIGMA. is proportional to the sum of DN + DE + DS + DW;
dot character storage means storing a plurality of different halftone dot characters of different size and shape;
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.
8. The photocomposing system of Claim 7 wherein said digital value produced by said arithmetic means is further responsive to the relationship of DN+ DS compared to DE+ DW.
9. 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 there-between 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 points within that cell area; and displaying each of said selected dot characters on an output medium at a certain position within the cell area of low detail cells and at a variable position within the cell area of high de-tail cells which position is dependent upon the digital representations of points within that cell area.
10. 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 to identify a cell as being of either high or low detail;
arithmetic means responsive to the digital representations of points with-in 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 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 com-parison 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.
arithmetic means responsive to the digital representations of points with-in 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 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 com-parison 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.
11. A photocomposing system for producing a halftone replica of an original image comprising in combination: means for scanning said orig-inal 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 forming a cell to identify a cell as being of either high or low detail; a digital mem-ory; 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 re-presentations of each of the points therein said digital memory; dot characters 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 associated with a low detail cell for selecting one of said halftone dot characters from said dot 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; out-put 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 accessed digital representations associated with a high detail cell for altering the hori-zontal and/or vertical position of the visible representation produced in the area of that cell.
12. The photocomposing system of Claim 11 wherein each of said cells contains four points successively displaced by 90° from one another and wherein the tonal densities of said four points are respectively re-presented by DN, DE, DS, DW and wherein said comparison means identifies a cell as being of high detail if the tonal density of any one of said four points differs by more than a threshold value from the tonal density of any other one of said points.
13. The photocomposing system of Claim 11 wherein each of said cells contains four points successively displaced by 90° from one another and wherein the tonal densities of said four points are respectively re-presented by DN, DE, DS, DW, and wherein said arithmetic means for pro-ducing said weighted value includes means responsive to DN + DS >> ? or D + DW >> ? where .SIGMA.= K (DN + DE + DS + DW).
14. The photocomposing system of Claim 11 wherein each of said cells contains four points successively displaced by 90° from one another and wherein the tonal densities of said four points are respectively re-presented by DN, DE, DS, DW and wherein said means for altering the hori-zontal position .DELTA.X proportional to L (DN - DS) and the vertical position .DELTA.Y proportional to L (DE - DN) where L represents the distance from the center of a cell to the center of one of said points.
15. In a photocomposing system for producing a halftone replica of an original image, an image read subassembly comprising: means for scan-ning 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 single digital weighted value for that cell; means for storing said single digital weighted value in said digital memory; means res-ponsive to the identification of a high detail cell for storing the digital representations of each of the points therein in said digital memory.
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 single digital weighted value for that cell; means for storing said single digital weighted value in said digital memory; means res-ponsive to the identification of a high detail cell for storing the digital representations of each of the points therein in said digital memory.
16. In a photocomposing system for producing a halftone replica of an original image in response to data stored in a digital memory re-presenting a low detail cell in the form of a single digital weighted value and a high detail cell in the form of multiple digital representa-tions 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 sel-ected for producing a visible representation of that dot character posit-ioned at substantially the center of a cell area; and means responsive to the accessing of 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.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US92464078A | 1978-07-14 | 1978-07-14 | |
| US924,640 | 1978-07-14 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1142858A true CA1142858A (en) | 1983-03-15 |
Family
ID=25450476
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000331790A Expired CA1142858A (en) | 1978-07-14 | 1979-07-13 | Method and apparatus for forming high resolution halftone images |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS5536896A (en) |
| CA (1) | CA1142858A (en) |
| DE (1) | DE2928378C2 (en) |
| FR (1) | FR2430850A1 (en) |
| GB (1) | GB2025730B (en) |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS57125580A (en) * | 1981-01-02 | 1982-08-04 | Ibm | Threshould processing mode switch |
| GB2103449B (en) * | 1981-06-29 | 1985-05-30 | Nippon Telegraph & Telephone | Method and apparatus for gray level signal processing |
| GB2101836B (en) * | 1981-07-18 | 1985-02-20 | Linotype Paul Ltd | Method for encoding image data |
| GB2170373B (en) * | 1984-12-28 | 1989-03-15 | Canon Kk | Image processing apparatus |
| US4797943A (en) * | 1985-11-29 | 1989-01-10 | Ricoh Company, Ltd. | Gradation data processing apparatus |
| FR2591367A1 (en) * | 1985-12-06 | 1987-06-12 | Canon Kk | Image data processing apparatus |
| DE3629462C2 (en) * | 1986-08-29 | 1996-05-15 | Agfa Gevaert Ag | Method and device for electronic image processing with intermediate storage |
| DE3808668C1 (en) * | 1988-03-15 | 1989-06-22 | Nixdorf Computer Ag, 4790 Paderborn, De | |
| US5122883A (en) * | 1990-03-02 | 1992-06-16 | Xerographic Laser Images Corporation | Raster scanning engine driver which independently locates engine drive signal transitions within each pixel |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1597773C3 (en) * | 1967-08-26 | 1974-09-19 | Dr.-Ing. Rudolf Hell Gmbh, 2300 Kiel | Method for setting rasterized halftone images |
| DE2262824C3 (en) * | 1972-12-22 | 1975-07-10 | Dr.-Ing. Rudolf Hell Gmbh, 2300 Kiel | Process for the rasterized reproduction of colored halftone images in single or multi-color printing |
| US3983319A (en) * | 1973-11-12 | 1976-09-28 | Printing Developments, Inc. | Electronic screening for image reproduction |
| US4084259A (en) * | 1973-11-30 | 1978-04-11 | The Mead Corporation | Apparatus for dot matrix recording |
| US4032978A (en) * | 1975-01-20 | 1977-06-28 | International Business Machines Corporation | Pseudo halftone print generator and method |
| US4051536A (en) * | 1975-03-14 | 1977-09-27 | Xerox Corporation | Electronic halftone imaging system |
-
1979
- 1979-07-13 CA CA000331790A patent/CA1142858A/en not_active Expired
- 1979-07-13 FR FR7918303A patent/FR2430850A1/en not_active Withdrawn
- 1979-07-13 DE DE19792928378 patent/DE2928378C2/en not_active Expired
- 1979-07-13 GB GB7924503A patent/GB2025730B/en not_active Expired
- 1979-07-13 JP JP8832179A patent/JPS5536896A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| GB2025730B (en) | 1983-02-16 |
| DE2928378A1 (en) | 1980-01-31 |
| GB2025730A (en) | 1980-01-23 |
| FR2430850A1 (en) | 1980-02-08 |
| JPS5536896A (en) | 1980-03-14 |
| DE2928378C2 (en) | 1985-02-14 |
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