CA1159373A - Method and apparatus for reduction of false contours in electrically screened images - Google Patents
Method and apparatus for reduction of false contours in electrically screened imagesInfo
- Publication number
- CA1159373A CA1159373A CA000361707A CA361707A CA1159373A CA 1159373 A CA1159373 A CA 1159373A CA 000361707 A CA000361707 A CA 000361707A CA 361707 A CA361707 A CA 361707A CA 1159373 A CA1159373 A CA 1159373A
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- CA
- Canada
- Prior art keywords
- screen
- signals
- signal
- image
- cell
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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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
- H04N1/40—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
Abstract
ABSTRACT OF THE DISCLOSURE
An electrical screening system for binary displays or binary graphic recording systems is dis-closed which suppresses false contours. The suppression is achieved by increasing the number of gray levels that a given m x n matrix of pixels can represent.
Each pixel can only represent one of two gray levels in a binary display or graphic system. A conventional m x n halftone cell is able to reproduce m x n + 1 gray levels. The extra gray levels above the m x n + 1 quantity are achieved for a given m x n halftone cell by dynamically changing the values of the m x n screen signals associated with a halftone cell. The amount of the change is limited to a value between zero and D inclusive where D is the difference between two adjacent screen signal values.
An electrical screening system for binary displays or binary graphic recording systems is dis-closed which suppresses false contours. The suppression is achieved by increasing the number of gray levels that a given m x n matrix of pixels can represent.
Each pixel can only represent one of two gray levels in a binary display or graphic system. A conventional m x n halftone cell is able to reproduce m x n + 1 gray levels. The extra gray levels above the m x n + 1 quantity are achieved for a given m x n halftone cell by dynamically changing the values of the m x n screen signals associated with a halftone cell. The amount of the change is limited to a value between zero and D inclusive where D is the difference between two adjacent screen signal values.
Description
3 a~ 3 METHOD AND APPARATUS FOR REDUCTION OF
FALSE CONTOURS IN ELECTRICALLY SCREENED IMAGES
Back~round This invention relates to electrical signal recording or writing systems for binary media. Speci-fically, this invention relates to halftone screening method and apapratus for suppressing false contours in continuous tone images simulated on binary media.
Binary media is intended to refer to media which has resolution elements, picture elements (pels) or pixels that are capable of assuming either of two states, e.g. black or white. Classically, continuous tone images are simulated on binary media by organizing groups of pixels into areas called halftone cells usually m x n matrices. The halftone cell has a gray level capability equal to the number of pixels in the halftone cell plus one (for all black or all white).
However, in low resolution displays (e.g. CRT and gas panel displays) and graphic recording systems (e.g.
xerographic, ink jet and electrostatic), the size of the halftone cell is noticeable to a human observer and can be objectionable. The more pixels included in a cell means that more gray levels can be repro-duced. ~ut, as the halftone cell becomes larger, the presence of the cell in the image becomes more objection-able to a human observer.
The process of simulating continuous tone or gray level images with the halftone cell technique has several limitations. One limitation is associated with the ability o~ the human eye to detect very small changes in density ~or intensity) when there are large areas Eor the eye ~o compare. False contour~ are imag~
de~ects that occur in large areas having compclratively constant den~ities. An example is a nearly uni~orm sky ln an outdoor photograph. A ~alse contour shows ~ ' ~
, :
3 ~ 3 up in the sky in a reproduction because subtle differ-ences in the density of ~he sky are smaller than the spacing between gray levels capable of being simultaed by the halftone cells of the screen.
Heretofore, false contours are suppressed by increasing the number of pixels within a gray scale by increasing the cell size. The suppression of false contours is of course achieved at the expense of a more noticeable screen pattern in the image.
Sum~ary Accordingly, a main object of this invention is to suppress false contours in simulated continuous tone images created with binary media withou~ lowering the quality of the image.
An object consistent with the foregoing is to increase the number of gray levels reproduced with a binary medium without increasing the number of pixels contained in a halftone cell.
Also, it is an object of this invention to vary the screen signal values associated with each pixel in a halftone cell from cell to cell to obtain over the entire image more gray levels than just those levels associated with a fixed number of pixels in a halftone cell.
The above and other objects oE this invention are realized by dynamically varying the values of ; screen signals associated with a halftone cell. The variations do not exceed the separation between adja-cent screen signal values. For example, a halftone cell composed of a 3 x 3 matrix of pixels is capable of simulating 10 gray levels. The number 10 is arrived at by counting the total number oE pixels in the cell, 9 in this case, and adding one. Irhe extra gray level i5 to take into consid~ration the gray level associated 3~ with all the pixels being set to the logical of e state, for example, all black or all white~
!
' ' ' ,, ' ' 9~3 1'3 Again by way of example, the 9 screen values associa~ed with the 9 pixel locations in the cell are evenly spaced over the tonal range to be reproauced. For the case where the screen signals are in the form of an eight bit binary number, th~
total tonal or density range available is 25~ units.
; Nine evenly spaced screen signal values start at one end of the density range with a value near 28 (appro-ximately 256 divided by 9) and progress to the other end by increments of 28 ending near 252.
In accordance with the present teachings, an electrical halftone screening apparatus is provided for recording on a binary graphic or display medium. The apparatus comprises storage means for storing a plurality of screen signals having m x n different values separated from acjacent values by at least an amount D with the m x n different screen signals being organized to correspond to a plurality of halftone cells of an m x n matrix of pixels on a medium. Adder means is provided coupled to the storage means to receive screen signals in a sequence corresponding to a raster pattern of pixels for changing the value of a screen signal by an amount between zero and the value D inclusive and combining means are provided coupled to the adder means and adapted to receive image signals representative of the gray level of a pixel in an original image for combining the image and changed screen signals to generate an output marking signal capable of setting a pixel on a medium to one of its two binary gray levels in response to the combining step to suppress false contours in an image reproduced by a medium.
In accordance with a further embodiment there is provided an electrical screening method for suppressing false contour~ in recordings made on a binary graphic or display media which comprises de~ining a halftone cell including a group of pixels organized in an m x n matrix with each pixel representing areas on a medium capable o~ assuming either oE
two gray levels, assigning m x n di~ierent values represen-3~73 -3a-tative of gray levels to m x n different screen signals with the values being separated from adjacent values by at least a min~lum difference D and organizing screen signals in a pattern corresponding to a halftone cell, changing the value of screen signals by an amount between zero and ~ inclusive and combining the changed screen signals with electrical image signals representative of the gray level of a pixel in an original image for generating an output marking signal capable of setting a corresponding pixel in a medium to one or the other of its two gray levels in response to the combining.
The present improvement includes changing the fixed screen signal values associated with a given halftone cell by an increment less than the spacing between screen signals. ~he change or modification to a screen signal must vary at least from cell to cell. Preferably, the change is made from screen signal to screen signal within every cell. A random number generator and a sequential, cyclic number gene-rator ~a counter) are two devices suitable for making the changes to the screen signals~
References The use of random noise in the facsimile art is reported by Lawrance Gilman ~oberts in a February, lg62 article titled "Picture Coding U~ing Pseudo-~andom Noi~e'l in the IRE Transactions on _n ormation Theory, pages 145 to 154. The idea set Xorth in this article is basically a data compression scheme. The continuous tone spectrum of electrlcal signals produced by a fac-simile scanner i5 randomly quantitized to reduce the ': 30 amount o~ in~ormation to be transmitted to a ~acsimile receiver. ~he basic concept is that an image o~ the original document at the transmit~er can be recreated at the receiver nearly as well from randomly varying slice9 0~ the continuous signal generated by a scan o~ the document as can be recrea~ed ~rom the noise plagued continuous signal sent to the receiver. More importantly, the bandwidth required to transmit the ~" ~ ~
;
;
3 i 3 .
data representative of the image is greatly reduced.
Put in other words, the amount of data rec~uired to be transmitted over a particular bandwidth channel to create an acceptable facsimile at the receiver can be significantly reduced by thresholding the video data with the randomly generated level. This means good copies at the receiver at higher speeds and lower costs.
In contrast, this invention involves com-bining a number generator with a hal~tone screen gen-erator. Halftone screen systems are reported by James M.
Barr~ et al in U.S. Patent 3,977,077; David Behane et al in U.S. Patent 3,604,846; C. N. Judice et al in an article titled "Using Ordered Dither To Display Continuous Tone Pictures On An AC Plasma Panel" at pages 161-169 of the Proceeding o~ the S.I.D., Vol.
15/4 Fourth Quarter, 1974, and an article by R. W.
Pryor et al titled "Bilevel Image Displays A New Approach"
at pages 127-131 of the Proceeding of the S.I.D., Volume 19/3 3rd Quarter 1978.
The combining of a varying signal with a screen signal ~or a given hal~tone cell is not sug-gested by the preceeding literature. For one, the display art and the graphic art employing electrical signal processing already understand the halftone cell as a quantization of the image input data and would consider the present scheme as redundant. Also, the expectation would be that the present combination would introduce noise into a resultant image that would not be o~fset by bene~its such as gray scale suppression.
Such expectations are incorrect and suprisingly, the Lmac~es created by the present. technique are greatly improved over prlor art screened halEtone images.
Descriptlon o~ the Draw~
Other objects and ~eatures o~ the invention are apparerlt Erom a complete readinq o~ the speci~i-3~ fV3 cation alone and in combination and the drawings which are:
Figure 1 is a schematic representation of a portion of a halftone screen for a graphic medium.
The smallest squares are pixels. A 3 x 3 halftone cell is employed in this screen. The growth pattern for the halftone cell is a spiral identifiable by tracing the pixels in the numbered sequence. The numbers are shown in every other halftone cell for 1 n ease of viewing and it should be undlerstood (for Figures
FALSE CONTOURS IN ELECTRICALLY SCREENED IMAGES
Back~round This invention relates to electrical signal recording or writing systems for binary media. Speci-fically, this invention relates to halftone screening method and apapratus for suppressing false contours in continuous tone images simulated on binary media.
Binary media is intended to refer to media which has resolution elements, picture elements (pels) or pixels that are capable of assuming either of two states, e.g. black or white. Classically, continuous tone images are simulated on binary media by organizing groups of pixels into areas called halftone cells usually m x n matrices. The halftone cell has a gray level capability equal to the number of pixels in the halftone cell plus one (for all black or all white).
However, in low resolution displays (e.g. CRT and gas panel displays) and graphic recording systems (e.g.
xerographic, ink jet and electrostatic), the size of the halftone cell is noticeable to a human observer and can be objectionable. The more pixels included in a cell means that more gray levels can be repro-duced. ~ut, as the halftone cell becomes larger, the presence of the cell in the image becomes more objection-able to a human observer.
The process of simulating continuous tone or gray level images with the halftone cell technique has several limitations. One limitation is associated with the ability o~ the human eye to detect very small changes in density ~or intensity) when there are large areas Eor the eye ~o compare. False contour~ are imag~
de~ects that occur in large areas having compclratively constant den~ities. An example is a nearly uni~orm sky ln an outdoor photograph. A ~alse contour shows ~ ' ~
, :
3 ~ 3 up in the sky in a reproduction because subtle differ-ences in the density of ~he sky are smaller than the spacing between gray levels capable of being simultaed by the halftone cells of the screen.
Heretofore, false contours are suppressed by increasing the number of pixels within a gray scale by increasing the cell size. The suppression of false contours is of course achieved at the expense of a more noticeable screen pattern in the image.
Sum~ary Accordingly, a main object of this invention is to suppress false contours in simulated continuous tone images created with binary media withou~ lowering the quality of the image.
An object consistent with the foregoing is to increase the number of gray levels reproduced with a binary medium without increasing the number of pixels contained in a halftone cell.
Also, it is an object of this invention to vary the screen signal values associated with each pixel in a halftone cell from cell to cell to obtain over the entire image more gray levels than just those levels associated with a fixed number of pixels in a halftone cell.
The above and other objects oE this invention are realized by dynamically varying the values of ; screen signals associated with a halftone cell. The variations do not exceed the separation between adja-cent screen signal values. For example, a halftone cell composed of a 3 x 3 matrix of pixels is capable of simulating 10 gray levels. The number 10 is arrived at by counting the total number oE pixels in the cell, 9 in this case, and adding one. Irhe extra gray level i5 to take into consid~ration the gray level associated 3~ with all the pixels being set to the logical of e state, for example, all black or all white~
!
' ' ' ,, ' ' 9~3 1'3 Again by way of example, the 9 screen values associa~ed with the 9 pixel locations in the cell are evenly spaced over the tonal range to be reproauced. For the case where the screen signals are in the form of an eight bit binary number, th~
total tonal or density range available is 25~ units.
; Nine evenly spaced screen signal values start at one end of the density range with a value near 28 (appro-ximately 256 divided by 9) and progress to the other end by increments of 28 ending near 252.
In accordance with the present teachings, an electrical halftone screening apparatus is provided for recording on a binary graphic or display medium. The apparatus comprises storage means for storing a plurality of screen signals having m x n different values separated from acjacent values by at least an amount D with the m x n different screen signals being organized to correspond to a plurality of halftone cells of an m x n matrix of pixels on a medium. Adder means is provided coupled to the storage means to receive screen signals in a sequence corresponding to a raster pattern of pixels for changing the value of a screen signal by an amount between zero and the value D inclusive and combining means are provided coupled to the adder means and adapted to receive image signals representative of the gray level of a pixel in an original image for combining the image and changed screen signals to generate an output marking signal capable of setting a pixel on a medium to one of its two binary gray levels in response to the combining step to suppress false contours in an image reproduced by a medium.
In accordance with a further embodiment there is provided an electrical screening method for suppressing false contour~ in recordings made on a binary graphic or display media which comprises de~ining a halftone cell including a group of pixels organized in an m x n matrix with each pixel representing areas on a medium capable o~ assuming either oE
two gray levels, assigning m x n di~ierent values represen-3~73 -3a-tative of gray levels to m x n different screen signals with the values being separated from adjacent values by at least a min~lum difference D and organizing screen signals in a pattern corresponding to a halftone cell, changing the value of screen signals by an amount between zero and ~ inclusive and combining the changed screen signals with electrical image signals representative of the gray level of a pixel in an original image for generating an output marking signal capable of setting a corresponding pixel in a medium to one or the other of its two gray levels in response to the combining.
The present improvement includes changing the fixed screen signal values associated with a given halftone cell by an increment less than the spacing between screen signals. ~he change or modification to a screen signal must vary at least from cell to cell. Preferably, the change is made from screen signal to screen signal within every cell. A random number generator and a sequential, cyclic number gene-rator ~a counter) are two devices suitable for making the changes to the screen signals~
References The use of random noise in the facsimile art is reported by Lawrance Gilman ~oberts in a February, lg62 article titled "Picture Coding U~ing Pseudo-~andom Noi~e'l in the IRE Transactions on _n ormation Theory, pages 145 to 154. The idea set Xorth in this article is basically a data compression scheme. The continuous tone spectrum of electrlcal signals produced by a fac-simile scanner i5 randomly quantitized to reduce the ': 30 amount o~ in~ormation to be transmitted to a ~acsimile receiver. ~he basic concept is that an image o~ the original document at the transmit~er can be recreated at the receiver nearly as well from randomly varying slice9 0~ the continuous signal generated by a scan o~ the document as can be recrea~ed ~rom the noise plagued continuous signal sent to the receiver. More importantly, the bandwidth required to transmit the ~" ~ ~
;
;
3 i 3 .
data representative of the image is greatly reduced.
Put in other words, the amount of data rec~uired to be transmitted over a particular bandwidth channel to create an acceptable facsimile at the receiver can be significantly reduced by thresholding the video data with the randomly generated level. This means good copies at the receiver at higher speeds and lower costs.
In contrast, this invention involves com-bining a number generator with a hal~tone screen gen-erator. Halftone screen systems are reported by James M.
Barr~ et al in U.S. Patent 3,977,077; David Behane et al in U.S. Patent 3,604,846; C. N. Judice et al in an article titled "Using Ordered Dither To Display Continuous Tone Pictures On An AC Plasma Panel" at pages 161-169 of the Proceeding o~ the S.I.D., Vol.
15/4 Fourth Quarter, 1974, and an article by R. W.
Pryor et al titled "Bilevel Image Displays A New Approach"
at pages 127-131 of the Proceeding of the S.I.D., Volume 19/3 3rd Quarter 1978.
The combining of a varying signal with a screen signal ~or a given hal~tone cell is not sug-gested by the preceeding literature. For one, the display art and the graphic art employing electrical signal processing already understand the halftone cell as a quantization of the image input data and would consider the present scheme as redundant. Also, the expectation would be that the present combination would introduce noise into a resultant image that would not be o~fset by bene~its such as gray scale suppression.
Such expectations are incorrect and suprisingly, the Lmac~es created by the present. technique are greatly improved over prlor art screened halEtone images.
Descriptlon o~ the Draw~
Other objects and ~eatures o~ the invention are apparerlt Erom a complete readinq o~ the speci~i-3~ fV3 cation alone and in combination and the drawings which are:
Figure 1 is a schematic representation of a portion of a halftone screen for a graphic medium.
The smallest squares are pixels. A 3 x 3 halftone cell is employed in this screen. The growth pattern for the halftone cell is a spiral identifiable by tracing the pixels in the numbered sequence. The numbers are shown in every other halftone cell for 1 n ease of viewing and it should be undlerstood (for Figures
2 and 3 as well) that even the un-numbered 3 x 3 squares contain like numbers. Each 3 x 3 cell can represent ten gray levels.
Figure 2 is a schematic representation of a portion of the halftone screen in Figure 1 with the screen signal values at every pixel in the screen changed randomly from pixel to pixel according to one embodiment of this invention.
Figure 3 is a schematic representation of a portion of a halftone cell of Figure 1 with the pixels or screen signal values within a halftone cell being changed by a fixed amount but with the change Erom cell to cell varying randomly to illustrate an-other embodiment of the invention.
Figure 4 is a schematic representation of a xerographic recording system in which the present screening method and apparatus are implemented.
Detailed Description A well known halEtone screen in the printing arts is a photograph oE a line screen roughly resem-bling a mesh wire ~ence. An electronic hal~tone - screen re~er~ to electrical means ~or altering video data e~isting in a raster scan pattern in a manner to approximate the wlre ~ence ma~k or overlay used ; ~5 in projection systems.
Figure 1 represents an electrical halftone ;~ screen. Screen 111 is a pattern by which a plurality : : ' .~ , ~ . '` , . . : , , . :
' ' ' ~ '' . ' 1~:
Figure 2 is a schematic representation of a portion of the halftone screen in Figure 1 with the screen signal values at every pixel in the screen changed randomly from pixel to pixel according to one embodiment of this invention.
Figure 3 is a schematic representation of a portion of a halftone cell of Figure 1 with the pixels or screen signal values within a halftone cell being changed by a fixed amount but with the change Erom cell to cell varying randomly to illustrate an-other embodiment of the invention.
Figure 4 is a schematic representation of a xerographic recording system in which the present screening method and apparatus are implemented.
Detailed Description A well known halEtone screen in the printing arts is a photograph oE a line screen roughly resem-bling a mesh wire ~ence. An electronic hal~tone - screen re~er~ to electrical means ~or altering video data e~isting in a raster scan pattern in a manner to approximate the wlre ~ence ma~k or overlay used ; ~5 in projection systems.
Figure 1 represents an electrical halftone ;~ screen. Screen 111 is a pattern by which a plurality : : ' .~ , ~ . '` , . . : , , . :
' ' ' ~ '' . ' 1~:
3 ~ 3 of electrical screen signals are organized to correspond to a raster scan pattern used by an electrical recorder.
Screen 111 is made up of a plurality of small squares 112 that correspond to an electrical screen signal and pixel locations in a rectangular raster scan appro-priate for an electrical recorder. Other raster patterns besides the rectangular pattern shown are possible.
Also, the screen 111 can have an angle between zero and 90 inclusive. For present purposes, the zero angle screen is all that is necessary to discuss to ade~uately describe the invention. The application of this invention to other screen angles is trival.
I'he pixels 112 are organized into groups of 3 x 3 pixels that define a halftone cell 113. The 3 x 3 cell is arbitrary and used as a convenient ce:Ll size to describe the invention. A cell of m x n di-mensions describes the general case. Each cell 113 includes nine pixels or screen signals. The numbers 1-9 within each cell identify the nine different screen signal values possible for the cell according to pre-vious practice~ The order or arrangement of the screen signals within the cell is called the growth pattern.
The cells shown have a spiral growth pattern identi~
fiable by tracing the numbers in either an ascending or descending sequence.
Figures 2 and 3 illustrate two embodiments of the present invention. In both Figures 2 and 3, the screen signals 122 and 132 differ from corresponding nominal screen signals 112 by an amount D that is a tenths place decimal. The decimal means that a given screen signal 122 or 132 di~E~rs by some value D that is a percentage o~ the diEference between two adjacent screen values 112. For example, 5.1 associated with a screen signal in Figures 2 or 3 means its value is larger than a corresponding screen signal in ~iyure 1 o~ the value 5 by one tenth the dif~erence between ' ~ ' . , ~: . . .
.
the values of screen signals 5 and 6 in Figure 1.
The decimal notation is made to clearly point out that the extra gray levels obtained by the present invention are obtained by altering or changing the fixed number of gray levels associated with a nor-mal screen 111. Also, the decimal notation is used to emphasize that the change made to a normal screen signal 112 are zero or less than the separation be~ween adjacent screen signal values.
The actual addition made to a normal screen signal 112 is easily implemented on a percent basis when the difference between the screen signals 112 are all equal, e.g. 28. In such case, a number gen-erator, e.g. a random number generator or an up-down counter, that produces numbers between zero and 27 can readily achieve most precentage changes. If the spacing between signals 112 are not equal, the spacing must be calculated and the limit on the largest number a generator can produce must be set to the calculated number.
In fact, the spacing between screen signal values may be logrithmic and therefore not the same between all the signals within a halftone cell. Also, other signal spacings-~i.e. that are not substantially fixed--ma~ be desirable in suited applications. The adaption of this inventlon to such modified screens should be apparent and is intended to be within the scope of this invention.
The screen 121 in Figure 2 is the same as screen 111 in Figure 1 except that each screen signal 122 has been increased by a randomly generatecl value represen~.ed by the decimal. (Not all va:Lues are shown or ease of reading the numbers~) The screen 131 in Figure 3 Is the same as 3S screen llL in Figure 1 except that each screen signal 132 within a hal~tone cell 132 has been increa.3ed by ' .
the same deci~al amount. The screen signals in differ-ent cells 133 have different randomly generated values added to them. That is, one cell has 0.1 added to each screen signal within the cell, another cell 0.5, s still another 0.6 and so on.
The screens 121 and 131 ha~e a hundred more gray levels than the screen 111 with 10 additional gray levels existing between each of the signal values in screen 111. The extra gray levels do not appear in each cell but appear a statistica:Lly large number of times over a full image, e.g. an image that is 8~5 x 11 inches. In practice, the space between adja-cent screen signals is divided into many more than ten increments. For example, in the embodiment of Figure 4, the spacing is divided into twenty-eight increments as is described below. The result even for the embodiments of Figures 2 and 3 are dramatic for suppressing false contours without otherwise de-grading image ~uality, such as by making the screen more noticeable to an observer.
The false contours are suppressed with screens according to this invention because the spacing between s~reen signals is subdivided and distributed over the entire screen e.g. an 8.5 x 11 inch surEace. The extra screen values located in the subdivided region enables the subtle density changes in a sky, for example, to be at least partially reproduced. This means the human eye has less discontinuity to detect against a large area oE relatively constant density.
S0 q'he screen in Figure 2 is generated dynam-; ically by adding a number to a standard screen signal like a signal 112. It should be understood that the changed values can be pre-construcked and stored in a memory. However, screens like screen 111 are re-petitive and as such only a portion oE the screen needs to be storecl in memory. ~he entire screen is generated by cyclically repeating the stored screen segment.
: ~ . ' . ' .
g By adding a small number to the stored values the screens of the present invention can be implemented without significantly adding to the memory requirements of a system.
Figure 4 is an exemplary graphic system for recording images from electrical signals. A graphic or recording medium in the system of Figure 4 is a photoconductive layer 140 on a rotating drum 141.
Other graphic systems include ink jet recorders of the type reported by Sweet in U.S. Patent 3,596,275, by Sweet and Cumming in U.S. Patent 3,373,437 and by Lewis in U .S. Patent 3, 298, 030 .
The surface of the photoconductor 140 is uniformly electrostatically charged by a corona generator (not sho~n) positioned adjacent the drum. The charged drum surface is exposed to a spot of light ge~erated at laser 142 and directed to the drum over the optical path identified by the dashed line 143.
A latent electrostatic image is created on the surface of drum 141 during the rotation of the drum. The spot from the laser 142 is turned on and off (effecitvely) by modulator 144 such as an acoustic-optical piezoelectric cyrstal device available from the Zenith Corporation. The modulator 144 permits or prevents the light emitted by the laser to pass thr~ugh it along the balance of the optical path 143.
An electrical signal re~erred to as an output marking signal is applied to modulator 144 to e~fect the on~
v~ ~tate o~ the 11~h~ ~ource at the drum 141.
rrhe laser spot is swept acro~R the drum 141 by a rotatlng polygon mirror L45. The mirror rotates at speeds as high as several hundreds o~ revolutions per mlnute (rpm). That rpm is multiplied by t~e number o~ faces on the polygorl (6 in the example ~hown).
The ~pot i5 able to travel the length of drum 141 in ~.. . .
' ' ~
~5~3~3 a length of time that is adeguate to consider the rotation of the drum 141 to bQ negligible. Between scans, the drum rotates a distance sufficient to posi-tion the spot one scan line away from the previous scan line. In this way, a full raster is created in a scan line by scan line fashion. Each scan line is subdivided in time so that a number of pixels in a given scan line are addressed by the moving laser spot.
Each on-off clocking of the modulator 144 defines a pixel within a scan line.
The light spot from the laser 142 discharges the charge on the drum. Consequently, a latent electro-static image is created on the surface of the drum corresponding to the electrical image and screening signals being employed. This latent image is made visible by depositing electrostatically charged toner particles ~ot shown) onto the drum surface by a develop-ment device located adjacent the drum (not shown).
The visible toner image is in turn electrostatically transferred to plain paper (not shown) to provide an image made up of black markings (the toner) on white paper/ for example. Conventionally, the toner is perW
manently bonded to the paper by heating and cooling ~he toner.
; 25 The electrical image signals are supplied from an appropriate source (no~ shown) to the video data bu~ of the system of Figure 4. The image signals may be created artifically by electrical equip-ment or by examining an original document on a plxel by pixel basis and generating a dlgital or analoy signal repre~entative o~ the reElection density ~or transparency) o~ a pixel.
Each image signal repre~ent9 the intensity or density o~ a pixel ln an electrical image. The lmage signals are applied to ~he video terminal 150 ; in a pixel by pixel, line by line sequence compatible : .
.. : . ~
, ! ' 3`~3 with the raster pattern being employed. In addition, a start scan signal is applied to the system of Figure
Screen 111 is made up of a plurality of small squares 112 that correspond to an electrical screen signal and pixel locations in a rectangular raster scan appro-priate for an electrical recorder. Other raster patterns besides the rectangular pattern shown are possible.
Also, the screen 111 can have an angle between zero and 90 inclusive. For present purposes, the zero angle screen is all that is necessary to discuss to ade~uately describe the invention. The application of this invention to other screen angles is trival.
I'he pixels 112 are organized into groups of 3 x 3 pixels that define a halftone cell 113. The 3 x 3 cell is arbitrary and used as a convenient ce:Ll size to describe the invention. A cell of m x n di-mensions describes the general case. Each cell 113 includes nine pixels or screen signals. The numbers 1-9 within each cell identify the nine different screen signal values possible for the cell according to pre-vious practice~ The order or arrangement of the screen signals within the cell is called the growth pattern.
The cells shown have a spiral growth pattern identi~
fiable by tracing the numbers in either an ascending or descending sequence.
Figures 2 and 3 illustrate two embodiments of the present invention. In both Figures 2 and 3, the screen signals 122 and 132 differ from corresponding nominal screen signals 112 by an amount D that is a tenths place decimal. The decimal means that a given screen signal 122 or 132 di~E~rs by some value D that is a percentage o~ the diEference between two adjacent screen values 112. For example, 5.1 associated with a screen signal in Figures 2 or 3 means its value is larger than a corresponding screen signal in ~iyure 1 o~ the value 5 by one tenth the dif~erence between ' ~ ' . , ~: . . .
.
the values of screen signals 5 and 6 in Figure 1.
The decimal notation is made to clearly point out that the extra gray levels obtained by the present invention are obtained by altering or changing the fixed number of gray levels associated with a nor-mal screen 111. Also, the decimal notation is used to emphasize that the change made to a normal screen signal 112 are zero or less than the separation be~ween adjacent screen signal values.
The actual addition made to a normal screen signal 112 is easily implemented on a percent basis when the difference between the screen signals 112 are all equal, e.g. 28. In such case, a number gen-erator, e.g. a random number generator or an up-down counter, that produces numbers between zero and 27 can readily achieve most precentage changes. If the spacing between signals 112 are not equal, the spacing must be calculated and the limit on the largest number a generator can produce must be set to the calculated number.
In fact, the spacing between screen signal values may be logrithmic and therefore not the same between all the signals within a halftone cell. Also, other signal spacings-~i.e. that are not substantially fixed--ma~ be desirable in suited applications. The adaption of this inventlon to such modified screens should be apparent and is intended to be within the scope of this invention.
The screen 121 in Figure 2 is the same as screen 111 in Figure 1 except that each screen signal 122 has been increased by a randomly generatecl value represen~.ed by the decimal. (Not all va:Lues are shown or ease of reading the numbers~) The screen 131 in Figure 3 Is the same as 3S screen llL in Figure 1 except that each screen signal 132 within a hal~tone cell 132 has been increa.3ed by ' .
the same deci~al amount. The screen signals in differ-ent cells 133 have different randomly generated values added to them. That is, one cell has 0.1 added to each screen signal within the cell, another cell 0.5, s still another 0.6 and so on.
The screens 121 and 131 ha~e a hundred more gray levels than the screen 111 with 10 additional gray levels existing between each of the signal values in screen 111. The extra gray levels do not appear in each cell but appear a statistica:Lly large number of times over a full image, e.g. an image that is 8~5 x 11 inches. In practice, the space between adja-cent screen signals is divided into many more than ten increments. For example, in the embodiment of Figure 4, the spacing is divided into twenty-eight increments as is described below. The result even for the embodiments of Figures 2 and 3 are dramatic for suppressing false contours without otherwise de-grading image ~uality, such as by making the screen more noticeable to an observer.
The false contours are suppressed with screens according to this invention because the spacing between s~reen signals is subdivided and distributed over the entire screen e.g. an 8.5 x 11 inch surEace. The extra screen values located in the subdivided region enables the subtle density changes in a sky, for example, to be at least partially reproduced. This means the human eye has less discontinuity to detect against a large area oE relatively constant density.
S0 q'he screen in Figure 2 is generated dynam-; ically by adding a number to a standard screen signal like a signal 112. It should be understood that the changed values can be pre-construcked and stored in a memory. However, screens like screen 111 are re-petitive and as such only a portion oE the screen needs to be storecl in memory. ~he entire screen is generated by cyclically repeating the stored screen segment.
: ~ . ' . ' .
g By adding a small number to the stored values the screens of the present invention can be implemented without significantly adding to the memory requirements of a system.
Figure 4 is an exemplary graphic system for recording images from electrical signals. A graphic or recording medium in the system of Figure 4 is a photoconductive layer 140 on a rotating drum 141.
Other graphic systems include ink jet recorders of the type reported by Sweet in U.S. Patent 3,596,275, by Sweet and Cumming in U.S. Patent 3,373,437 and by Lewis in U .S. Patent 3, 298, 030 .
The surface of the photoconductor 140 is uniformly electrostatically charged by a corona generator (not sho~n) positioned adjacent the drum. The charged drum surface is exposed to a spot of light ge~erated at laser 142 and directed to the drum over the optical path identified by the dashed line 143.
A latent electrostatic image is created on the surface of drum 141 during the rotation of the drum. The spot from the laser 142 is turned on and off (effecitvely) by modulator 144 such as an acoustic-optical piezoelectric cyrstal device available from the Zenith Corporation. The modulator 144 permits or prevents the light emitted by the laser to pass thr~ugh it along the balance of the optical path 143.
An electrical signal re~erred to as an output marking signal is applied to modulator 144 to e~fect the on~
v~ ~tate o~ the 11~h~ ~ource at the drum 141.
rrhe laser spot is swept acro~R the drum 141 by a rotatlng polygon mirror L45. The mirror rotates at speeds as high as several hundreds o~ revolutions per mlnute (rpm). That rpm is multiplied by t~e number o~ faces on the polygorl (6 in the example ~hown).
The ~pot i5 able to travel the length of drum 141 in ~.. . .
' ' ~
~5~3~3 a length of time that is adeguate to consider the rotation of the drum 141 to bQ negligible. Between scans, the drum rotates a distance sufficient to posi-tion the spot one scan line away from the previous scan line. In this way, a full raster is created in a scan line by scan line fashion. Each scan line is subdivided in time so that a number of pixels in a given scan line are addressed by the moving laser spot.
Each on-off clocking of the modulator 144 defines a pixel within a scan line.
The light spot from the laser 142 discharges the charge on the drum. Consequently, a latent electro-static image is created on the surface of the drum corresponding to the electrical image and screening signals being employed. This latent image is made visible by depositing electrostatically charged toner particles ~ot shown) onto the drum surface by a develop-ment device located adjacent the drum (not shown).
The visible toner image is in turn electrostatically transferred to plain paper (not shown) to provide an image made up of black markings (the toner) on white paper/ for example. Conventionally, the toner is perW
manently bonded to the paper by heating and cooling ~he toner.
; 25 The electrical image signals are supplied from an appropriate source (no~ shown) to the video data bu~ of the system of Figure 4. The image signals may be created artifically by electrical equip-ment or by examining an original document on a plxel by pixel basis and generating a dlgital or analoy signal repre~entative o~ the reElection density ~or transparency) o~ a pixel.
Each image signal repre~ent9 the intensity or density o~ a pixel ln an electrical image. The lmage signals are applied to ~he video terminal 150 ; in a pixel by pixel, line by line sequence compatible : .
.. : . ~
, ! ' 3`~3 with the raster pattern being employed. In addition, a start scan signal is applied to the system of Figure
4 at termina] 151 to align the video signals with the rotation of drum 141.
The video or image signal, start of scan signal and a system clock signal at terminal 152 are applied to the AND gate 153. This gate applies the video data to the combining circuit 160. Simulta-neously, the start of scan signal is applied to the address register 154 which serially addresses stored screen signals in the Random Output Memory (ROM) 155.
The screen signals are stored as eight bit numbers in ROM 155. The difference between adjacent screen signals values is about 28, radix 10, for the example used throughout which is 11100, radix 2. Consequently, the five least significant bits of the eight bit binary number representative of a screen signal is what is dynamically varied in this embodiment. The change is limited to numbers ranging from zero to 27. A five place binary number can generate the numbers zero through 31 so the number generator must be prevented from generating the numbers 28-31.
Changes are made to the screen signals in this embodiment with a five bit random number generator 156. An eight bit screen sig~al and a five bit random number are synchronously applied to a digital adder 157 through AND gates (or enable gates) 158 and 159 by the system clock. The adder 157 adds the binary number from the random generator to the five least 3~ sigrliEicant bLts of the screen signal number. The sum is a new screen ~ignal corresponding to the screen signals in ~creen 121 of Figure ~ The screen of Figure 3 ls more complLcated to generate because the cells 132 extend over three scan lines. To implement the screen of Figure 3, memor~ is used to hold random numbers until a full hal~tone cell is generated which ;
. . ': :.' - : ' : . ., :: . ,: :.
t~3 ~ 3 amounts to three scan lines of screen signals for the 3 x 3, zero angle screen of this example.
A two level output marking signal is gen-erated by circuit 160 in response to the sum from adder 157 (the screen signal) and the image signal from gate 153. Both the image and screen signals are eight bit binary numbers in electrical signal form. The com-bining circuit, in this emboidment, compares the binary values of the two signals and puts the output marking signal to an "On'l level if the image signal is less than the screen signal value and to the "Off" level if the i~age is equal to or greater than the screen signal value.
The output signal is applied to the modulator 144 over lead line 161. The application of the output marking signal to the modulator is gated by the system clock through the AND (or enable) gate 162.
The "Onl' state of the marking signal permits the light from laser 142 to pass ~hrough the modulator 144 and proceed to the drum 141. At the drum, the light spot discharges a small pixel size area. The discharge of the surface means that toner is not attracted to the drum at that location. The "Off"
state of the marking signal prevents the laser light from passing the modulator and means that toner is attracted to the drum at the given pixel location in the latent electrostatic image.
The combining circuit 160 in other embodi-ments may develop an output signal by multiplying the image and screen signal values together or with a con-stant. Al~o, techniques involving adcling the image and screen signals together are known which enable a deci~ion to be made concerning the density oE a pixel represented by the ima~e slgnals.
The rotation o~ the polygon mirror 145 and oE the drum 141 is done with separate motors. ~he two motors and their control circuitry are represented ... , ~ . . :. :
: ., ,.;.:.
by the block 163. The rotation speeds of the polygon and the drum are synchronized to each other and the video data rate by means of the system clock. The clock is shown being applied to the motor control circuitry.
Other embodiments of the invention will be apparent from the foregoing description. The scope of the invention is intended to embrace those embodi-ments.
,:
. ' !, ' ,, : , ' ., ' .
' ' ' ' '. , .' . ' :'' ' ' ' . , : ~
The video or image signal, start of scan signal and a system clock signal at terminal 152 are applied to the AND gate 153. This gate applies the video data to the combining circuit 160. Simulta-neously, the start of scan signal is applied to the address register 154 which serially addresses stored screen signals in the Random Output Memory (ROM) 155.
The screen signals are stored as eight bit numbers in ROM 155. The difference between adjacent screen signals values is about 28, radix 10, for the example used throughout which is 11100, radix 2. Consequently, the five least significant bits of the eight bit binary number representative of a screen signal is what is dynamically varied in this embodiment. The change is limited to numbers ranging from zero to 27. A five place binary number can generate the numbers zero through 31 so the number generator must be prevented from generating the numbers 28-31.
Changes are made to the screen signals in this embodiment with a five bit random number generator 156. An eight bit screen sig~al and a five bit random number are synchronously applied to a digital adder 157 through AND gates (or enable gates) 158 and 159 by the system clock. The adder 157 adds the binary number from the random generator to the five least 3~ sigrliEicant bLts of the screen signal number. The sum is a new screen ~ignal corresponding to the screen signals in ~creen 121 of Figure ~ The screen of Figure 3 ls more complLcated to generate because the cells 132 extend over three scan lines. To implement the screen of Figure 3, memor~ is used to hold random numbers until a full hal~tone cell is generated which ;
. . ': :.' - : ' : . ., :: . ,: :.
t~3 ~ 3 amounts to three scan lines of screen signals for the 3 x 3, zero angle screen of this example.
A two level output marking signal is gen-erated by circuit 160 in response to the sum from adder 157 (the screen signal) and the image signal from gate 153. Both the image and screen signals are eight bit binary numbers in electrical signal form. The com-bining circuit, in this emboidment, compares the binary values of the two signals and puts the output marking signal to an "On'l level if the image signal is less than the screen signal value and to the "Off" level if the i~age is equal to or greater than the screen signal value.
The output signal is applied to the modulator 144 over lead line 161. The application of the output marking signal to the modulator is gated by the system clock through the AND (or enable) gate 162.
The "Onl' state of the marking signal permits the light from laser 142 to pass ~hrough the modulator 144 and proceed to the drum 141. At the drum, the light spot discharges a small pixel size area. The discharge of the surface means that toner is not attracted to the drum at that location. The "Off"
state of the marking signal prevents the laser light from passing the modulator and means that toner is attracted to the drum at the given pixel location in the latent electrostatic image.
The combining circuit 160 in other embodi-ments may develop an output signal by multiplying the image and screen signal values together or with a con-stant. Al~o, techniques involving adcling the image and screen signals together are known which enable a deci~ion to be made concerning the density oE a pixel represented by the ima~e slgnals.
The rotation o~ the polygon mirror 145 and oE the drum 141 is done with separate motors. ~he two motors and their control circuitry are represented ... , ~ . . :. :
: ., ,.;.:.
by the block 163. The rotation speeds of the polygon and the drum are synchronized to each other and the video data rate by means of the system clock. The clock is shown being applied to the motor control circuitry.
Other embodiments of the invention will be apparent from the foregoing description. The scope of the invention is intended to embrace those embodi-ments.
,:
. ' !, ' ,, : , ' ., ' .
' ' ' ' '. , .' . ' :'' ' ' ' . , : ~
Claims (16)
1. An electrical screening method for suppressing false contours in recordings made on binary graphic or display media comprising defining a halftone cell including a group of pixels organized in an m x n matrix with each pixel representing areas on a medium capable of assuming either of two gray levels, assigning m x n different values represen-tative of gray levels to m x n different screen signals with the values being separated from adjacent values by at least a minimum difference D and organizing the screen signals in a pattern corresponding to a halftone cell, changing the value of screen signals by an amount between zero and D inclusive, and combining the changed screen signals with electrical image signals representative of the gray level of a pixel in an original image for generating an output marking signal capable of setting a corres-ponding pixel in a medium to one or the other of its two gray levels in response to the combining.
2. The method of Claim 1 wherein the screen signals are arranged in halftone cells that are oriented at an angle between zero and ninety degrees inclusive.
3. The method of Claim 1 wherein the change to the screen signals within a given halftone cell are the same but differ from cell to cell.
4. The method of Claim 1 wherein the change to the screen signals within a halftone cell are different from screen signal to screen signal.
5. The method of Claim 1 wherein the change to the screen signal is randomly selected within the limits of zero to the value D.
6. The method of Claim 1 wherein the change to the screen signal is systemically changed by increments between the limits of zero and the value D.
7. The method of Claim 1 wherein the combining step includes comparing the magnitudes of image and changed screen signals and setting the output marking signal to one of two levels representative of which is the greater.
8. Electrical halftone screening apparatus for recording on a binary graphic or display medium comprising storage means for storing a plurality of screen signals having m x n different values separated from adjacent values by at least an amount D with the m x n different screen signals being organized to correspond to a plurality of halftone cells of an m x n matrix of pixels on a medium, adder means coupled to the storage means to receive screen signals in a sequence corresponding to a raster pattern of pixels for changing the value of a screen signal by an amount between zero and the value D inclusive and combining means coupled to the adder means and adapted to receive image signals representative of the gray level of a pixel in an original image for combining the image and changed screen signals to generate an output marking signal capable of setting a pixel on a medium to one of its two binary gray levels in response to the combining step to suppress false contours in an image reproduced by a medium.
9. The apparatus of Claim 8 wherein a random number generator is coupled to said adder means for adding a random number to each of the screen signals received from the storage means.
10. The apparatus of Claim 8 wherein a cyclic counter means is coupled to said adder means for adding a cyclically reoccurring numbers to each of the screen signals received from the storage means.
11. The apparatus of Claim 8 wherein the halftone cell pattern is organized at an angle between zero and ninety degrees inclusive.
12. The apparatus of Claim 8 wherein the combining means includes means for generating a two level output signal that assumes one level when an image signal is greater than a screen signal and the other level when less than the screen signal.
13. The apparatus of Claim 8 further including an electrophotographic recording means coupled to receive the output marking signals for creation of a latent electrostatic image on a photoconductive member.
14. The apparatus of Claim 13 wherein the electrophotographic recording means includes development means for developing latent electrostatic images with a toner material.
15. The apparatus of Claim 8 further including an ink jet recording means coupled to receive the output marking signals and to create images with ink drops on paper in response to the output signals.
16. The apparatus of Claim 8 further including a CRT display means coupled to receive the output marking signals and to create images in response to the output signals.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US091,282 | 1979-11-05 | ||
| US06/091,282 US4245258A (en) | 1979-11-05 | 1979-11-05 | Method and apparatus for reduction of false contours in electrically screened images |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1159373A true CA1159373A (en) | 1983-12-27 |
Family
ID=22226987
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000361707A Expired CA1159373A (en) | 1979-11-05 | 1980-10-07 | Method and apparatus for reduction of false contours in electrically screened images |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US4245258A (en) |
| CA (1) | CA1159373A (en) |
| GB (1) | GB2066612B (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4326258A (en) * | 1980-01-31 | 1982-04-20 | Ncr Canada Ltd - Ncr Canada Ltee | Method and apparatus for reducing the gray scale resolution of a digitized image |
| US4449150A (en) * | 1981-01-19 | 1984-05-15 | Ricoh Company, Ltd. | Method of processing medium tone picture |
| JPS57142072A (en) * | 1981-02-27 | 1982-09-02 | Canon Inc | Video signal processor |
| JPS57160264A (en) * | 1981-03-30 | 1982-10-02 | Canon Inc | Recorder of half tone picture |
| JPH0620232B2 (en) * | 1981-03-30 | 1994-03-16 | キヤノン株式会社 | Image processing device |
| US5666444A (en) * | 1982-04-06 | 1997-09-09 | Canon Kabushiki Kaisha | Image processing apparatus |
| GB2120896B (en) * | 1982-04-06 | 1987-01-28 | Canon Kk | Half-tone facsimile system |
| US4682216A (en) * | 1983-03-08 | 1987-07-21 | Canon Kabushiki Kaisha | Color image picture forming process and apparatus which improves the quality of the black portions of the picture |
| ATE29815T1 (en) * | 1983-11-14 | 1987-10-15 | Hell Rudolf Dr Ing Gmbh | METHOD AND EQUIPMENT FOR THE PRODUCTION OF SCREENED PRINTING FORMS. |
| JPS60157375A (en) * | 1983-12-21 | 1985-08-17 | Fuji Xerox Co Ltd | Halftone expressing system |
| US4680596A (en) * | 1984-08-02 | 1987-07-14 | Metromedia Company | Method and apparatus for controlling ink-jet color printing heads |
| US4577235A (en) * | 1984-08-20 | 1986-03-18 | The Mead Corporation | Text/continuous tone image decision processor |
| JPS6163893A (en) * | 1984-09-06 | 1986-04-02 | 株式会社日立製作所 | Display of false halftone image of display unit |
| US4673987A (en) * | 1985-04-22 | 1987-06-16 | Ibm Corporation | Compressed block code for facsimile transmission |
| US4873537A (en) * | 1985-06-08 | 1989-10-10 | Canon Kabushiki Kaisha | Image recording apparatus for producing a plurality of pulse width modulated signals on the basis of image data |
| US5175807A (en) * | 1986-12-04 | 1992-12-29 | Quantel Limited | Video signal processing with added probabilistic dither |
| JPS6444681A (en) * | 1987-08-12 | 1989-02-17 | Fuji Photo Film Co Ltd | Dot screen forming method |
| US4920501A (en) * | 1988-10-19 | 1990-04-24 | Eastman Kodak Company | Digital halftoning with minimum visual modulation patterns |
| US4916545A (en) * | 1988-11-16 | 1990-04-10 | Eastman Kodak Company | Electronic graphic arts screener that suppresses Moire patterns using pseudo-random font selection |
| US4918622A (en) * | 1988-11-16 | 1990-04-17 | Eastman Kodak Company | Electronic graphic arts screener |
| US4977458A (en) * | 1988-11-16 | 1990-12-11 | Eastman Kodak Company | Apparatus for addressing a font to suppress Moire patterns occurring thereby and a method for use therein |
| US5274473A (en) * | 1989-08-04 | 1993-12-28 | Intergraph Corporation | Rapid variable angle digital screening |
| US4959730A (en) * | 1990-01-02 | 1990-09-25 | Eastman Kodak Company | False density contour suppression using randomly modified input signals for comparison to threshold values |
| US4945422A (en) * | 1990-01-02 | 1990-07-31 | Eastman Kodak Company | False density contour suppression using stored random probabilities to form print/no-print decisions |
| US5214517A (en) * | 1990-02-07 | 1993-05-25 | Eastman Kodak Company | Digital halftoning with correlated minimum visual modulation patterns |
| US5341228A (en) * | 1990-12-04 | 1994-08-23 | Research Corporation Technologies | Method and apparatus for halftone rendering of a gray scale image using a blue noise mask |
| US5276535A (en) * | 1991-12-24 | 1994-01-04 | Levien Raphael L | Method and apparatus for halftoning of images using grayscale error diffusion |
| US5331429A (en) * | 1992-02-07 | 1994-07-19 | Levien Raphael L | Digital generation of halftone images with error diffusion and frequency matched periodic screen rulings |
| US5311602A (en) * | 1992-02-25 | 1994-05-10 | Unisys Corporation | Circuit for compressing digital images while retaining image fidelity |
| US5313309A (en) * | 1992-06-05 | 1994-05-17 | Eastman Kodak Company | Method and apparatus for printing halftones with a gray level printer with contour suppression and/or minimization of moire patterns |
| US5838462A (en) * | 1996-04-01 | 1998-11-17 | Xerox Corporation | Hybrid imaging system |
| US6226103B1 (en) | 1998-09-02 | 2001-05-01 | Xerox Corporation | Method of producing spot colors appearance with process color printing |
| US7110144B2 (en) * | 2001-09-05 | 2006-09-19 | Xerox Corporation | System and method of performing halftone gamma correction in a printing environment |
| US8064722B1 (en) * | 2006-03-07 | 2011-11-22 | The United States Of America As Represented By The Secretary Of The Navy | Method and system for analyzing signal-vector data for pattern recognition from first order sensors |
| US7667875B2 (en) * | 2006-06-09 | 2010-02-23 | Xerox Corporation | Textured HT screens for improved uniformity appearance |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3294896A (en) * | 1963-07-24 | 1966-12-27 | Bell Telephone Labor Inc | Digital encoder for facsimile transmission |
| US3739082A (en) * | 1972-02-29 | 1973-06-12 | Us Army | Ordered dither system |
| GB1448112A (en) * | 1973-02-13 | 1976-09-02 | Int Publishing Corp Ltd | Electronic screening |
| US4205341A (en) * | 1978-01-24 | 1980-05-27 | Nippon Telegraph And Telephone Public Corporation | Picture signal coding apparatus |
| US4184206A (en) * | 1978-03-07 | 1980-01-15 | Hughes Aircraft Company | Subpixel X-Y coordinate encoding |
-
1979
- 1979-11-05 US US06/091,282 patent/US4245258A/en not_active Expired - Lifetime
-
1980
- 1980-10-07 CA CA000361707A patent/CA1159373A/en not_active Expired
- 1980-11-05 GB GB8035563A patent/GB2066612B/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2066612B (en) | 1983-08-24 |
| US4245258A (en) | 1981-01-13 |
| GB2066612A (en) | 1981-07-08 |
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