GB2258783A

GB2258783A - Colour printer data signals with undercolour removal

Info

Publication number: GB2258783A
Application number: GB9215236A
Authority: GB
Inventors: Dennis J Merna
Original assignee: Ricoh Printing Systems America Inc
Current assignee: Ricoh Printing Systems America Inc
Priority date: 1991-08-14
Filing date: 1992-07-17
Publication date: 1993-02-17
Also published as: DE4226518A1; GB9215236D0; JPH05336355A

Description

6 1 DERIVATION-OF COLOR PRINTER DATA SIGNALS

BACKGROUND OF THE INVENTION:

The present invention relates to the generation of data signals for controlling the operation of a color printer, and particularly a four color dot matrix printer utilizing black ink and three subtractive primary color inks.

State-of-the-art color printers operate by applying individual dots of ink to a sheet at selected points of a rectangular matrix in a pattern determined by print data and having the form of printed text or pictorial or graphics images. With attainable horizontal and vertical resolutions of 300 dots per inch (DPI) or more, sharply defined characters or high resolution images can be generated.

when a printer is capable of printing with inks producing three different primary colors and black, a wide range of hues can theoretically be produced. One approach to producing apparent colors different from any one of the three primary colors is to place two or three of the primary colors atop one another or adjacent one another so as to allow the spatial integration characteristic of the human eye to "combine" the primary colors into a number of secondary colors. However, the number of different apparent hues which can be generated in this manner is, in the present state of technology, limited because it is difficult to vary the size or color saturation of each individual primary color dot. When all of the dots of each primary color have the same size and color saturation, only a single apparent secondary color will result from the juxtaposition or superposition of any two primary color dots.

2 SUMMARY OF THE INVENTION

It is a primary object of the present invention to increase the range of apparent colors which can be produced by color dot matrix printers.

Another object of the invention is to enable both the hue and saturation of apparent colors produced by a dot matrix printer to be varied over a wide range.

The.above and other objects are achieved, according to the present invention, by a method for generating printer control signals for a dot matrix color printer capable of printing patterns composed of dots of black ink and first, second and third subtractive primary color inks at locations arranged in a rectangular matrix on a sheet, which patterns constitute visual reproductions of a source image composed of a rectangular matrix of picture elements, each picture element having a hue and intensity defined by the intensities of three primary color components, comprising:

deriving three signals for each picture element of the source image, each signal having a respective value representative of the intensity of a respective subtractive primary color component corresponding to the intensity of a respective primary color component of the picture element of the source image; and % generating, on the basis of the three signal values for each picture element of the source image, printer control signals for effecting printing of ink dots at a selected group of n adjacent locations, 30. forming a portion of the rectangular matrix, on the sheet such that B Min (X,, Y1, Z1) X2 X1 - B1 Y2 Y1 - B, and Z2 Z1 B1 1 1m g 3 where X,, Y1 and Z1 are integers each having a value between 0 and n and each at least approximately proportional to the intensity of a respective one of the first, second and third subtractive primary colors, B is the number of the selected group of locations which each receives a black ink dot, and X2, Y2 and Y3 are, respectively, the number of the selected group of locations which each receives a dot.of a respective subtractive primary color ink, and wherein primary color ink dots are placed only at locations which do not receive a black ink dot.

4 BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a block diagram of a preferred embodiment of a system for implementing the present invention.

Figure 2 is a block diagram of a component of the system of Figure 1.

Figure 3 is a pictorial diagram illustrating a group of memory locations of each of three color memories forming part of the system of Figure 1.

1 DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the concept of representing each pixel of a scanned image by a rectangular, preferably square, matrix of printed image pixels, each matrix of printed image pixels being known as a superpel. Each superpel would typically be constituted by a 4x4 or 8x8 dot matrix on a printed sheet, although other matrix sizes could be selected. In view of the high resolution permitted by existing printers, of the order of 300 DPI in both the horizontal and vertical directions, the resolution in superpels on a printed sheet can be made to correspond to the horizontal and vertical resolution of a video image, which typically is of the order of 75 pixels per inch, by forming each superpel as a 4x4 dot matrix.

Within each superpel, the number of dots of ink of each color can be selected to produce a wide variety of hues and saturations. Color saturation can be controlled by appropriate selection of the number of matrix dots in a superpel which are left blank.

For each pixel of a superpel at which all three subtractive primary colors are to be printed, these are preferably replaced by black ink to produce greater contrast. At all other pixel locations, either one or two primary colors will be printed or no ink will be deposited, leaving a white region.

When each superpel is composed of an array having one of the sizes indicated above, and the printer operates with horizontal and vertical resolutions of 300 DPI, the spatial integration behavior of the human eye will cause the viewer to interpret the dots of each superpel as a single image element having a hue and saturation dependent on the pattern of dots placed at that location.

Figure 1 illustrates a system constructed to implement the present invention and composed of a conventional video signal source 2 which scans a scene and produces three primary color signals, in this case 6 -10 the additive primaries red (R), green (G) and blue (B). For each picture element (pixel) of th scene, each of the output signals from source 2 has a value representative of the intensity of the R, G, or B component thereof. Typically, the signals produced by source 2 will be in analog form.

These signals are supplied to respective signal processing devices 4, each of which performs the following operations:

1) the signal element associated with each scene pixel is converted into digital form and given a value with respect to a scale of zero to 100 representing the intensity of the respective color component in the scene pixel, zero representing minimum intensity and 100 representing maximum intensity; 2) each digitized signal element is then subtracted from a value of 100 to represent the intensity of a corresponding one of the subtractive primary colors cyan (C), magenta (M) and yellow (Y).

The intensity values of the resulting subtractive primary color components will then be as follows:

C = 100 - R M = 100 - G Y = 100 - B; 3) the resulting intensity value for the respective C, M or Y signal is converted into a signal representative of the value of the associated subtractive primary color signal divided by 100 and multiplied by the number of dot matrix points forming one superpel of the printed image, which corresponds to one pixel of the scene image; and 4) each signal produced in step 3) may be converted to a train of pulses equal in number to the integer closest to the value represented by that signal.

The resulting pulse trains are then used to wte binary a corresponding number of bit rll locations of a respective source image-pixel region of a 7 corresponding one of three raster image memories 6. The reading and writing operations of memories 6 will be controlled in a conventional manner by a suitable control circuit 7.

one suitable embodiment of each device 4 is illustrated in Figure 2. one of the signals produced by source 2 is supplied to an analog/digital converter 14, wherein it is.converted into a digital signal u having a value of between zero and 100. This signal is supplied to a digital subtraction circuit 16 which subtracts the digitized signal u from a value of 100, to produce the difference signal v. Signal v is then applied to a calculating circuit, or digital function generator, 18 producing a digital output equal to (v/100)P, rounded is to the nearest integer value, where P is equal to the number of locations in the.associated memory 6 assigned to a single pixel of the initial video image, the number of memory locations corresponding to the number printing dot matrix points in a superpel. The resulting signal is then supplied to a pulse generator 20 which generates, with appropriate timing, a stream of pulses equal in number to the value produced by circuit 18. The pulses produced by generator 20 are then delivered to set a corresponding number of bit locations in the corresponding memory 6 to the binary 11111 state. The same process takes place in parallel for each of the primary color signals.

Thus, each primary color pixel value produced by signal source 2 determines the number of binary "ill's inserted in the associated superpel, or 4x4 memory location matrix, of the corresponding memory 6.

Reverting to Figure 1, after a complete image, or image block, has been processed in this manner, and the resulting bit values stored in memories 6, the locations of each memory 6 are read out successively to a first AND gate 8 in a manner such each location of one memory is read out simultaneously with readout of a corresponding location in each other memory. In a additionj the output from each memory 6 is simultaneously delivered to a respective one of three further AND gates 10 each having a negating input connected to the output of AND gate 8. The output of each AND gate 8,10 is supplied to a respective input of a conventional print head driving circuit for controlling the delivery of dots of a respective ink color in a printer. The timing relation among the various output signal trains will be controlled in a known manner dependent on the configuration of the particular,print head being employed.

When a binary 11111 appears simultaneously at the outputs of all three memories 6, an output pulse will be produced by gate 8 and all gates 10 will be inhibited, or disabled. The output signal from gate 8 is supplied to control the production of black ink pulses. When the output from any one memory 6 represents a binary 11011, then no output will be produced by- gate 8 and gates 10 will be enabled to conduct all C, M and Y signals being generated, these signals being supplied to control the printing performed with cyan,,magenta and yellow inks, respectively.

Figure 3 may be considered a pictorial representation of corresponding portions of each of memories 6. It will be appreciated that the physical arrangement of such a memory will not necessarily be in the form a rectangular matrix, but it is donventional to visualize a memory in this manner. For purposes of illustration, the upper left-hand corner of the matrix of memory locations of each memory is illustrated. The 4x4 matrix shown 6r each memory corresponds to one pixel of the scene, or source, image to be reproduced and to one superpel constituted by a rectangular matrix of 16 ink dot locations of the corresponding image representation to be produced by the printer.

The group of bit values illustrated in Figure 3 has been derived on the basis of the following example. For a selected pixel of the source image, or 1 1 9 scene, and based on a scale of zero to 100, the signals produced by source 2 have the following values: R=33, G=58 and B=63. These will yield the following subtractive primary color intensity values: C=67, M=42 and B=37. When each of the latter values is divided by 100, multiplied by 16 and rounded to the nearest integer, the resulting values are C=11, M=7 and Y=6. Based on these values, devices 4 will cause a binary 111" to be stored at 11 locations of the associated superpel of the cyan memory, at 7 locations of the associated superpel of the magenta memory and at 6 locations of the superpel of the yellow memory.

These memories may then be read in a pattern determined-by the sequence in which print control is signals must be supplied to the print head control system. Typically, each row of each memory can be read in turn, with corresponding memory bit locations being read simultaneously. Whenever the values simultaneously read out from the three memories all represent a binary 20 11191, a signal indicating the production of a black dot will be produced at the output of AND gate a of Figure 1. Otherwise, cyan, magenta or yellow dots will be produced at the outputs of respective ones of AND gates 10 when the respective memory locations contain a binary fill$.

In the specific example shown in Figure 3, a black dot will be produced in response to scanning of the first row of the illustrated superpel and the first two bit locations of the second row. Cyan and magenta dots will be superposed on the sheet being printed at a position corresponding to the third bit location in the second row of each memory and dots of cyan ink will be produced on the sheet being printed at positions corresponding to the fourth bit location in the second row and the first three bit locations in the third row of the memories. No yellow ink dots will be produced and the positions of the sheet being printed corresponding to the remaining memory locations of the superpel will not be printed.

The arrangement described herein corresponds to the under-color removal technique in which, at any given matrix point of the sheet being printed, no more than two primary colors will be printed and any point where all three primary colors would otherwise be printed is printed with black ink. Specifically, the technique disclosed herein corresponds to the establishment of relative values for the four colors equal to the following:

BLK min (C1,M1,Y1); C2 Cl - BLK; M2 M, - BLK; and Y2 Yl - BLK, where Cl, M, and Yl are the values calculated in circuits 4 and C2, M2 and Y2 are the values supplied to,the print head driving circuit.

The use of a scale of zero to 100 provides normalized values.which can be readily adapted to a variety of logic systems.

While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.

The presently disclosed embodiments are therefore to be considered in all respects as illustrative and rrot restrictive, the scope of the invention being indicated by the appended claims, rather than the foregging description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

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Claims

WHAT IS CLAIMED IS:

1. A method for geherating printer control signals for a dot matrix color printer capable of printing patterns composed of dots of black ink and first, second and third subtractive primary color inks at locations arranged in a rectangular matrix on a sheet, which patterns constitute visual reproductions of a source image composed of a rectangular matrix of picture elements, each picture element having a hue, saturation and intensity defined by the intensities of three primary color components, comprising: deriving three signals for each picture element of the source image, each signal having a value representative of the intensity of a respective subtractive primary color component corresponding to the intensity of a respective primary color component of the picture element of the source image; and generating, on the basis of the three signal values for each picture element of the source image, printer control signals for effecting printing of ink dots at a selected group of n adjacent locations, forming a portion of the rectangular matrix, on the sheet such that B Min (X,, Y1, Z1) X2 X1 - B1 Y2 Y1 - B, and Z2 Z1 - Bl 1 where X,, Y1 and ZI are integers each having a value between 0 and n and each at least approximitely proportional to the intensity of a respective one of the first, second and third subtractive primary colors, B is the number of the selected group of locations which each receives a black ink dot., and 1 12 X2f Y2 and Y 3'are, respectively, the number of the selected group'of locations which each receives a dot of a respective subtractive primary color ink, and wherein primary color ink dots are placed only at locations which do not receive a black ink dot.

2. A method as defined in claim 1 wherein said step of deriving comprises: providing three memory units each associated with a respective subtractive primary color and each having a plurality of groups of memory locations, with each group of binary memory locations consisting of n locations and being associated with a respective picture element of the source image, and with each memory location of one memory unit being associated with a respective memory location of each of the other memory units; and for each picture element of the source image, setting X,, Y1 and Z1, respectively, memory locations of the associated group of locations of each respective memory unit in a selected binary state.

3. A method as defined in claim 2 wherein said step of generating comprises: reading the binary memory locations of the three memory units in a manner such that the memory locations of each memory unit are read in sequence and associated memory locations of the three memory units are read simultaneously; and producing a first printer control signal for controlling the formation of black ink dots, the first control signal containing a pulse for producing a black ink dot in response to each simultaneous reading of three associated memory locations which are all in the selected binary state.

14

4. A method as defined in claim 3 wherein said step of generating further comprises: producing second, third and fourth printer control signals each for controlling the formation of dots of ink of a different respective one of the three subtractive primary colors, each of the second, third and fourth printer control signals containing a pulse for producing a dot of ink of the respective subtractive primary color in response to the reading of a memory location of the associated memory unit which is in the selected binary state and the simultaneous reading of at least one other memory location which is not in the selected binary state.

5. A method as defined in claim 4 wherein the primary color components of the source image are additive primary colors each having an intensity between zero and a maximum intensity, and said step of deriving comprises giving each signal a value corresponding to the difference between the maximum intensity and the actual intensity of a corresponding additive primary color.

6. A method as defined in claim 5 wherein the additive primary colors are red, green and blue and the subtractive primary colors are cyan, magenta and yellow.

7. Apparatus for generating printer control signals for a dot matrix color printer capable of printing patterns composed of dots of black ink and first, second and third subtractive primary color inks at locations arranged in a rectangular matrix on a sheet, which patterns constitute visual reproductions of a source image composed of a rectangular matrix of picture elements, each picture element having a hue, saturation and intensity defined by the intensities of three primary color components, comprising:

first circuit means connected for deriving three signals for each picture element of the source image, each signal having a value representative of the intensity of a respective subtractive primary color component corresponding to the intensity of a respective primary color component of the picture element of the source image; and second circuit means connected to receive the signals derived by said first circuit means for generating, on the basis of the three signal values for each picture element of the source image, printer control signals for effecting printing of ink dots at a selected group of n adjacent locations, forming a portion of the rectangular inatrix, on the sheet such that B Min (X,, Y1, Z1) X2 Xl - Bl Y2 Y1 - B, and Z2 Z1 - B1 where Xl, Y1 and ZI are integers each having a value between 0 and n and each at least approximately proportional to the intensity of a respective one of the first, second and third subtractive primary colors, B is the number of the selected group of locations which each receives a black ink dot, and 1 16 X2, Y2 and Y3 are, respectively, the number of the selected group of locations which each receives a dot of a respective subtractive primary color ink, and wherein primary color ink dots are placed only at locations which do not receive a black ink dot.

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8. Apparatus as-defined in claim 7 wherein said first circuit means comprises: three memory units each associated with a respective subtractive primary color and each having a plurality of groups of memory locations, with each group of binary memory locations consisting of n locations and being associated with a respective picture element of the source image, and with each memory location of one memory unit being associated with a respective memory location of each of the other memory units; and means for setting X,, Y1 and Z1, respectively, memory locations of a group of locations of each respective memory unit associated with each picture element of the source image in a selected binary state.

9. Apparatus as defined in claim 8 where said second circuit means comprises: means for reading the binary memory locations of the three memory units in a manner such that the memory locations of each memory unit are read in sequence and associated memory locations of the three memory units are read simultaneously; and means for producing a first printer control signal for controlling the formation of black ink dots, the first control signal containing a pulse for producing a black ink dot in response to each simultaneous reading of three associated memory locations which are all in the selected binary state.

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10. Apparatus as defined in claim 9 wherein said second circuit means further comprises: means for producing second, third and fourth printer control signals each for controlling the formation of dots of ink of a different respective one of the three subtractive primary colors, each of the second. third and fourth printer control signals containing.a pulse for producing a dot of ink of the. respective subtractive primary color in response to the reading of a memory location of the associated memory unit which is in the selected binary state and the simultaneous reading of at least one other memory location which is not in the selected binary state.

11. Apparatus as defined in claim 10 wherein the primary color components of the source image are additive primary colors each having an intensity between zero and a maximum intensity. and first circuit means comprises means for giving each signal a value corresponding to the difference between the maximum intensity and the actual intensity of a corresponding additive primary color.

12. Apparatus as defined in claim 11 wherein the additive primary colors are red, green and blue and the subtractive primary colors are cyan, magenta and yellow.

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