System and method for performing intensity-variable printing

Abstract: A system and method of printing an image on a substrate based on an input of detailed image intensity information for each of discrete pixel areas of the substrate, includes or performs steps of processing the detailed image intensity information into less detailed image intensity information; dithering the less detailed image intensity information to approximate the intensity resolution of the detailed image intensity information; and printing the dithered, less detailed, image intensity information onto the substrate. The disclosed system will use less memory space than a comparable system which prints directly from the detailed image intensity information, will minimize patterning effects, and will correct for artifacts which might otherwise be printed.

Description: This invention relates to printing methods and systems of the type in which discrete areas or pixels are varied in intensity to create an image on a substrate. More specifically, the invention relates to a system or method for intensity-variable printing which provides smoother image quality with less memory use than systems heretofore known. Intensity-variable printing is, for the purposes of this document, defined as any type of printing in which colorant may be deposited on a base unit area, or pixel, in more than one intensity or degree of dilution. Diffusion printing, which is defined as including both common dye-diffusion and sublimation dye printing techniques, is one type of intensity-variable printing. Other intensity-variable printing techniques may include continuous ink jet printing, variable spot-sized phase change ink jet printing, variable dot electrophotography, variable dot thermal transfer printing, laser-induced dye diffusion transfer processes, dry silver technology and photographic printing, as well as others. Dye diffusion printing involves the transfer of a dye colorant from a carrier, such as a polymer ribbon, onto a specialized substrate surface, such as a polyester sheet or a coated sheet of ordinary paper, in a controlled manner to generate an image. A thermal transfer print engine having an array of electrically actuatable heating elements is typically positioned so that the ribbon is juxtaposed between the print head elements and the substrate. The amount or intensity of dye deposited at a single location or pixel on the substrate can be varied by adjusting the electrical input to the print head elements. When one or more of the print head elements is heated, the dye/carrier structure is heated to a temperature sufficient to cause migration of a commensurate amount of dye to the substrate. By using a ribbon or ribbons having the three primary or subtractive colors, or those colors plus black, highly subtle colorant gradations may be achieved on the substrate. Images of near photo quality have been produced using dye diffusion technology. One disadvantage that is present in some such systems is the tendency to produce unwanted patterning and printer-created artefacts. For example, patterning might result if more than one of the possible inputted intensity-specifying values is assigned to a single power input level for the heating elements of the print head, as would be the case if a large range of intensity-specifying values were to be compressed over a smaller range of selected power input levels. US-A-3 681,650 discloses a method for raster production of half tone pictures wherein a picture is scanned photo-electrically dot by dot and voltage values are generated and quantified to represent brightness ranges. An alternating voltage is superimposed on the voltage values before they are quantified to generate a mixing of dot sizes indicative of one brightness range with dot sizes indicative of another adjacent brightness range and to cause the mixing to occur in such a pattern as to make transition between brightness ranges more pleasing to the eye. US-A-4 578,714 discloses a method of printing as detailed in the pre-characterising portion of the independent method claim in this application. In designing such systems, it is also desirable to keep printer memory requirements to a minimum. Memory adds to the cost of the printer, and printer speed and performance will tend to decrease as the degree of memory storage and manipulation escalates. It is clear there has existed a long and unfilled need in the prior art for an intensity-variable printing system and method which abates patterning and printer-created artefacts, which provides smoother shading of colors, and which uses less memory space than systems and methods heretofore known. Accordingly, it is an object of the invention to provide an intensity-variable printing system and method which will tend to ameliorate patterning and printer-created artefacts. It is further an object of the invention to provide an intensity-variable printing system and method, which provides a smoother shading of colors than systems presently in use. It is yet further an object of the invention to provide an intensity-variable printing system and method which uses less memory space than systems and methods of this type heretofore known. According to a first aspect of the present invention there is provided a method of printing on a one-pixel area of a substrate (12) based on detailed intensity information (Y detailed , M detailed , C detailed , K detailed ) for the one-pixel area (r,c), comprising: (a) processing the detailed image intensity information into a first approximated intensity value V left having a lower level of detail than that of the detailed intensity information; (b) determining according to a dithering process whether to add one bit to the first approximated intensity value to produce a second approximated intensity value (V left + 1); and (c) printing on the one-pixel area at the intensity (P(r,c)) which has been determined in step (b); characterised in that the dithering process of step (b) comprises (d) determining a differential value (V right ) which represents the difference between the first approximated value and the detailed image intensity information; (e) comparing the differential value with a predetermined value (d (rmod4, cmod4)) which has been set for the one-pixel area according to a dither cell (d) which has been conceptually superimposed over the substrate; and (f) determining whether to use the first approximated value or the second approximated value on the basis of such comparison. According to a second aspect of the present invention there is provided a system for printing an image on a substrate based on detailed intensity information (Y detailed , M detailed , C detailed , K detailed ) for each one-pixel area (r,c)of the substrate, comprising: (a) processing means for processing the detailed image intensity information into a first approximated intensity value (V left ) having a lower level of detail than that of the detailed intensity information; (b) dithering means for determining according to a dithering process whether to add one bit to the first approximated intensity value to produce a second approximated intensity value (V left + 1); and (c) printing means for printing on the one-pixel area at the intensity (P(r,c)) which has been determined in step (b); characterised in that the dithering means of step (b) comprises (d) means for determining a differential value (V right ) which represents the difference between the first approximated value and the detailed image intensity information; (e) comparing means for comparing the differential value with a predetermined value (d (rmod4, cmod4)) which has been set for the one-pixel area according to a dither cell (d) which has been conceptually superimposed over the substrate; and (f) determining means for determining whether to use the first approximated value or the second approximated value on the basis of such comparison. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:- FIGURE 1 is a schematic diagram depicting an intensity variable printing system constructed according to a preferred embodiment of the invention; FIGURE 2 is a flow chart depicting operation of the system depicted in Fig. 1; FIGURE 3 is a diagrammatical depiction of a dither cell according to the invention superimposed over a substrate upon which an image is to be printed; and FIGURE 4 is a flowchart depicting a portion of the process illustrated in Fig. 2, in a greater level of detail. Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to Figure 1, an intensity-variable printing system 10 according to a preferred embodiment of the invention is preferably a diffusion-type printing system. System 10, designed to print an image on a substrate 12, includes a printhead element 16, which is responsive to commands from a printer controller 18. Printhead 16, according to the preferred embodiment of the invention, includes a linear array of heating elements 20, each of which may be heated in response to a command from the printer controller 18 by electrical resistance. Each heating element is intended to be aligned with discrete unit areas or pixels on the substrate 12. A printer ribbon 24 is interposed between heating elements 20 and the substrate 12. Printer ribbon 24 includes a carrier layer, and a colorant layer containing a dye or colorant that is designed to migrate to the substrate upon application of heat from one or more of the heating elements 20. For dye-diffusion printing, the ribbon 24 contacts the substrate 12, and the colorant migrates into the substrate on a molecular level. For sublimation dye printing, there is no contact between the ribbon and the paper, and heat from the heating elements causes the colorant to sublimate, or phase from solid to gas, adjacent to the substrate, causing absorption in to the substrate 12. The surface of substrate 12 must be chemically compatible with the dye of colorant for printing to be successful, as is well known to those having ordinary skill in this technology. System 10 is preferably a color printing system, and ribbon 24 preferably includes different portions, each having colorant impregnated thereon, which correspond to the three primary or subtractive colors. Optionally, a black portion may also be included. More than one ribbon 24 may also be used to accomplish transfer of the different colorants to the substrate, if desired. A support roller 22 or equivalent surface is provided to support substrate 12 in parallel spaced relationship with respect to heating elements 20. By selectively controlling the heating elements 20 on printhead 16, printer controller 18 is constructed to control the placement of an image onto the substrate 12, as will be described in greater detail below. Preferably, printhead 16 is obtainable from Sharp Electronics Corporation as Model No. JX561. Preferably, ribbon 24 is the printer ribbon which is obtainable from Sharp Electronics under Model No. A4JX61R3T, which is a three color ribbon having a cyan component, a magenta component, and a yellow component. Alternative ribbon models may include Sharp Electronics Model No. A4JX61R4T, which is a four color ribbon having a cyan component, a magenta component, a yellow component, and a black component. A black ribbon, which may be used alone or in conjunction with the three or four color ribbons, is obtainable from Sharp Electronics under Model No. A4JX61RBT. Printhead element 16 is capable of accepting 256 electrical levels of input for the purpose of specifying how much heat will be applied to each of the heating elements 20. In effect, this allows print head element 16 to specify, for each pixel, 256 levels of intensity for each of the primary colors, giving the system 10 an almost unlimited spectrum of colors that may be printed. The 256 electrical levels of input are communicated to print head element 16 in an eight-bit per pixel binary format. Of course, an eight-bit binary format can express precisely 256 different values. The relationship between the amount of electrical power which is applied to a heating element 20 and the perceived intensity of the resulting colorant transfer to the substrate 12 is not a linear one, and it has been found that not all of the 256 possible electrical input levels to the heating element 20 will result in an amount of dye deposition which will be visible on the substrate 12. Furthermore, the electrical input levels that do result in a visible colorant deposition do not necessarily result in a linear increase in visible colorant concentration; the relationship is a nonlinear one that depends on a number of different factors and is best determined empirically. More specifically, the inventors have found that, out of the 256 possible electrical levels of input for the heating elements 20 of print head element 16, the lowest 80 levels will not produce a visible image, and the highest 10-15 levels produce complete saturation. To provide the desired relationship between the input value which specifies pixel intensity and the electrical signal to the corresponding heating element, a 256-level gamma table is conventionally used. A gamma table thus converts the pixel intensity values to the electrical levels that are required by the printer. Printer controller 18 is designed to accept image information input from a unit, such as a personal computer, which specifies the image intensity for each pixel on the substrate 12 in terms of an eight-bit binary value. To ensure that any non-zero manifestation of this value does in fact result in visible colorant deposition, the inventors first experimented with compressing the 256 values into the approximately (255-80) electrical levels which represented the visible colorant deposition range. This, unfortunately, resulted in two of the input values being assigned to several of the different electrical levels, which created artifacts such as a banding effect in images such as sweeps. To eliminate such problems, the preferred embodiment of the invention utilizes a 16 level (instead of a 256 level) gamma table which is linear in the spectrum of visible electrical input for each primary color and which fully spans the range of electrical inputs from no ink to full saturation. To create the effect of 256 distinct levels of intensity, the invention uses a process called dithering, which spatially interpolates between the 16 distinct gamma table values. One benefit of this process is that the pixel values of each of the primary colors can be represented in the form of four-bit values instead of the eight-bit values required by the normal rendering method. Referring now to Fig. 2, it will be seen that printer controller 18 is constructed and arranged to sequentially process arrays of four eight-bit input values of the rounding-off process Y detailed M detailed , C detailed , and K detailed , which represent, respectively, the specified intensity for each pixel area for the primary colors of yellow (Y), magenta (M), cyan (C), and black (K). The information represented by the eight-bit values is then processed into less detailed image intensity information, which is then dithered to approximate the intensity resolution of the detailed image intensity information. The dithering step, in effect, injects a degree of pseudo-randomness into how the eight-bit values are "rounded" off into the four bit approximated values, which eliminates patterning and other related created artifacts that might otherwise happen as a result of the rounding-off process. This dithered information, which is preferably in four-bit per pixel format, is then processed through an output transformation step, which adjusts for the non-linearity between electrical level input and print intensity output by assigning, through an output gamma table, predetermined electrical input values to the heating elements 20 for each four-bit dithered signal input. The pixel of information is then printed on substrate 12 at the intended intensity. The processing step is preferably performed by processing the detailed, eight-bit, image intensity information into a first approximated intensity value and a differential value which represents the difference between the first approximated value and the detailed image intensity information. The dithering step is preferably performed by comparing the differential value with a predetermined value which has been setup for the one pixel area; and determining whether to use the first approximated value or a second approximated value on the basis of such comparison. Referring to Fig. 4, which represents the process depicted in Fig. 2 in a greater level of detail, the processing of a two dimensional array V will be discussed. Two dimensional array V may represent intensity information for yellow, magenta, cyan or black, and is sized so as to store such intensity information for the pixels in rows 0 through R, and columns 0 through C. The information in array V is in eight-bit format representing any of 256 different intensity levels for the color component which is represented therein. Looking briefly to Fig. 3, it will be seen that a dithering cell d is logically replicated from the top left corner of the substrate 12 across the substrate and then down the substrate. This can be done without physically replicating the dithering cell by simply using modulus arithmetic to reference elements of the dithering cell while processing individual pixels on the substrate 12. Assuming that r specifies the current row of the page, that c specifies the current column, and that d(i,j) specifies an element of the dithering cell, then d(r mod 4, c mod 4) corresponds to the pixel at row r and column c of the page. As may be seen in Fig. 4, each value V(r,c) is first multiplied by a constant K to compress the possible range of values for the eight-bit signal into what can be approximated by a four-bit signal. More specifically, each eight-bit value V(r,c) is multiplied by a constant K which is equal to 240 divided by 256. After this step, the resulting eight-bit value can be no greater than the digital representation of 240, which is [11110000]. Preferably, dither cell d is arranged as a magic square, meaning that the sum of the values in diagonals, rows and columns will be the same. This minimizes patterning to the greatest extent possible. At this point, two four-bit values, v left and v right , are calculated. v left is set to equal the four leftmost bits of v(r,c) and v right is set to equal the four rightmost bits of v(r,c). At this point, printer controller 18 inquires as to whether v left is equal to 15, which in binary form is represented as [1111]. If v left does equal 15, then a four-bit value P(r,c) in a two dimensional array P is set to equal v left . If v left is not equal to 15, printer controller 18 then inquires whether v right is greater than d(r mod 4, c mod 4). If v right is greater, then four-bit value P(r,c) is set equal to v left plus one. If v right is not greater, then P(r,c) is set equal to v left . This completes the dithering step which is represented in Fig. 2. An example of the processing and dithering sequences may be expressed as follows: Example V(10,15) = 11010010 (V detailed ) K = 240/256 V(10,15) = (11010010) * (240/256) = 11000101 V left = 1100(≠ 1111) V right = 0101 d(10 mod 4, 15 mod 4) = d(2,3) = 9 = 1001 0101 is not > 1001 P(10,15) = V left = 1100 The intensity of P(10,15), then, will be 1100, or 12 in base 10 format. In the output transformation step, each of the four-bit values P(r,c) are replaced by an eight-bit output of print driver value which has been predetermined to correspond to the particular four-bit value contained in P(r,c). The following table illustrates the preferred values of print head element output which correspond to the respective 16 electrical input values. As will be seen in the table, the print drive or printhead element output values are expressed in eight-bit format, having possible values of 0-255. output Transformation Mapping A. Plain Paper Colorant Deposition Input Four-Bit Eight-Bit Output Yellow Magenta Cyan Black 0 46 46 43 42 1 78 78 68 81 2 87 87 76 93 3 97 97 87 104 4 107 107 100 115 5 116 116 111 124 6 124 124 123 134 7 133 133 132 145 8 142 142 141 157 9 153 153 152 169 10 165 165 164 182 11 179 179 178 194 12 193 193 192 206 13 210 210 210 221 14 230 230 230 236 15 255 255 255 255 A. Colorant Deposition on Overhead Transparency Input Four-Bit Eight-Bit Output Yellow Magenta Cyan Black 0 46 45 48 56 1 86 86 76 95 2 104 104 94 115 3 116 116 107 130 4 127 127 119 142 5 139 139 133 153 6 150 150 145 164 7 159 159 155 176 8 171 171 168 185 9 179 179 176 195 10 190 190 188 205 11 200 200 199 216 12 212 212 212 223 13 225 225 225 233 14 239 239 239 242 15 255 255 255 255 This corrects for the non-linearity between the electrical input to the heating elements 20 in printhead 16 and the perceptible intensity of the colorant that is transferred to the substrate 12.

Claims: A method of printing on a one-pixel area of a substrate (12) based on detailed intensity information (Y detailed , M detailed , C detailed , K detailed ) for the one-pixel area (r,c), comprising: (a) processing the detailed image intensity information into a first approximated intensity value V left having a lower level of detail than that of the detailed intensity information; (b) determining according to a dithering process whether to add one bit to the first approximated intensity value to produce a second approximated intensity value (V left + 1); and (c) printing on the one-pixel area at the intensity (P(r,c)) which has been determined in step (b); characterised in that the dithering process of step (b) comprises (d) determining a differential value (V right ) which represents the difference between the first approximated value and the detailed image intensity information; (e) comparing the differential value with a predetermined value (d (rmod4, cmod4)) which has been set for the one-pixel area according to a dither cell (d) which has been conceptually superimposed over the substrate; and (f) determining whether to use the first approximated value or the second approximated value on the basis of such comparison. A method as claimed in Claim 1 wherein the detailed image intensity information comprises an eight-bit value (V detailed ) representing 256-level image intensity information for the one-pixel area. A method as claimed in claim 2 wherein the first approximated intensity value consists of the leftmost four bits (V left ) of the eight-bit value, and the differential consists of the rightmost four bits (V right ) of the eight-bit value. A method as claimed in any preceding claim wherein the predetermined value in step (e) is part of a magic-square dither cell. A method according to any preceding claim, wherein step (e) is accomplished by using a 4-by-4 dither cell. A method as claimed in any preceding claim wherein step (f) comprises selecting the second approximated value if the differential value is greater than the predetermined value, and selecting the first approximated value if the differential value is not greater than the predetermined value. A method as claimed in any preceding claim and including multiplying the detailed intensity information by a constant (K) prior to step (a). A method as claimed in any preceding claim wherein the printing step is effected using a diffusion type printer (16). A method as claimed in any preceding claim wherein sufficient repetition of steps thereof is practised to accomplish printing of at least three primary colour components (Y,M,C) on the substrate. A method as claimed in Claim 2 wherein the less detailed image intensity information comprises four-bit values representing 16-level image intensity information for each pixel area on the substrate. A method as claimed in Claim 2 wherein step (a) comprises disregarding at least one of the rightmost bits in the eight-bit values. A method as claimed in Claim 11 wherein step (a) comprises disregarding the four rightmost bits in the eight-bit values. A method as claimed in any preceding claim including the step of transforming the value determined in step (f) into a format which has the same specificity of detail as the detailed image intensity information. A method as claimed in Claim 13 wherein the detailed image intensity information and the printer acceptable format both comprise eight-bit values representing 256-level image intensity information for each pixel area on the substrate. A method as claimed in Claims 13 and 14 wherein the step of transforming corrects for any non-linearity between the value determined in step (f) and the perceptible intensity of the colorant that is transferred to the substrate with a given hardware configuration. A method as claimed in Claim 15 wherein the transforming step includes considering whether the substrate will be paper or overhead transparency material prior to making said transformation. A system for printing an image on a substrate based on detailed intensity information (Y detailed , M detailed , C detailed , K detailed ) for each one-pixel area (r,c)of the substrate, comprising: (a) processing means for processing the detailed image intensity information into a first approximated intensity value (V left ) having a lower level of detail than that of the detailed intensity information; (b) dithering means for determining according to a dithering process whether to add one bit to the first approximated intensity value to produce a second approximated intensity value (V left + 1); and (c) printing means for printing on the one-pixel area at the intensity (P(r,c)) which has been determined in step (b); characterised in that the dithering means of step (b) comprises (d) means for determining a differential value (V right ) which represents the difference between the first approximated value and the detailed image intensity information; (e) comparing means for comparing the differential value with a predetermined value (d (rmod4, cmod4)) which has been set for the one-pixel area according to a dither cell (d) which has been conceptually superimposed over the substrate; and (f) determining means for determining whether to use the first approximated value or the second approximated value on the basis of such comparison.

Inventors: CLARK JEFFERY J, CROSBY BRIAN G, LEWENDAL BO, SHAVER CHRISTOPHER D, THORNTON GREGORY P, WELBORN PATRICK E, CLARK, JEFFERY J., CROSBY, BRIAN G., LEWENDAL, BO, SHAVER, CHRISTOPHER D., THORNTON, GREGORY P., WELBORN, PATRICK E.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1993
Main classification: H04N1
IPC codes: B41J2, H04N1, B41M5