AU704935B2

AU704935B2 - Halftone printing area patterns

Info

Publication number: AU704935B2
Application number: AU62145/96A
Authority: AU
Inventors: William Frederick Ashworth
Original assignee: Megadot Systems Ltd
Current assignee: Megadot Systems Ltd
Priority date: 1991-08-13
Filing date: 1996-08-19
Publication date: 1999-05-06
Anticipated expiration: 2012-08-13
Also published as: AU6214596A

Description

Q:\OPER\GCP\62145.SPE- 16/3/99 -2- HALFTONE PRINTING AREA PATTERNS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to printing processes and in particular to printing area patterns used when preparing halftone images. These patterns include shapes which reduce the perceptibility of certain moire effects and various tone jumps which are often seen.

DESCRIPTION OF THE PRIOR ART In most printing processes it is only possible to apply a single tone of each *Oo.

se: 15 available ink colour to the print media. Tone variation is then achieved by breaking up each image into fine dots of varying size on a halftone screen grid. Colour variation is normally achieved by superimposing screens of the primary colours cyan, magenta 00 and yellow, plus black for definition. Ideally human vision integrates the dots over a well prepared image into an accurate impression of the original scene. A final image will almost always include a number of compromises between practical limitations and defects in the printing process, and what can actually be perceived by the human eye and brain.

Round dots at 450 to vertical are least perceptible for a given spacing or screen ruling (or dot frequency), and single colour images are conventionally printed in this manner. Other dot shapes or combinations of shapes such as square and "elliptical" ::00 (diamond shape) are sometimes used, but all have generally straight or outwardly curved edges around the full length of their perimeters. The dots are -3- 15 20 usually created in square cells forming rows spaced at between about 30/cm for newsprint and about 60/cm for higher quality images. In light tones the dots remain distinct on a light background provided by the print medium, but merge in darker tones which then appear as light dots on a dark background. The printed and non-printed areas of an image therefore appear to reverse from dots to background and background to dots respectively as tone darkens.

When printing colour images undesirable moir6 effects in the form of large and small scale patterns are often seen due to periodic alignment of the dots as a whole and of their edges. The large scale patterns are typically bands which intersect to form squares on the order of tens or more dots along each side. This effect is largely removed by suitable relative rotation of the colour screens such as by cyan 150, magenta 450, yellow 900 and black 75' anticlockwise from horizontal. Complex mathematical procedures are often used to establish suitable angles. Placing the screens without relative rotation or offset can cause colour shifts in an image where colours of differing opacity, particularly black and yellow, overlap consistently over a large region. Subtle colour shifts may occur in any case due to mis-registration of the screens during their superposition. The small scale patterns of moir6 effects are typically rosettes on the order of a few dots width, which cause perceptible speckling of otherwise uniformly coloured areas.

This effect has proved more difficult to remove.

A further problem often arises in printing halftone images, known as dot gain, amid tones where adjacent dots are so sufficiently large as to become linked by imprecision in their reproduction. For example in lithography, which includes offset printing, a greasy ink is confined to printing areas of an image plate by dampening the surrounding non-printing areas with water. Unfortunately surface tension at the ink/water interfaces can cause or enhance bridging between closely

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spaced printing areas creating sudden tone jumps. Ink absorption on poor quality print paper can also lead to bridging. Round and square dots formed in square cells naturally meet their nearest neighbours at 78% and 50% printing area densities respectively. Dot gain causes bridging at slightly lower densities and enhances bridging at slightly higher densities creating discontinuities in regions of an intended smoothly varying tone. This effect is also difficult to remove completely.

Preparation of halftone images is largely carried out using computer controlled devices such as scanners and imagesetters. A photograph or other artwork to be reproduced is scanned and the original scene is stored in electronic memory or output directly. The images can be manipulated and/or combined with text before a printing medium such as a film or plate is produced. It is normally only in the final output stages that an image is converted to halftone dot screens.

The manipulations are complex software operations which may be varied to suit particular images. Similar software is used in other electronic printing and imagesetting processes such as desktop publishing. Precise control of the dot patterns is necessary in preparing acceptable images, and the computations required for high quality images are often extensive and time consuming. For example, software methods for reducing moire effects are disclosed in US 4,084,183, (congruent screens), US 4,894,726 (quasi periodic screens), EP 370271 (elongated conventional dots), WO 90/10991 (rectilinear screen transposition) and WO 90/06034 (pseudo random variation of dot shapes). Some images are still prepared using conventional photomechanical equipment such as contact screens. Good summaries of known dot patterns, their various problems and moire effects are to be found in Colour Screening Technology; A Tutorial on the Basic Issues, The Seybold Report on Desktop Publishing, Vol 6, No. 2,

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5*55 S S Q:\OPER\GCP\62145.SPE- 16/3/99 October 1991, Seybold Publications Inc., PA, USA, and Desktop To Press, Issue 9, February 1992, Peter Fink Communications Inc., CA, USA.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an alternative printing area pattern which can be used to reduce rosette moire, dot gain and tone jump effects in halftone images.

According to the present invention there is provided a method of preparing halftone screens prior to printing a colored image, comprising: receiving information representing color and tone variation in a previously recorded image; processing the information to create halftone screens representing S primary colors in the recorded image, with each screen having a pattern of printing 15 areas, by: creating cyan and magenta screens to have respective patterns S S of printing areas with inwardly curved edges, and (ii) creating each primary color screen to have the respective pattern of printing areas at an angle of approximately 450 900 or 1350 to the pattern of any other primary color screen; and 55: producing halftone screens representing the primary colors in a form S appropriate for subsequent printing of the colored image.

The invention also provides a method of printing halftone screens to form a colored image, comprising: receiving information representing primary color halftone screens, wherein cyan and magenta screens have patterns of printing areas with inwardly curved edges; orienting each primary color screen to place the respective pattern of printing areas at an angle which is approximately 450, 900 or 1350 to the pattern of any other primary color screen; and superimposing and printing the oriented primary color screens to form the colored image.

Q: \OPER\GCP\62145,SPE 16/3/99 -6- The invention also provides a computer controlled apparatus for preparing halftone screens representing a colored image, comprising: means for receiving information representing color and tone variation in a previously recorded image; means for processing the information to create halftone screens representing primary colors in the recorded image, each screen having a pattern of printing areas; and means for producing halftone screens representing primary colors in a form appropriate for subsequent printing of the colored image; wherein the means for processing creates cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and each primary color screen at an angle which places the respective pattern at approximately 450, 900 or 1350 to the pattern of any other primary color screen.

The invention also provides a computer controlled apparatus for preparing 15 halftone screens representing a colored image, comprising: means for scanning a previously recorded image to produce information o representing color and tone variation in the recorded image; means for processing the information to create halftone screens representing the primary colors and black tone variation in the image; and means for producing the halftone screens in or on an output medium; wherein the means for processing includes means for creating cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and creates each primary color screen to place the respective pattern at an 00 angle which is approximately a multiple of 450 to the pattern any other primary color screen.

In light tones the printing areas may resemble pin cushions having pronounced oo cusps. In dark tones the printing areas may be effectively merged to create non- Sprinting areas which may be oval shaped.

As tone varies from light to dark, the pin cushions may be increasingly elongated along one direction and meet their nearest neighbours in two distinct stages, S. first in the direction of elongation and then in a direction substantially perpendicular to it. The printing area screen is typically formed from a square cell so there are typically Q:\OPER\GCP\62145.SPE- 16/3/99 -6Afour nearest neighbours symmetrically placed at equal distances. As tone continues to darken after the printing areas having joined, the elongation is gradually decreased so that the oval shaped non-printing dots approach round printing areas. The pattern may also be considered overall as a reversed elliptical printing area pattern in which the ellipses vary from round to a maximum ellipticity and back to round across the full range of tones. When a number of screens are superimposed the inwardly curved edges of the printing areas do not align so readily to form perceptible rosettes as do conventional dots. Further, the elongation may be varied so that printing area gain occurs in tones where its perceptibility also is minimised.

At the present time as this specification is prepared, the optimum uses and ramifications of the invention have not been fully explored. In preparing colored images it has been found that the primary color and black screens are best placed so that their printing area patterns lie at approximately 450 separations from each other.

S- For example cyan 450 magenta 1350, yellow 900 and black 00. Screens which are 0o-: 15 separated by 900 may generally be regarded alternatively as being at 00 so that in this o example there are essentially only two angles for computation of printing area patterns, 00 and 450 This represents a considerable computational simplification over S conventional screen angles.

A method and apparatus relating to the printing area elongation has been claimed in AU 668,868 from which the present specification has been divided. A method and apparatus relating to 450 screen separation is claimed in the present 9. 5 specification.

The cyan and magenta screens will normally have a deliberate offset or misregistration from each other to avoid possible color shifts. Yellow and black screens may also be offset from each other, and may be printed with little or no elongation of the printing areas. The yellow and/or black screen rulings may also be increased 5e*9 S and/or decreased relative to cyan and magenta. Final determination of these S possibilities awaits full software implementation of the invention and will depend on particular images.

In the description which follows, the terms dot or dots are used but it is to be understood that this is intended to include printed areas where adjacent dots merge 1 with one another.

Q:\OPER\GCP\62145.SPE 16/3/99 6B BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention will be described with reference to the drawings of which: Figures la, lb and 1c show coarse conventional round, square and elliptical dot patterns respectively, varying uniformly between light and dark tones; 9 00 0

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9 0@ *9 99 4 49 *o 9 -7- .o e 15 Figure 2 is an illuminated globe image using a conventional variable round dot pattern; Figure 3 shows reversal of a coarse conventional round dot pattern to produce printing areas having inwardly curved edges; Figures 4a and 4b show reversal of a variable oval dot pattern in which the dots vary from round to oval and back to round; Figures 5a, 5b and 5c show example variable oval dot outlines superimposed upon common centres; Figures 6a and 6b show respectively two conventional round dot screens overlapping at 30' and two reversed dot screens according to the invention overlapping at 900; Figures 7a to 7i show example dot patterns for various screen combinations according to the invention; Figures 8a to 8i show corresponding conventional round dot patterns for contrast with figures 7a to 7i; Figure 9 is a flowchart indicating a general process in which the invention may be implemented; and Figure 10 is a schematic diagram of hardware on which the invention may be implemented.

a DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to these figures it will be seen that the dot patterns are magnified about 10 to 100 times in black and white for purposes of their description. The visual effects resulting from integration by the human eye over a halftone image at normal scales are not evident but should be appreciated by a skilled reader.

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-8- Particularly the improvements to be obtained in colour images by inwards curvature of printing dot edges in light to middle tones and to be obtained in single color images by dot elongation will be appreciated.

The various forms of computer hardware and software used to implement dot patterns according to the invention will also be known to the skilled reader, or at least will be available for consideration through the references given above. For example, a range of desktop publishing and high end scanner equipment and software is available through suppliers such as Adobe, Agfa, Crosfield, Linotype- Hell and Scitex. As new patterns are developed the hardware and/or software may be correspondingly upgraded. A typical process of preparing a halftone image, particularly a colored image as implemented on their equipment, is outlined in the flowchart of figure 9. Typical hardware is indicated in Figure In general terms, a scene is electronically scanned from a film or other artwork to be printed, or recorded in some other digital process, and the data obtained is stored as pixel based color and intensity information. The pixels are generally aligned with the vertical and horizontal directions of movement of the scanner. The data is then processed into a standardised format such as known under the trade mark POSTSCRIPT and from there into up to four halftone screens which represent the primary colors and black as required. These screens are created from the pixel information by various raster image processor programs which calculate the dot shapes, dot frequencies and screen rotation angles. An operator normally has a range of dot patterns available through software installed on the equipment. The operator selects appropriate shapes, frequencies and angles in reaching an acceptable image during proofing. In traditional printing operations each screen is then output individually, to create up to four films or plates which are used to print multiple copies of the final black and white or 95 .20 .9 20 -9colored image. This part of the process is known as imagesetting in the case of desktop publishing and as scanner output in the case of "high end" systems. In other computer based operations, such as ink jet of laser printing, it is possible to output a colored image directly.

Figures la, lb and ic show conventional round, square and "elliptical" dot patterns respectively, varying smoothly in size from about 10% print area density in light tones to about 90% area density in dark tones. Round dots have outwardly curved edges by definition whereas square or diamond shaped dots have flat edges between four points. The square patterns are invertible in that the printed areas in light tones have similar shapes to the non-printed areas in dark tones and vice versa.

Figure 2 demonstrates a more complicated dot pattern in which conventional round dots in light tones become square dots in middle tones with reversal of the round dots in dark tones. This is often referred to as the Euclidean dot pattern. The edge curvature of printed areas correspondingly changes from outward to flat to inward respectively. Moir6 effects do not arise in Figures 1 and 2 as only a single screen is present in each case. Dot gain is not apparent due to the magnification.

Figure 3 shows a reversed round dot pattern varying uniformly from light to dark tones. The dots in light tones have inwardly curved edges between cusps distributed on two planes of mirror symmetry. Their points could perhaps be flattened or rounded as desired. Each dot meets its four nearest neighbours simultaneously at around 22% area density. Otherwise the inwards curvature extends along substantially the full length of all printed areas. The non-printed areas have corresponding outwardly curved edges and separate to appear as round dots in the dark tones. Overall the pattern is seen to be effectively a

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15 20 *4 r oi reversal or negative of the conventional round dot pattern in Figure la throughout the entire range of tones, and in this respect may be considered a pattern of light round dots on a dark printed background. By virtue of this reversal all print area edges are circles or circular arcs but could be other smooth curves as desired, or as required to link with the end points. Also the dots need not be fourfold symmetrical as shown. Rosette moir6 effects in coloured images can be reduced by printing dot patterns having inwardly curved edges as will be evident from later figures in which two screens are superimposed. Tone jumps due to dot gain remain a problem however, as with the conventional round dot pattern.

Figures 4a and 4b show a reversed variable oval dot pattern according to the invention. In this form the generally pin cushion shaped dots are elongated along a plane of mirror symmetry in light quarter to middle tones, so that nearest neighbours meet first vertically along the direction of tone darkening and then second horizontally in the corresponding perpendicular direction. Each dot meets two opposing pairs in two distinct stages of around 22% and 36% area density.

This pattern is seen to be effectively a reversal or negative of a round dot pattern in which the dots become oval in middle tones. Figure 4a shows a full range of tone from 0% to 100% while Figure 4b shows more detail of the fourfold/twofold or alternatively the round/oval variation from about 15% to 39% area density.

Rosette moir6 effects can be reduced as with the pattern of Figure 3, but now by controlling dot elongation the perceptibility of any tone jumps can also be reduced by splitting dot gain into two less obvious stages which in turn may be shifted among a range of tones.

Figures 5a, 5b and 5c show example variations of dot edges according to the invention superimposed on common centres. The diagonally lined squares facilitate comparison and measurement in production. The edges are not 15 2 -11 necessarily mathematical ellipses but may take any suitable oval or approximately similar form as can be generated by computer. Figure 5a shows the gentle roundoval-round variation of Figures 4a and 4b. Figures 5b and 5c show more severe distortions resulting in greater separation of the dot gain stages. These Figures are contour patterns which have been used in manual preparation of threshold matrices as described on pages 15 and 16 of the Seybold Report mentioned above. The process is also described in the appendix to this specification.

Figures 6a and 6b are an illustration of the reduction in rosette moire effect which can be achieved according to the present invention. Figure 6a shows rosettes created by superimposing two conventional round dot screens at 300, as is common with cyan and magenta in coloured images. Figure 6b shows a smoother variation of similar tones created by superimposing two reversed variable oval dot screens at 900 according to the invention. Preparing colored images using all four possible screens is less straightforward.

It has been found that the primary color screens are apparently best placed at 450 angular separations, such as cyan 450 magenta 135' and yellow 90', with black placed at 0O or 900 Further, that not every screen need be printed according to the invention. For example, at least cyan and magenta should use elongated dots of the reversed variable oval dot pattern, while yellow may use only the reversed round dot pattern. Black may only need the reversed round dot pattern, or may even give satisfactory images using the conventional round dot pattern itself.

It has also been found that black dots are often not necessary in preparing a well defined colored image. For example, the primary color screens may be used alone at the angles and witn the patterns mentioned above.

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a *aaa a a 20 -12- It is also generally required that the cyan and magenta screens should be offset from each other to avoid possible color shifts. The offset should be about half a cell parallel with either of the screen directions, for best results in view of the accidental offsets which often occur during printing. Yellow and black screens should also sometimes be offset to avoid color shifts. The nature of these offsets has yet to be fully explored but will be evident to a skilled reader working on a particular image.

In some images the possibility of color shifts and moire effects has been reduced with the 450 angles and reversed patterns mentioned above, by decreasing the black screen ruling (increasing the dot frequency) by a factor of about cosine 450 (about 0.71) relative to cyan and magenta. In a smaller number of cases the yellow screen ruling has been correspondingly increased by this factor. Again the nature of these adjustments to the spacings has yet to be fully explored, but will be evident to a skilled reader working on particular images.

Figures 7a to 7i show the dot patterns of a test which has been carried out to date according to the invention. Figures 7a to 7d represent individual screens of cyan 1350, magenta 450 yellow 0 and black 0/900, all at tone values where the printing and non-printing areas have linked with two or four of their nearest neighbours. Cyan and magenta use the reversed variable oval dot pattern. Yellow and black use simply the reversed round dot pattern. The black screen ruling has been reduced by a factor of about 0.71 relative to the others. Figures 7e to 7g show screens superimposed in pairs, namely cyan/yellow, magenta/yellow and cyan/magenta. In figures 7f and 7g the cyan and magenta have been offset by half a cell parallel to their screen directions, which are diagonal on the page as shown. The visible effect is somewhat unrealistic in that yellow normally has substantially less impact on the eye than the darker colors but here all colors must -13be shown equally in black. Figure 7h shows the cyan, magenta and yellow screens superimposed. A very slight moire effect is apparent primarily because the colors must be shown in black as mentioned. Figure 7i shows the black screen of figure 7d superimposed on figure 7h. Again there is a slight moire effect due to the overall black color representation whereas under normal printing circumstances the colors would have different impacts and the final image screen would be virtually moire free.

Figures 8a to 8i show an attempt to contrast with figures 7a to 7i, the conventional round dot patterns which would typically have been used. Figures 8a to 8d represent individual screens of cyan 150, magenta 45 yellow 00 and black 750 Figures 8e to 8g show these screens superimposed in pairs, namely cyan/yellow, magenta/yellow and cyan/magenta. Figure 8h shows the cyan, magenta and yellow screens superimposed. Figure 8i shows the black screen of i figure 8d superimposed on figure 8h. Both large and small scale moire effects can 15 be seen in these figures, although the effects are not so apparent as they normally would be in a complete colored image at normal screen rulings. The effects of dot gain are not at all apparent due to the artificial manner in which these figures must be presented.

Figure 10 is a highly schematic diagram of high end computer controlled apparatus which might be used by a human operator in implementing the process of Figure 9. The operator interfaces with the apparatus through a keyboard or 4*eeeC other appropriate hardware and software 140. A recorded scene such as a photograph is passed under a scanner 110 to produce pixel data. Data is stored in memory 130 typically a hard disc. A computer processor 100 is then used to select and modify dot patterns in the pixel data. Halftone screens in primary 14 colours or black are output through a printer/image setter 120. The screens are combined in preparation of a high quality halftone image.

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APPENDIX

This is a simple explanation of the process by which half-toning may take place on a computer, or computerised equipment such as an imagesetter. The equipment is known technology, with the present invention relating to variations of dot shape and screen angles implemented on the equipment.

The fundamental operation is the application of a half toning pattern to a continuous tone original image to produce a binary (black or white) image in such a way that the visual effect approximates the continuous tone original image. An identical process is carried out for each primary col our or black screen. Thi s process revolves around the threshold matrix: a grid of numbers or grey levels that specifies the size and shape of each printed dot desired for various levels of grey in the image to be printed.

The table below is one possible threshold matrix to implement a dot shape according to the invention on a 16x16 grid as would be used, for instance, for a 150 Ipi screen on a printer operating at 2400 dots per inch.

253 248 234 197 156 113 103 99 97 1021 112 158 196 233 246~ 252 249 239 207 170 132 104 85 80 82 86 106 133 169 206 2i37 245 242 223 185 145 117 90 7B 71 70 76 91 11 6 1744 184 220 243 228 201 173, 142 109 73 62 57 56 63 74 108 141 172 203 231 2251 192 165 138 931 53 511 43 341 45 52 941 137 1671 195 2271 217 189 161 131 67 38 29 19 18 241 37 64 130 160 190 218 212 181 155 124 48 25 13 8 7 14 26 49 126 152 183 213 209 176 151 120 32 17 5 2 3 4 16 35 122 148 177 210 211, 178 1T50 -123 361 20 9 01 1 !11 211 39 121 149 179, 208 214 180 153 125 461 301 15 6 101 12 27 50 1271 154 1821 215 216 188 162 128 651 33 28 23 22 31 41 66 129 163 191 219 224 193 166 139 921 55 47 40 42 44 54 95 136 164 194 226 230, 200 174 140 1101 75 60 i58 59 61 72 111 143 175 202 229 241 221 186 146 118 88, 771 681 691 79 89 i119 1T47 187 222 240 247 236 205 168 134 105 87 811 831 84 107 135 171 24 38244 254 2501 235 198 157 1151 1011 981 961 100 114 159 199 232 251 255 16- The interpretation of this table is that, starting from a completely black pattern, the first pixels to be whitened are in a roughly circular pattern in the middle. By the time 100 pixels have been whitened, the white area extends fully to the top and bottom of the pattern, but only halfway to the left and right sides (note the sudden jump from values in the mid-30's to more than 120 as you proceed left or right from the middle in the middle rows).

The process of creating a threshold matrix such as the one above consists of two steps: deciding what shape dots you want, and how the shape should change at various dot sizes. This process will affect the overall look of the printed image, and the exact shape selected will determine various properties of the printed image, such as tone-jump and for multi-colour printing, moire i: 15 patterns.

too This is the subject addressed by the present invention.

t o o creating the threshold array in a standard fashion. This can be performed 20 manually, or by a mathematical algorithm.

The manual process consists of overlaying a grid with "contour lines" such as those shown in Figures 5a, 5b, 5c which determine the desired dot shapes at various grey levels 20%, 30%, etc) and writing down the numbers 0, 1, 2, 3 in the cells enclosed by each contour line in turn, until the entire table is filled in.

The mathematical process which can be used for sufficiently simple shapes consists of developing a mathematical function (called a spot function) that takes x and y coordinates within the halftone cell and returns a value indicating the "height" of that point. The heights so generated are then sorted by the computer and assigned the numbers 0, 1, 2, 3 as above.

Example mathematical functions include f(x,y) x for a vertical line pattern, f(x,y) y for a horizontal line pattern, and f(x,y) (X 2 for a conventional circular dot screen.

A manual process was used in developing the present invention through the prototype stages described in the provisional and complete specifications.

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S Once you have your threshold matrix, the actual algorithm used to apply it is simply to consider each cell in a particular halftone screen in turn and compare the number written there against the desired grey level (also expressed as a number from 0 to 255, with 0 being black and 255 being white) in the image to be printed.

If the number from the table is larger than the desired grey level then that pixel is printed as white, otherwise it is printed as black.

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5555 This process can be carried out in any of a number of ways: S i photographically, by arranging for light from the desired image to shine through a transparent sheet imprinted in the above pattern, and thence 18 onto high-contrast film that will quite suddenly change from transparent to opaque at some fixed level of exposure to light.

by a program on a computer workstation, with the resulting black and white result stored on a hard disk, or displayed on a TV monitor, or sent as a bitmap to a simple dot matrix, ink jet, laser, or other printer.

by a program built into a sophisticated computerised printer. Many printers have several halftone screen patterns built into their software. In order to be useful for the various dot shapes, it must be possible for the user to specify the pattern to be used for half toning.

The most common high-end output devices in the pre-press industry today are based on a computer programming language called PostScript, and we have 15 concentrated our efforts on these patterns, though the inventions described in the specification apply equally well to other printing and display devices.

In printers that use Level 2 PostScript, it is possible for the user to directly specify the threshold matrix to be used to the printer, as in this example of one of the 20 prototype shapes (this is the same table as previously shown): dict begin /HalftoneType 3 /Width 16 def /Height 16 def /Thresholds 256 def string def [253 249 242 228 225 217 212 209 211 214 216 224 230 241 247 254 248 239 223 201 192 189 181 176 178 180 188 193 200 221 236 250 234 207 185 173 165 161 155 151 150 153 162 166 174 186 205 235 197 170 145 142 138 131 124 120 123 125 128 139 140 146 168 198 156 132 117 109 93 67 48 32 36 46 65 92 110 118 134 157 113 104 90 73 53 38 25 17 20 30 33 55 75 88 105 115 103 85 78 62 51 29 13 5 9 15 28 47 60 77 87 101 102 86 76 63 45 24 14 4 11 12 31 44 61 79 84 100 112 106 91 74 52 37 26 16 21 27 41 54 72 89 107 114 158 133 116 108 94 64 49 35 39 50 66 95 111 119 135 159 196 169 144 141 137 130 126 122 121 127 129 136 143 147 171 199 233 206 184 172 167 160 152 148 149 154 163 164 175 187 204 232 246 237 220 203 195 190 183 177 179 182 191 194 202 222 238 251 252 245 243 231 227 218 213 210 208 215 219 226 229 240 244 255] sq 6 .00 6

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6* 0 1 255 {2 copy get Thresholds 3 end sethalftone -1 roll put} for If the desired dot shape is simple enough to be reduced to a mathematical function, it is also possible to specify this directly to the PostScript printer. For instance, the standard circular dot f(x,y) (x 2 y 2 can be created as follows: 150 0 {dup mul exch dup mul add sqr 0e 4 ''2 06 S This will create a 150 Ipi screen at an angle of zero degrees.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

6* 0 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of preparing halftone screens prior to printing a colored image, comprising: receiving information representing color and tone variation in a previously recorded image; processing the information to create halftone screens representing primary colors in the recorded image, with each screen having a pattern of printing areas, by: creating cyan and magenta screens to have respective patterns of printing areas with inwardly curved edges, and (ii) creating each primary color screen to have the respective pattern of printing areas at an angle of approximately 450, 90 or 1350 to the pattern of any other primary color screen; and 15 producing halftone screens representing the primary colors in a form appropriate for subsequent printing of the colored image.

2. A method according to claim 1 further comprising: creating the cyan and magenta screens having printing areas in 20 light tones formed as dots which are increasingly elongated with darkening tones.

3. A method according to claim 1 further comprising: creating the cyan and magenta screens having non printing areas in dark tones formed as dots which are increasingly elongated with lightening tones.

Claims

4. A method according to claim 2 or 3 further comprising: joining the dots in two distinct stages as tone darkens or lightens respectively.
5. A method according to claim 1 further comprising: creating cyan and magenta screens which are relatively offset.
6. A method according to claim 1 further comprising: creating cyan and magenta screens having printing areas which in light tones are inwardly curved on every edge.
7. A method according to claim 1 further comprising: creating every primary color screen having printing areas with inwardly curved edges.
8. A method according to claim 1 further comprising: creating a black screen having a pattern of printing areas with inwardly curved edges. *0
9. A method according to claim 1 further comprising: S°creating the cyan and magenta screens to have respective patterns •go* at approximately 900 to each other.
10. A method according to claim 8 further comprising: creating yellow and black screens to have respective patterns at approximately 90" to each other. I -22-
11. A method according to claim 8 further comprising: creating the black screen to have a pattern at an angle which is approximately a multiple of 450 to any other screen.
12. A method according to claim 11 further comprising: creating the black screen with a screen ruling which is reduced by a factor of approximately 0.71 relative to the cyan and magenta screens.
13. A method of printing halftone screens to form a colored image, comprising: receiving information representing primary color halftone screens, wherein cyan and magenta screens have patterns of printing areas with inwardly curved edges; orienting each primary color screen to place the respective 15 pattern of printing areas at an angle which is approximately 45', 900 or 1350 to the pattern of any other primary color screen; and S.i superimposing and printing the oriented primary color screens to form the colored image.
14. A method according to claim 13 further comprising: receiving information representing a black halftone screen; orienting the black halftone screen to place a respective pattern of printing areas at an angle which is approximately a multiple of 450 to the pattern S•of any other screen; and printing the oriented black color screen with the primary color screens to form the colored image. -23- A computer controlled apparatus for preparing halftone screens representing a colored image, comprising: means for receiving information representing color and tone variation in a previously recorded image; means for processing the information to create halftone screens representing primary colors in the recorded image, each screen having a pattern of printing areas; and means for producing halftone screens representing primary colors in a form appropriate for subsequent printing of the colored image; wherein the means for processing creates cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and each primary color screen at an angle which places the respective pattern at approximately 450 900 or 1350 to the pattern of any other primary color screen. 15 16. Apparatus according to claim 15 wherein the means for processing includes means for producing tone variation in the primary color halftone screens by creating printing areas which vary in elongation between light and dark tones.
17. Apparatus according to claim 15 wherein the means for processing 20 includes means for producing tone variation in the primary color halftone screens •by creating non-printing areas which vary in elongation between dark and light tones.
18. Apparatus according to claim 16 or 17 wherein the means for processing includes means for joining the printing or non-printing areas in two distinct stages as tone darkness or lightens respectively. -24-
19. A computer controlled apparatus for preparing halftone screens representing a colored image, comprising: means for scanning a previously recorded image to produce information representing color and tone variation in the recorded image; means for processing the information to create halftone screens representing the primary colors and black tone variation in the image; and means for producing the halftone screens in or on an output medium; wherein the means for processing includes means for creating cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and creates each primary color screen to place the respective pattern at an angle which is approximately a multiple of 450 to the pattern any other primary color screen.
20. A colored halftone image wherein tone variation in at least cyan 9 and magenta is produced by patterns of printed areas having inwardly curved 9 edges, and in which the primary color patterns are oriented at angles of approximately 450, 900 or 1350 to one another. 99 99 6 9999
21. A process of preparing a halftone image wherein tone variation in cyan and magenta screens is produced by creating patterns of printing areas which have inwardly curved edges, and wherein the pattern of each primary color S: screen is placed at an angle of approximately 450 900 or 1350 to any other primary color screen. primary color screen. Q:\OPER\GCP\62145.SPE- 16/3/99
22. A method according to any one of claims 1 to 14 substantially as herein described with respect to the accompanying drawings.
23. Apparatus according to any one of claims 15 to 19 and substantially as here described with respect to the accompanying drawings.
24. A coloured halftone image made in accordance with the method as claimed in any one of claims 1 to 14 or by the apparatus as claimed in any one of claims 15 to 19. a *9S C. *e DATED this 16th day of March, 1999 MEGADOT SYSTEMS LIMITED By its Patent Attorneys DAVIES COLLISON CAVE C S. C S C a a ABSTRACT A method of preparing halftone screens prior to printing a colored image, comprising: receiving information representing color and tone variation in a previously recorded image; processing the information to create halftone screens representing primary colors in the recorded image, with each screen having a pattern of printing areas, by: creating cyan and magenta screens to have respective patterns of printing areas with inwardly curved edges, and (ii) creating each primary color screen to have the respective pattern of *:printing areas at an angel of approximately 450, 900 or 1350 to the pattern of any other 20 primary color screen; and producing halftone screens representing the primary colors in a form •appropriate for subsequent printing of the colored image. oo o