AU2006346021B2 - Method and apparatus for image manipulation via a dither matrix - Google Patents

Description

WO 2008/006131 PCT/AU2006/000975 1 METHOD AND APPARATUS FOR IMAGE MANIPULATION VIA A DITHER MATRIX 5 FIELD OF THE INVENTION The present invention relates to digital image processing. In particular, the invention relates to enhancing digital colour images while processing them for printing to a media substrate. 10 CROSS REFERENCES TO RELATED APPLICATIONS Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/ Patent Applications filed by the applicant or assignee of the present invention: 09/517539 6566858 6331946 6246970 6442525 09/517384 09/505951 6374354 09/517608 6816968 6757832 6334190 6745331 09/517541 10/203559 10/203560 10/203564 10/636263 10/636283 10/866608 10/902889 10/902833 10/940653 10/942858 10/727181 10/727162 10/727163 10/727245 10/727204 10/727233 10/727280 10/727157 10/727178 10/727210 10/727257 10/727238 10/727251 10/727159 10/727180 10/727179 10/727192 10/727274 10/727164 10/727161 10/727198 10/727158 10/754536 10/754938 10/727227 10/727160 10/934720 11/212702 11/272491 10/296522 6795215 10/296535 09/575109 6805419 6859289 6977751 6398332 6394573 6622923 6747760 6921144 10/884881 10/943941 10/949294 11/039866 11/123011 6986560 7008033 11/148237 11/248435 11/248426 10/922846 10/922845 10/854521 10/854522 10/854488 10/854487 10/854503 10/854504 10/854509 10/854510 10/854496 10/854497 10/854495 10/854498 10/854511 10/854512 10/854525 10/854526 10/854516 10/854508 10/854507 10/854515 10/854506 10/854505 10/854493 10/854494 10/854489 10/854490 10/854492 10/854491 10/854528 10/854523 10/854527 10/854524 10/854520 10/854514 10/854519 10/854513 10/854499 10/854501 10/854500 10/854502 10/854518 10/854517 10/934628 11/212823 10/728804 10/728952 10/728806 6991322 10/728790 WO 2008/006131 PCT/AU2006/000975 2 10/728884 10/728970 10/728784 10/728783 10/728925 6962402 10/728803 10/728780 10/728779 10/773189 10/773204 10/773198 10/773199 6830318 10/773201 10/773191 10/773183 10/773195 10/773196 10/773186 10/773200 10/773185 10/773192 10/773197 10/773203 10/773187 10/773202 10/773188 10/773194 10/773193 10/773184 11/008118 11/060751 11/060805 11/188017 11/298773 11/298774 11/329157 6623101 6406129 6505916 6457809 6550895 6457812 10/296434 6428133 6746105 10/407212 10/407207 10/683064 10/683041 6750901 6476863 6788336 11/097308 11/097309 11/097335 11/097299 11/097310 11/097213 11/210687 11/097212 11/212637 11/246687 11/246718 11/246685 11/246686 11/246703 11/246691 11/246711 11/246690 11/246712 11/246717 11/246709 11/246700 11/246701 11/246702 11/246668 11/246697 11/246698 11/246699 11/246675 11/246674 11/246667 11/246684 11/246672 11/246673 11/246683 11/246682 10/760272 10/760273 10/760187 10/760182 10/760188 10/760218 10/760217 10/760216 10/760233 10/760246 10/760212 10/760243 10/760201 10/760185 10/760253 10/760255 10/760209 10/760208 10/760194 10/760238 10/760234 10/760235 10/760183 10/760189 10/760262 10/760232 10/760231 10/760200 10/760190 10/760191 10/760227 10/760207 10/760181 10/815625 10/815624 10/815628 10/913375 10/913373 10/913374 10/913372 10/913377 10/913378 10/913380 10/913379 10/913376 10/913381 10/986402 11/172816 11/172815 11/172814 11/003786 11/003616 11/003418 11/003334 11/003600 11/003404 11/003419 11/003700 11/003601 11/003618 11/003615 11/003337 11/003698 11/003420 6984017 11/003699 11/071473 11/003463 11/003701 11/003683 11/003614 11/003702 11/003684 11/003619 11/003617 11/293800 11/293802 11/293801 11/293808 11/293809 11/246676 11/246677 11/246678 11/246679 11/246680 11/246681 11/246714 11/246713 11/246689 11/246671 11/246670 11/246669 11/246704 11/246710 11/246688 11/246716 11/246715 11/246707 11/246706 11/246705 11/246708 11/246693 11/246692 11/246696 11/246695 11/246694 11/293832 11/293838 11/293825 11/293841 11/293799 11/293796 11/293797 11/293798 10/760254 10/760210 10/760202 10/760197 10/760198 10/760249 10/760263 10/760196 10/760247 10/760223 10/760264 10/760244 10/760245 10/760222 10/760248 10/760236 10/760192 10/760203 10/760204 10/760205 10/760206 10/760267 10/760270 10/760259 10/760271 10/760275 10/760274 10/760268 WO 2008/006131 PCT/AU2006/000975 3 10/760184 10/760195 10/760186 10/760261 10/760258 11/293804 11/293840 11/293803 11/293833 11/293834 11/293835 11/293836 11/293837 11/293792 11/293794 11/293839 11/293826 11/293829 11/293830 11/293827 11/293828 11/293795 11/293823 11/293824 11/293831 11/293815 11/293819 11/293818 11/293817 11/293816 11/014764 11/014763 11/014748 11/014747 11/014761 11/014760 11/014757 11/014714 11/014713 11/014762 11/014724 11/014723 11/014756 11/014736 11/014759 11/014758 11/014725 11/014739 11/014738 11/014737 11/014726 11/014745 11/014712 11/014715 11/014751 11/014735 11/014734 11/014719 11/014750 11/014749 11/014746 11/014769 11/014729 11/014743 11/014733 11/014754 11/014755 11/014765 11/014766 11/014740 11/014720 11/014753 11/014752 11/014744 11/014741 11/014768 11/014767 11/014718 11/014717 11/014716 11/014732 11/014742 11/097268 11/097185 11/097184 11/293820 11/293813 11/293822 11/293812 11/293821 11/293814 11/293793 11/293842 11/293811 11/293807 11/293806 11/293805 11/293810 09/575197 09/575195 09/575159 09/575123 6825945 09/575165 6813039 6987506 09/575131 6980318 6816274 09/575139 09/575186 6681045 6728000 09/575145 09/575192 09/575181 09/575193 09/575183 6789194 6789191 6644642 6502614 6622999 6669385 6549935 09/575187 6727996 6591884 6439706 6760119 09/575198 6290349 6428155 6785016 09/575174 09/575163 6737591 09/575154 09/575129 6830196 6832717 6957768 09/575162 09/575172 09/575170 09/575171 09/575161 The disclosures of these applications and patents are incorporated herein by reference. BACKGROUND OF THE INVENTION 5 A market has developed for inkjet printers that can dock directly with digital cameras. These printers are expected to download and print the photographs quickly and with a minimum of user input. A significant part of the delay between docking the camera and printing the photographs, is the processing time needed to prepare the image data for the printhead. 10 WO 2008/006131 PCT/AU2006/000975 4 The major processing tasks are JPEG decompression, colour space conversion, image rotation and halftoning to convert the colour values into dots. These tasks are briefly explained below. 5 Colour can be specified using three independent variables. The variables are essentially coordinates in a colour space. The same colour can be specified in different colour spaces using different variables. Each colour space has a particular use or application. RGB (red, green, blue) is the natural colour space for devices that emit light such as television screens or computer monitors. CMY (cyan, magenta, yellow) is the natural colour space for 10 devices that display images with reflected light such as printed images from a printer. YCRCB (luminance, chrominance red, chrominance blue) separates the luminance from the chrominance (usually abbreviated to 'chroma') channels for more convenient data compression. Humans are more sensitive to luminance than chroma so any change in chrominance resulting from compression and subsequent decompression will be less 15 noticeable than an equivalent change in luminance. This means both chroma channels can be heavily compressed as long as the luminance is lightly compressed. With two of the three channels heavily compressed, YCC image data is more efficiently handled by the processor. 20 Digital cameras capture images natively in RGB. For efficient storage, the images are converted to YCC and compressed. Image data downloaded from a camera is typically in sYCC which is a widely recognized standard form of YCC. This must then be colour space converted when it is output to a screen or printer.

WO 2008/006131 PCT/AU2006/000975 5 If the image is downloaded to a printer, the data is converted to the pritner's colour space and the separate colour channels are halftoned with a dither matrix. Halftoning exploits the eye's perception of a spatial average of printed dots to reproduce contone (continuous tone) images. Inkjet printers can either print a dot at any one of the addressable locations 5 on the media, or not. However, dots dispersed over area of white (say) paper, will appear to eye as a contone shade somewhere between white and the dot colour, depending on the number of dots. The dither matrix covers a small area of the image at a time. The matrix has a range of 10 threshold values dispersed throughout its sites. The contone colour levels for each pixel are compared to the spatially corresponding threshold values within the matrix. If the contone level exceeds the threshold value, a dot of that colour is printed (or equivalently, a dot is printed if the contone level is greater than or equal to the threshold, or less than, or less than or equal to the threshold value). This will produce many micro-differences 15 between the contone and halftone image, but the eye is largely insensitive to these high frequency differences. To produce a colour image, the separate halftoned images for each of the three colour channels are superimposed by the printer on the media. Printers typically have cyan, 20 magenta, yellow and sometimes black (to conserve the other inks and provide a 'truer' black). This is abbreviated to CMYK (Cyan, Magenta, Yellow and blacK). If the print resolution, or dots per inch (dpi) is high enough, halfioning can reproduce any colour in the printer's gamut (palette of printable colours). Accordingly, the individual dots in CMY(K) space are colour averaged by the eye to reproduce the colours of the original image. 25 WO 2008/006131 PCT/AU2006/000975 6 Downloaded images can be manipulated and enhanced on a computer prior to printing. However, if downloaded directly from a camera to a photo printer, the user does not have an opportunity to manually enhance and view the images prior to printing. Even so, it is possible to incorporate some relatively basic image enhancement into the printer that can 5 be automatically and uniformly applied to the images prior to printing. One common and relatively basic image enhancement technique is histogram expansion. It improves the colour contrast by expanding the range of colours present in the raw image data so that it is more evenly spread across the entire range of available colours. To do 10 this, it is necessary to collect image statistics and build histograms for each colour channel. This involves the collection of the three colour levels for each pixel and recording the number of pixels that fall into a range of discrete colour level intervals to build the histograms. Usually the original image will have histograms with at least one sparsely populated region. By re-assigning all the pixels in the sparsely populated region into one 15 of the colour levels, the rest of the histogram can expand into the vacated region. Spreading the pixels more evenly across the histogram improves the colour contrast. As the number of pixels in the sparsely populated regions is insignificant, reassigning them to a single colour level has little, if any, detrimental effect on the image. So in the vast majority of cases, the net effect of histogram expansion is an enhancement of the image. 20 Unfortunately, the processing tasks associated with histogram expansion can delay printing. In some printing applications, there is an expectation that the printer will start printing the downloaded images almost instantly. Photograph printers that dock directly with the camera are one such example. The captured images will typically download from 25 the camera upon docking and automatically print the images to 6 inch by 4 inch photo WO 2008/006131 PCT/AU2006/000975 7 grade paper. Users prefer, if not expect, to see their photos being printed within a few seconds. More importantly, users expect good quality prints, but, as discussed above, computationally intensive image enhancement is counter to quickly initiating the printing of downloaded photos. 5 SUMMARY OF THE INVENTION According to a first aspect, the present invention provides a method of manipulating contone image data to be halftoned with a dither matrix, the method comprising: determining at least one characteristic of the contone image data; 10 using the at least one characteristic to derive a secondary dither matrix from a predetermined primary dither matrix; and, halftoning the contone image data with the secondary dither matrix. Manipulating the dither matrix can be equivalent to performing image enhancement 15 manipulations to the input image data. However, it far less computationally intensive to manipulate the dither matrix instead of all the raw image data. For example, if the dither matrix is (say) 64 x 64, with each element being an 8 bit value, it has roughly 4 kilobytes of data. By comparison, a digital photograph (6 inches by 4 inches) at 3 mega-pixel native resolution, is about 10 megabytes of data. So, in this example, the number of 20 manipulations to the dither matrix is about three orders of magnitude less than that needed for the image data to achieve the same net effect. As discussed above, histogram expansions are a very common image enhancement technique and simply compressing the range of threshold values in the dither matrix as an inverse to the desired expansion of the histogram provides an equivalent result with far less processing. Furthermore, as the data 25 in the dither matrix is a tiny fraction of the input data, the dither matrix can be given added WO 2008/006131 PCT/AU2006/000975 8 complexity, or granularity, in order to achieve a better result than a normal histogram expansion, while still providing greater computation efficiency. According to a second aspect, the present invention provides a print engine controller for 5 an inkjet printer, the print engine controller comprising: a processor for receiving contone image data; memory storing a predetermined primary dither matrix; wherein, the processor is configured to determine at least one characteristic of the contone image data and derive a secondary dither matrix from the primary dither matrix using the 10 at least one characteristic of the contone image data; such that, the contone image data is halftoned with the secondary dither matrix prior to printing. Preferably, the contone image data has colour level values for pixels in the image, the 15 colour level values having a certain distribution within a predetermined range of discrete colour levels, and, the at least one characteristic of the contone image data relates to said certain distribution. Preferably, the primary dither matrix has a range of threshold values and the secondary 20 dither matrix has a compressed range of threshold values for comparison to the colour level values of the contone image data during halftoning. In a further preferred form, the at least one characteristic related to the certain distribution is the minimum number of contiguous discrete colour levels containing a predetermined portion of the pixels, divided by the total number of levels in the predetermined range of WO 2008/006131 PCT/AU2006/000975 9 discrete colour levels. In some embodiments, the predetermined portion of the pixels is greater than 90%. Optionally, the at least one characteristic related to the certain distribution is: 5 (Lmax - Lmin)/Ltotai Where: Lmax is the maximum number of discrete colour levels if a top-most portion of the colour level values of the contone image data is disregarded; Lm. is the minimum number of discrete colour levels that contain a bottom-most 10 number of the colour level values of the contone image data; and, Ltotai is the total number of levels in the range of discrete colour levels. In these embodiments, the top-most portion may be the highest 5% of the colour level values of the contone image data. Likewise, the bottom-most portion may be the lowest 15 5% of the colour level values of the contone image data. In a less aggressive enhancement, the top and bottom-most portions may be 1%. In preferred embodiments, the threshold values in the compressed range in the secondary matrix are determined in accordance with the following algorithm: 20 Tnew= Lin + Tol.(ILna - Lmin)/Ltotal where: Tne, is the compressed threshold values in the secondary dither matrix; and, Told is the threshold value in the primary dither matrix.

WO 2008/006131 PCT/AU2006/000975 10 Optionally, at least some of the thresholds in the primary dither matrix are not whole numbers and the compressed threshold values in the secondary matrix are rounded or truncated to the nearest whole numbers. In other options, the threshold values occur in the primary dither matrix a predetermined number of times and the compressed threshold 5 values occur in the secondary dither matrix a greater number of times, the greater number being approximately equal to Ltota/(LmarLmin) multiplied by the predetermined number associated with the threshold values of the primary matrix, or only one of two colliding threshold values from the primary matrix, that correspond to the compressed threshold. 10 In some preferred embodiments the processor only samples a portion of the pixels of the contone image data to determine Lmi and Lmax. Optionally, the colour levels values are an eight bit binary number such that there are 256 (28) levels in the range of discrete colour levels. Optionally, the dither matrix is 64 x 64 15 and the threshold levels range from 1 to 255 prior to compression. BRIEF DESCRIPTION OF THE DRAWINGS 20 The invention will now be described, by way of example only, with reference to the preferred embodiments shown in the accompanying drawings in which: Figure 1 shows a print engine pipeline in accordance with the present invention; Figure 2 shows a dither matrix partially completed with threshold values; 25 Figure 3 shows a histogram of the image data for one of the colour channels; WO 2008/006131 PCT/AU2006/000975 11 Figure 4 shows the histogram of Figure 3 expanded to enhance colour contrast; Figure 5 shows the dither matrix with compressed threshold values calculated to two decimal places; and, Figure 6 shows the dither matrix with compressed threshold values rounded to the nearest 5 whole number. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As discussed in the Background to the Invention, photograph printers have recently been 10 developed that dock directly with a digital camera and automatically printed the captured images. They are expected to print the images quickly and with photographic quality. Furthermore, these printers will only offer rudimentary image enhancement options, if any at all. Users that want more complex image enhancement of their photos will download the images to desktops or laptops and manipulate them with PhotoShopTM or similar 15 software (note that PhotoShop is a trademark of Adobe Systems Inc). As the present invention offers basic image enhancement with computational efficiency, it is well suited to photo printers. In light of this, it will be described with specific reference to this application. However, skilled workers will readily appreciate that the invention is 20 not limited to photo printers and is suitable for a wide range of applications. Figure 1 shows a print engine pipeline for the image data from the camera 1 to the printhead 13. When the camera docks with the printer, the images are downloaded to the print engine controller (PEC) 2 as EXIF (exchangeable image file data) JPEG (joint 25 photographic expert group) files in sYCC colour space (or standard YCC colour space).

WO 2008/006131 PCT/AU2006/000975 12 The PEC 2 decompresses the images with a contone decoder unit (CDU) 3. If the image is too large 4, it is downsampled 5 as it is decoded. As the pixel data from each JPEG MCU (minimum coding unit) becomes available, it is 5 rotated and converted to the specific CMY colour space 7 of the printhead 13. Once the data is in the CMY space, the PEC 2 can collect image statistics and build a histogram 8. The collection of image statistics involves building a histogram of the number of occurrences of each color level. Once the histograms for the image are known, 10 the degree of histogram expansion can be determined. This can be done in any number of ways and one particular method will be discussed below with reference to Figures 3 and 4. Expanding the histogram requires new maximum and minimum colour levels to be determined 9. That is, a minimum level, Lmi, is determined and mapped to 0. All levels 15 between 0 and Lmin are also mapped to 0. Likewise, Lma is determined and mapped to the highest colour level value. For example, if the colour levels are 8 bit numbers, the highest is 255. Every level between Lm and 255 is also mapped to 255. However, as the present invention manipulates the dither matrix to enhance the image 20 rather than the colour levels in the image data, the only characteristic of the histogram that the PEC 2 needs is the degree of histogram expansion that would be caused by the Li and Lm.a, determined by the chosen method. There is no need to map any of the colour levels in the image data to new levels.

WO 2008/006131 PCT/AU2006/000975 13 The histogram is expanded by a factor of 255/(LmaxLmin). Hence, the corresponding compression of the range of threshold values in the dither matrix is given by: Te,= Lmin + Told.(Lmax - Lmin)/255 EQ(1) Where: 5 Tne, is the compressed threshold; and, Told is the original threshold value. If the dither matrix size is 64 x 64, then compressing the threshold values involves the manipulation of about 4kB of data, whereas the equivalent expansion of the input colour 10 levels is a manipulation of about 10MB of data, or possibly more depending on image resolution. Compressing the dither matrix is more computationally efficient than expanding the image data by several orders of magnitude. This can dramatically reduce any delay between docking the camera and printing the downloading images, and it also allows more complex image enhance techniques via the dither matrix while still remaining 15 far less computationally intensive than performing equivalent techniques on the input data. Once the compressed dither matrix values have been calculated 10, the CMY colour values determined at stage 7 of the print engine pipeline, are compared directly with the compressed threshold values of the dither matrix to produce a halftone image of each 20 colour channel 11. It will be appreciated that the same matrix can be used for each channel, or separate dither matrices derived from respective histograms could be used for each colour channel. The halfioned images are sent to the printhead 13 for printing 12 to complete the pipeline. 25 WO 2008/006131 PCT/AU2006/000975 14 Figure 2 shows an example of a 64 by 64 dither matrix. For simplicity, it is only partially completed with the range of threshold values. If the threshold values are 8 bit (corresponding to 8 bit colour levels), there are 255 thresholds. All 255 threshold levels appear in the matrix a number of times and the number of times a particular threshold 5 occurs depends on the characteristics of the printhead and the aim to achieve a perceptually uniform colour space. As discussed above, the dither matrix is repeatedly tiled across the contone image and the separate colour levels for pixel are compared to a corresponding threshold in the dither 10 matrix. If the colour level exceeds the threshold, the printhead will eject a drop (of that particular colour) at that location, and if it is less than the threshold, no drop is ejected. Because the eye spatially averages the colour, it does not see the high frequency differences between the contone image and the halftoned image. 15 Referring now to Figure 4, the histogram for input image data is shown. Lmn and Lmax can be derived in any number of ways. For example, many images will not have a pixel at level 0, or a pixel at level 256. In this case, Lmin and Lmax can simply be the highest and lowest colour levels sampled. However, this does not take into account the tendency for the highest and lowest sampled colour levels to be outliers from the histogram distribution. 20 Hence, using outliers as Lmi and Lmax usually means the histogram is not expanded as much as it should. A better approach is to select a portion of the pixels at either end of the histogram and set the highest of the low end portion to be Lmin, and the lowest of the high end portion to be 25 Lmax. Some experimentation may be required to optimize for each print engine pipeline, WO 2008/006131 PCT/AU2006/000975 15 but taking the top and bottom 1/256 (or approximately 0.4%) of the histogram will usually account for any outliers. In other words, 0.4% of the samples are below Lm and 0.4% of the samples are above Lm.. This approach is likely to provide visually better results than simply taking the extreme-levels however, it is slightly more computationally intensive. 5 There is also a risk of over expansion by taking a percentage from the top and bottom of the histogram. Too much expansion can introduce visible contouring in areas of with a ramped colour gradient (because of the large colour differences between adjacent pixels after expansion). To guard against this, the processor might impose a maximum allowable expansion. 10 Once Lm and Lm. have been determined, any samples at Lm or below are mapped to 0, and any sample at Lmax or more are mapped to 256. As shown in Figure 5, the remainder of the histogram is then expanded between 0 and 255. There are spikes at 0 and 256 because they now contain all the original Lmin and Lma samples, as well as the 15 outliers. However, this is not likely to have any detrimental effect on image quality. Traditionally, histrogram expansion has been done with the level mapping function given by: L new = 256.(Loid - Lin)/(Lmax-Lmin) EQ(2) 20 This function would be applied to the input levels to determine the expanded levels, the expanded levels then compared to the threshold values of the dither matrix. Therefore, it is equivalent and computationally easier to compress the range of threshold values by the inverse of the expansion that would otherwise be performed on the histogram. The algorithm for mapping each of the original threshold values to new threshold values is 25 given by equation 1 discussed above.

WO 2008/006131 PCT/AU2006/000975 16 Figure 5 shows the dither matrix of Figure 2 compressed in accordance with equation 1. Unfortunately, the compressed threshold values must be rounded or truncated to whole numbers as the hardware requires the dithering process to be a whole number comparison. 5 Hence the threshold values of Figure 2 are rounded to give the whole number thresholds shown in Figure 6. Manipulating the input levels via the dither matrix also presents an opportunity to improve the image enhancement beyond that offered by histogram expansion. 10 During histogram expansion, mapping the old levels to new levels via equation 2 involves rounding as the new levels need to be integers (because of the printer hardware). Hence, some of the levels in the expanded histogram have no samples in them. As shown in Figure 4, these appear as gaps in the histogram. The colour difference between samples on 15 either side of a gap is greater than the difference between the same samples in the unexpanded histogram. These increased colour differences are more likely to produce visible contouring in the printed image. Likewise, rounding (or truncating) the compressed threshold values in the dither matrix 20 causes some of the thresholds to collide. For example, the shaded thresholds in the compressed matrix of Figure 6 are pairs of equal value whereas, the corresponding threshold in the original matrix (see Figure 2) were not equal. Consequently, the number of dots added when moving from one tone level to the next will not be smooth. Again, this increases the risk of visible contouring in the printed image. 25 WO 2008/006131 PCT/AU2006/000975 17 Increasing the granularity of the dither matrix (that is, using threshold values greater than 8 bits), would avoid collisions and smooth out the number of dots added to the halftone image with each subsequent tone level. Unfortunately, in most printers, the comparison of the dither matrix with the contone CMY levels is a hardware function that requires the 5 threshold values to be 8 bit integers only. Alternatively, the software can reconstruct the original dither matrix at a higher granularity for a perceptually smooth transition between tone levels. This will effectively add sub levels to each threshold so any collisions will between 2 sub-levels and consequently 10 involve far fewer dots. The problem of threshold collision and the solution of higher granularity are illustrated in the tables below. Firstly, Table 1 shows the number of times some threshold values appear in the original matrix and the compressed matrix. The 64 by 64 matrix has 4096 elements 15 so each of the 1 to 255 original threshold values will occur 4096/255, or about 16 times in the original matrix. When the matrix is compressed, rounding causes some of the original thresholds to map to the same compressed threshold. These colliding thresholds appear 32 times in the compressed matrix (e.g. compressed threshold 38 in Figure 6). So the number of dots added when moving from tone level 37 to 38, will be twice that of moving 38 to 39, 20 or 36 to 37. Hence the increased risk of visually perceptible contouring. Original Threshold Occurrences in Compressed Occurrences in Original Matrix Threshold Compressed Matrix (Rounded) 34 16 46 16 WO 2008/006131 PCT/AU2006/000975 18 35 16 36 16 47 32 37 16 48 16 38 16 49 16 39 16 40 16 50 32 Table 1: Inconsistent Occurrences of Compressed Thresholds because of Threshold Collision 5 For the purposes of this example, we will assume the thresholds in the compressed matrix should occur 21 times each (4096/(Lmax-Lmin) = 21.005 - so one threshold would occur 22 times) for least risk of visible contours. By adding granularity to the original matrix, the occurrences of the compressed thresholds can be more uniform. For example, if the original threshold values are 12 bit (or rather 8.2 bit) the original matrix essentially gains 10 extra threshold levels. Table 2 sets out the finer grained original thresholds and the smoothing effect this has on the compressed matrix. Original Threshold Occurrences in Compressed Occurrences in Original Matrix Threshold Compressed Matrix (Rounded) 35.00 4 47 20 35.25 4 47 35.50 4 47 35.75 4 47 WO 2008/006131 PCT/AU2006/000975 19 36.00 4 47 36.25 4 48 20 36.50 4 48 36.75 4 48 37.00 4 48 37.25 4 48 37.50 4 49 20 37.75 4 49 38.00 4 49 38.25 4 49 38.50 4 49 38.75 4 50 24 39.00 4 50 39.25 4 50 39.50 4 50 39.75 4 50 40.00 4 50 With the increased granularity, there are smaller inconsistencies in the number of times the compressed thresholds appear in the final matrix. Most compressed thresholds occur 20 times with about a quarter appearing 24 times. Therefore, moving from one tone level to 5 the next is smoother and there is far less likelihood of any visible contouring. Instead of increasing the granularity of the original matrix, it may be simpler to multiply the number of times each threshold appears in the original matrix by the inverse of the WO 2008/006131 PCT/AU2006/000975 20 compression to determine the number of times the compressed threshold appears in the final matrix. Referring again to the above example, the inverse of the compression is 256/(Lmax - Li) = 1.313. If an original threshold occurred 16 times in the matrix, the compressed threshold might ideally occur 1.313x16 = 21 (with one threshold occurring 22 5 times). Of course, if two original thresholds are colliding at a single compressed threshold, the occurrences of only one of the original thresholds should be used to determine how many times the compressed threshold occurs. Enhancing the image by manipulating the dither matrix means that gathering image 10 statistics can be reasonably heavily optimized because the impact of granularity issues on the final matrix (and therefore the printed image) is relatively minor. For example, in some applications, the histogram need not have 256 levels. 64 levels (6 bit) may be adequate. When building the histogram, it may not be necessary to collect statistics for every pixel. Sampling as little as 1 in 256 pixels can be within acceptable output errors. 15 When calculating Lmax and Li, portions of the image that can no longer have any impact on the end points of the histogram, may be ignored. Likewise, reasonable results may be obtained by computing or looking up a minimum and maximum value from each colour point, rather than each of the individual CMY values. All these optimizations serve to reduce the processing burden on the PEC, and so shorten the time between docking the 20 camera and printing the images. The invention has been described here by way of example only. Skilled workers in this field will readily recognize many variations and modifications that do not depart from the spirit and scope of the broad inventive concept. 25

Claims (16)

1. A method of manipulating contone image data to be halftoned with a dither matrix, the contone image data having colour level values for pixels in an image, the colour level values having a certain distribution within a predetermined range of discrete colour levels, 5 the method comprising: determining a ratio of the minimum number of contiguous discrete colour levels containing a predetermined portion of the pixels, to the total number of levels in the predetermined range of discrete colour levels; using the ratio to derive a secondary dither matrix from a predetermined primary 10 dither matrix; and, halftoning the contone image data with the secondary dither matrix.

2. A method according to claim 1 wherein the primary dither matrix has a range of 15 threshold values and the secondary dither matrix has a compressed range of threshold values for comparison to the colour level values of the contone image data during halftoning. 20

3. A method according to claim 1 wherein the predetermined portion of the pixels is greater than 90%.

4. A method according to claim 1 wherein the at least one characteristic related to the certain distribution is: 25 (Lmax - Lmin)/Ltotai KIPOO1 22 where: Lmax is the maximum number of discrete colour levels if a top-most portion of the colour level values of the contone image data is disregarded; Lmin is the minimum number of discrete colour levels that contain a bottom-most 5 number of the colour level values of the contone image data; and, Ltotai is the total number of levels in the range of discrete colour levels.

5. A method according to claim 4 wherein the top-most portion are the highest 5% of the colour level values of the contone image data and the bottom-most portion may be the 10 lowest 5% of the colour level values of the contone image data.

6. A method according to claim 5 wherein the top and bottom-most portions are 1% of the colour level values of the contone image data. 15

7. A method according to claim 6 wherein the threshold values in the compressed range in the secondary matrix are determined in accordance with the following algorithm: Tcw = Lmin + Told.(Lmax - Lmin)/Ltotal where: Tnc, is the compressed threshold values in the secondary dither matrix; and, 20 Told is the threshold value in the primary dither matrix.

8. A method according to claim 7 wherein at least some of the thresholds in the primary dither matrix are not whole numbers and the compressed threshold values in the secondary matrix are rounded or truncated to the nearest whole numbers. In other options, 25 the threshold values occur in the primary dither matrix a predetermined number of times KIPOO1 23 and the compressed threshold values occur in the secondary dither matrix a greater number of times, the greater number being approximately equal to Ltotal/(Lmax-Lmin) multiplied by the predetermined number associated with the threshold values of the primary matrix, or only one of two colliding threshold values from the primary matrix, that correspond to the 5 compressed threshold.

9. A print engine controller for an inkjet printer, the print engine controller comprising: a processor for receiving contone image data having colour level values for pixels 10 in an image, the colour level values having a certain distribution within a predetermined range of discrete colour levels; memory storing a predetermined primary dither matrix; wherein, the processor is configured to determine a ratio of the minimum number of contiguous discrete colour levels containing a predetermined portion of the pixels, to the 15 total number of levels in the predetermined range of discrete colour levels, and derive a secondary dither matrix from the primary dither matrix using the ratio; such that, the contone image data is halftoned with the secondary dither matrix prior to printing. 20

10. A print engine controller according to claim 9 wherein the primary dither matrix has a range of threshold values and the secondary dither matrix has a compressed range of threshold values for comparison to the colour level values of the contone image data during halftoning. 25

11. A print engine controller according to claim 9 wherein the ratio is: KIPOO1 24 (Lmax - Lmin)/Ltotal where: Lmax is the maximum number of discrete colour levels if a top-most portion of the colour level values of the contone image data is disregarded; 5 Lmin is the minimum number of discrete colour levels that contain a bottom-most number of the colour level values of the contone image data; and, Ltotal is the total number of levels in the range of discrete colour levels.

12. A print engine controller according to claim 11 wherein the top-most portion are 10 the highest 5% of the colour level values of the contone image data and the bottom-most portion are the lowest 5% of the colour level values of the contone image data.

13. A print engine controller according to claim 11 wherein the threshold values in the compressed range in the secondary matrix are determined in accordance with the following 15 algorithm: Tcw = Lmin + Told.(Lmax - Lmin)/Ltotal where: Tnc, is the compressed threshold values in the secondary dither matrix; and, Told is the threshold value in the primary dither matrix. 20

14. A print engine controller according to claim 11 wherein at least some of the thresholds in the primary dither matrix are not whole numbers and the compressed threshold values in the secondary matrix are rounded or truncated to the nearest whole numbers. 25 KIPOO1 25

15. A print engine controller according to claim 11 wherein the threshold values occur in the primary dither matrix a predetermined number of times and the compressed threshold values occur in the secondary dither matrix a greater number of times, the greater number being approximately equal to Ltotaj/(Lmax-Lmin) multiplied by the predetermined 5 number associated with the threshold values of the primary matrix, or only one of two colliding threshold values from the primary matrix, that correspond to the compressed threshold.

16. A print engine controller according to claim 11 wherein the processor only samples 10 a portion of the pixels of the contone image data to determine Lmin and Lmax. KIPOO1