A COMPARATIVE STUDY OF LOSSLESS CODING TECHNIQUES FOR SCREENED CONTINUOUS-TONE IMAGES 1; 2 Koen Denecker Peter De Neve ELIS, University of Gent, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium 1 2 Email: [email protected] e [email protected] e ABSTRACT of twotwo-dimensional black-and-white oriented adaptive co ders: BILEVEL co ding (by Witten et al.) and JBIG The huge sizes of screened colour-separated photographic (latest fax-standard). images makes lossless compression very b ene cial for b oth The compression eciency of these co ders is investigated storage and transmission. Because of the sp ecial struc- using six test images, representing halftones at di erent ture induced by the half-tone dots, the compression results resolutions, at di erent screening angles and at di erent obtained on the CCITT test images might not apply to screening techniques. high-resolution screened images and the default parameters of existing compression algorithms may not b e optimal. In 2. DIGITAL COLOUR SCREENING this pap er we compare the p erformance of di erent classes of lossless co ders: general-purp ose one-dimensional co ders, The most common pro cedure in colour screening technology non-adaptivetwo-dimensional black-and-white co ders and is as follows [4 ]. Firstly, the CMYK colour image is sepa- adaptive two-dimensional co ders. Firstly, exp eriments on rated into its four colour channels (cyan, magenta, yellow a set of test images screened under di erent conditions and black). Each colour separation is a greyscale image, showed that MGBILEVEL and JBIG p erform b est with re- which is then screened under a di erent angle (e.g., 45 for sp ect to compression eciency; the di erence with the other the most covering colour black, 15 and 75 for cyan and co ders is signi cant. Secondly,weinvestigated the in uence magenta, and 0 for the least visible colour yellow) in order of the screening metho d (sto chastic or classical screening) to avoid the app earance of moire patterns. The screening and screening resolution on the compression ratio for these step itself converts the original greyscale image into dots techniques. placed on a rectangular grid. The sizes of the dots are determined by the intensity of the greyscale original. This typ e of screening is called classic screening. Another typ e of 1. INTRODUCTION screening, stochastic screening, pro duces equally sized dots at varying distances in order to repro duce the grey tones, The repro duction of continuous-tone images using the four giving the dots a random-like app earance. printing inks (CMYK) consists of splitting the image into four colour separations and screening each colour separa- The screening parameters are illustrated in gure 1. It tion under a di erent angle. New applications, such as shows enlargements of the same detail of the cyan separa- printing-on-demand, require an almost identical image to tion of the \Musicians" test image screened under di erent b e screened many times; therefore, storing the unchanged screening metho ds and screening resolutions. The details part as a screened halftone using lossless compression tech- are part of the test images in our research. niques instead of screening it again and again, may save There are two imp ortant parameters for pro ducing digital printing time as well as storage space. Transmitting such screens: the screening resolution at which the laser sp ots are images at the very latest moment also b ene t from com- pro duced, varying from ab out 300 dpi to ab out 5000 dpi, pression. Moreover, the sizes of the images (often ab out and the screen ruling (only in the case of classic screens and 100 MByte) and the time pressure in the application area expressed in lines p er inch), indicating the size of the grid (e.g., printing newspap ers) make lossless compression even and varying from ab out 75 lpi for newspap ers to 300 lpi for more desirable. ne art repro ductions. Screening a contone image at high In this pap er, we have compared the compression e- resolution increases the image data size drastically, making ciency of several lossless compression schemes. As a refer- the need for compression more urgent (e.g., the \Musicians" ence, wehave exp erimented with four general-purp ose byte- test CMYK image, 16 Mbyte large, pro duces 200 Mbyte oriented co ders: GZIP (which is a dictionary-based loss- after halftoning at a fairly high resolution of 2540 dpi). less co der), STAT (an optimized PPM-based compressor by F. Bellard), TIFF PackBits (a simple run-length compres- 3. LOSSLESS COMPRESSION SCHEMES sor) and TIFF LZW (Lemp el-Ziv-Welch, a dictionary-based Since each pixel of a screened image is either white or compression scheme like GZIP). Then, we have used two black, a screened image is actually is a bilevel image, al- nonadaptive co ders which accompany the TIFF-standard, b eit with a very sp ecial structure. The investigated co ders namely TIFF Group 3 and Group 4 compression (the for- can b e divided into three classes: one-dimensional general- mer published fax-standards by CCITT [1 , 2, 3]). Finally, purp ose co ders (GZIP, STAT, TIFF PackBits and TIFF wehave also investigated the compression eciency (using LZW), two-dimensional nonadaptive co ders (TIFF Group 3 standard options and after ne-tuning of the parameters) and Group 4) and two-dimensional adaptive context-based Research Assistant with the NFWO, Brussels, Belgium co ders (BILEVEL and JBIG). (a) (b) (c) (d) Figure 1. Magni cation of the same detail of the \Musicians" image: (a) classic screening at 1270 dpi; (b) sto chastic screening at 1270 dpi; (c) classic screening at 2540 dpi and (d) sto chastic screening at 2540 dpi. 3.1. General-purp ose 1-D byte-oriented co ders 3.2. Nonadaptive 2-D co ders TIFF Group 3 and TIFF Group 4 are ocially known as This rst class of co ders do esn't exploit the two-dimensional CCITT Recommendations T.4 [1 ] and resp. T.6 [2]. These correlation present in images. Neither do es it take into compression schemes are not byte-oriented but treat each account the physical interpretation of the bytes: they are line of the image as an alternating sequence of black runs merely interpreted as symb ols of an alphab et, disregarding and white runs; only the run-lengths are transmitted us- the numerical value asso ciated with these bytes. On the ing co de words de ned by the standard, hence the co ders other hand, they are able to adapt themselves one wayor are nonadaptive. The co de words are chosen to b e optimal another to the information stream. for scanned textual images, but the nonadaptivity could GZIP (a free and wide-spread compressor by GNU) uses cause it to p erform badly on images with sp ecial structures. Lemp el-Ziv co ding (often denoted as LZ77 [5 ]), which is Group 3 enco ding basically is one-dimensional but provides a form of sliding-window compression. While running additional options such like two-dimensional enco din and through the image le, the window consisting of the last byte-aligning. Group 4 is a simpli ed or streamlined ver- n bytes (e.g., n = 1024) is used as a dictionary and each sion of the Group 3 standard in which only two-dimensional new symb ol is replaced by an index to this sliding-window co ding is allowed. dictionary. STAT uses a context (i.e., the string consisting of the 3.3. Adaptive 2-D context-based co ders last 4 symb ols) to feed an arithmetic co der [6 ]. Since it is These co ders use a context, i.e., an amount of previous pix- imp ossible to construct complete conditional probabilities els, to condition the value of each new pixel. Since these with a context of 4 bytes, only these sp eci c contexts which co ders will prove to b e the most ecient ones, we will de- arise more than once are taken into account. This approach scrib e them in more detail than the other. is called PPM-based, which stands for Prediction byPartial 3.3.1. JBIG Matching [7 ]. JBIG (Joint Bilevel Image Group) is the newest CCITT TIFF Packbits replaces consequent runs of identical sym- lossless facsimile compression standard develop ed by the b ols by the numb er of symb ols and the value of the symb ol; Joint Bilevel Image Group [3]. It is an adaptive co der which this technique is called run-length co ding. It is a very sim- supp orts progressive transmission. It uses the Q-co der as a ple and fast algorithm. statistical co der but the fact that it's patented by IBM is the TIFF LZW (Lemp el-Ziv-Welch) [8 ] is a dictionary-based most imp ortant explanation for the very small p opularity. technique like GZIP. The technique uses previously co ded text as a dictionary. A dictionary consisting of strings is built, in suchaway that each shorter string also exists in Organization of the image data The image is sepa- the library. The enco ded les consists of indexes to the rated into resolution layers in case of progressive transmis- longest matching strings. sion, bitplanes in case of greyscale images and horizontal AA ? ? ?? Figure 2. JBIG provides two di erenttyp es of con- text templates. Figure 3. Two context templates are provided: the left one contains 10 pixels and is always used, the strip es.