Study On Lossless Compression Algorithms For Gray Halftone

The lossless compression algorithms for current gray halftone usually haveproblems of higher complexity and lower ratio.By analyzing the basic theory of typicalhalftoning algorithms and characteristics of halftone, lossless compression algorithms ofordered dither and error diffused halftone are designed.Main contents of the paper asfollows:(1) Black block skipping coding for ordered dither halftone is presented in thepaper. Firstly, by partitioning and xor calculation, halftone is converted into binaryimage where isolated white pixel is surrounded by large area of black pixels,then thebinary image is compressed by black block skipping coding.In order to achieveeffective compression for non-black block, specific short code word is adopted of only awhite pixel of non-black block and direct coding is adopted of the remaining types ofnon-black block.(2) Combined with characteristics of ordered dither halftone, sub-blocks scanningmethod of halftone is designed to achieve the purpose of increasing the black and whiterun length.On the basis,an adaptive run length coding of ordered dither halftone ispresented. The adaptive run length coding that the shorter prefix code is adopted ofshort run length and the adaptive allocation number of bits that represent run length isadopted of long run length is designed to solve the defect which the basic run lengthcoding is not well adapted to the actual run.(3) The error is mainly extended to the bottom right corner of error diffusedhalftone. Therefore, a new scanning method of error diffused halftone is designed andcontinuous multiple pixels is determined for image encoded symbol, then huffmanencoding is adopted to compress error diffused halftone. Finally, the experimental results show that new algorithms are effective.