THE USE OF DIFFERENCE COLOUR MODELS WITH COMPONENT DIFFERENCE OFFSETS TO IMPROVE THE COMPRESSION OF PNG IMAGES
DOI:
https://doi.org/10.15588/1607-3274-2026-2-17Keywords:
lossless image compression, PNG graphic format, difference colour models with integer coefficientsAbstract
Context. Today, static difference colour models with integer coefficients are used to improve the efficiency of lossless imagecompression in graphic formats and archivers. These models improve compression, b ut do not consider the level of cross-correlation between different pairs of colour components of pixels in each image. Therefore, the development of methods for the formation and use of difference colour models for individual images in order to improve their compression by intercomponent decimation is currently an urgent scientific task.
Objective. To develop methods and algorithms for the transition to difference colour models with integer coefficients and difference offsets to reduce compression ratios in the process of lossless compression of RGB images in modern graphic formats, in particular in PNG format.
Method. Depending on the coding time constraints, the paper proposes to use 4, 16, 19 or 49 alternative difference colour models with difference offsets to select the most efficient model for each image. Prediction of the compression efficiency due to the use of the next alternative difference colour model is performed using entropy. The differences in the colour models are shifted so that the centre of the interval with the maximum number of these differences is shifted to the middle of the range of possible values. The effectiveness of three methods of determining the centre of this interval is investigated: without considering the deviations of component brightnesses, using the difference in component medians, and by determining the centre of the interval with the maximum number of component differences after their sequential search.
Results. Our experiments have shown that, for example, applying difference colour models with integer coefficients to whole images in the process of sequential lossless compression, in particular, in the PNG graphic format we modified, allows reducing
compression ratios of photorealistic images of the ACT set by 0.19–1.06 bpb. Shifting the differences to the differences of the medians of individual components or centring the intervals of component differences provides an additional 0.01–0.02 bpb compression ratio reduction on average. Thus, difference colour models with integer coefficients and difference offsets can significantly increase the compression efficiency of lossless three-component photorealistic images in formats that use predictors and therefore can be implemented in the next versions of these formats at the standard level.
Conclusions. In graphic formats, to reduce the lossless image compression ratio, in addition to decorrelation of individual component data, it is advisable to perform intercomponent decorrelation by switching to difference colour models with integer coefficients with difference offsets, which provide fast decoding. To maximise the reduction in compression ratio due to the application of the selected colour model, the midpoints of the difference intervals of the basic components R, G, B should be shifted to the middle of the range of possible values. When, for photorealistic images, due to strict limitations on encoding time or encoder size, it is impossible to select a difference colour model from among 49, 19 or 16 alternative ones, this choice should be made among three models: G – R + 128, G, G – B + 128; R, R – G + 128, B – G + 128, or R – G + 128, B – G + 128, B. For synthesised images, it is not advisable to switch to difference colour models with integer coefficients
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