Hybrid image compression with implicit fractal terms

The performance of any block based image coder can be improved by applying fractal terms to selected blocks. Two novel methods are used to achieve this. Firstly the coder determines whether a local fractal term will improve each image block by examining its rate/distortion contribution, so that only beneficial fractal terms are used. Secondly, the decoder deduces the offset parameters for the local fractal transform from the basis functions alone, by inferring the dominant edge position, so that no offset information is required. To illustrate the method, we use a quadtree decomposed image with a truncated DCT basis. Using a standard test image, the proportion of the picture area enhanced by fractals decreases from 16.1% at 0.6 bpp to 8.1% at a high compression ratio of 80:1 (0.1bpp). The fractal terms contribute less than 5% of the compressed code in all cases. The PSNR is improved slightly, and edge detail is visually enhanced.