Lossless compression of already compressed textures

Texture compression helps rendering by reducing the footprint in graphics memory, thus allowing for more textures, and by lowering the number of memory accesses between the graphics processor and memory, increasing performance and lowering power consumption. Compared to image compression methods like JPEG however, textures codecs are typically much less efficient, which is a problem when downloading the texture over a network or reading it from disk. Therefore, in this paper we investigate lossless compression of already compressed textures. By predicting compression parameters in the image domain instead of in the parameter domain, a more efficient representation is obtained compared to using general compression such as ZIP or LZMA. This works well also for pixel indices that have previously proved hard to compress. A 4-bit-per-pixel format can thus be compressed to around 2.3 bits per pixel (bpp), or 9.6% of the original size, compared to around 3.0 bpp when using ZIP or 2.8 bpp using LZMA. Compressing the original images with JPEG to the same quality also gives 2.3 bpp, meaning that texture compression followed by our packing is on par with JPEG in terms of compression efficiency.