Video decoding is very branchy and not easily decomposable. e.g. each frame depends on decoding the previous frame. They are however relatively easy to implemen

Video decoding is very branchy and not easily decomposable. e.g. each frame depends on decoding the previous frame. They are however relatively easy to implement in custom hardware (part of the design goals of the standard committee) so in most cases the GPU accelerated decompression does not actually use the standard shaders/pipeline etc... but a separate section of silicon.