I wrote the Vulkan ProRes backend. The bitstream decoder was implemented from scratch, for a number of reasons.

First, the original code was reverse-engineered, before Apple published an SMPTE document describing the bitstream syntax. Second, I tried my best at optimizing the code for GPU hardware. And finally, I wanted take the learning opportunity :)

And to answer the parent's question, the shaders are written in pure GLSL. For instance, this is the ProRes bitstream decoder in question: https://code.ffmpeg.org/FFmpeg/FFmpeg/src/branch/master/liba...