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Originally Posted by Chainmax
Forgive my complete and utter ingorance, but it can be done in a lossless way? How?
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Entropy coding via CABAC or CAVLC is done as last step, after ME and quantization have taken place. Let's say it's a lossless process that just "writes down to the bitstream" the already-computed data (transformed&quantized coefficients, macroblock types, motion vectors, plus additional side-infos). All you need to do is to decode the CAVLC/CABAC stream and reencode it to the other format, while signalling in the slice headers that you've switched entropy coder type. Nothing will be lost in the process, since detail removal (which comes from quantization) need not be performed again. You just redo the very last step in your encoding chain (see what I've written above).