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由 Fabian Giesen 提交于
The original AC decoding logic handled ZRL (runs of 16 zeros) incorrectly. The problem is that the original flow set r=16 and skipped the final coeff write when s=0. This is not actually correct. The problem is the intervening "refinement" bits. With the original logic, even once we decrement r to 0, we keep reading more refinement bits for subsequent coefficients until we find the next currently-unsent AC in the current scan. That is, it works as if it was trying to place 17 new AC values, and only bails at the last minute from actually setting that 17th value. This is wrong. Once we've found the 16th previously-unsent AC, we need to stop reading refinement bits, otherwise we get out of sync with the bit stream (which expects us to read a huffman code next). The easiest fix is to just do what the JPEG standard implicitly assumes anyway: treat ZRL as a run of 15 zeros followed by an explicit magnitude-zero AC coeff. (That is, leave s=0 and actually write s). So this is what this fix does.
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