[FFmpeg-devel] [PATCH] avcodec/x86/hpeldsp: fix half pel interpolation

Jerome Borsboom jerome.borsboom at carpalis.nl
Sat Apr 28 11:44:54 EEST 2018

> This patch is not correct.
> this code is not used if AV_CODEC_FLAG_BITEXACT is set, because it is
> not bit exact ...
> Also the case where the off by 1 error occurs is a rare corner case,
> Compared to the errors introduced by the IDCT this is not significant
> If you want to optimize the bit exact version, feel free to do so.
> It may be faster to use xor -1 before and after avg for this though

Thank you for the review. VC-1 spec is defined bit exact, including the
inverse transform. This code is used by the VC-1 decoder and as I was under
the impression that the VC-1 decoder ought to be bit exact without any additional
command line options, I tried to resolve the issue.

As this code is guarded by AV_CODEC_FLAG_BITEXACT, i agree that the
proposed solution is not appropriate. The underlying question remains though. Should
the VC-1 decoder fully conform to spec or do we allow small deviations?

In addition, the VC-1 fate tests put the -bitexact option after the input file (-i). I
could only get the bitexeact working if it was put before the input file. Thus, I
think the current vc-1 fate-tests do not use the bit exact version.

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