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

Hendrik Leppkes h.leppkes at gmail.com
Sat Apr 28 12:42:17 EEST 2018

On Sat, Apr 28, 2018 at 10:44 AM, Jerome Borsboom
<jerome.borsboom at carpalis.nl> wrote:
>> 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?

If the spec is bitexact, then our decoder should be by default as
well. Perhaps such things should be flipped around and use bitexact by
default unless the "fast" flag is specified.

- Hendrik

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