[FFmpeg-devel] [PATCH 2/3] lavc: add a framework to fix alignment problems.

Hendrik Leppkes h.leppkes at gmail.com
Thu May 11 00:06:17 EEST 2017

On Wed, May 10, 2017 at 10:28 PM, Nicolas George <george at nsup.org> wrote:
> Le primidi 21 floréal, an CCXXV, Michael Niedermayer a écrit :
>> fixed, and now your patchset seems to cause a 1% slowdown of the
>> example above
> Thanks. The slowdown is to be expected, since this filter used
> non-aligned frames which are now realigned. I guess you can get back the
> previous speed by setting "avctx->alignment = 0" in snow_encoder.init().
> The default in this patchset is 5 to fix all the alignment problems, at
> the cost of some slowdown in corner cases such as this one. I am not
> qualified to decide the best way of setting an optimal value to get
> speed without crashes. My opinion would be:
> - have avctx->alignment default to 0;
> - have all codecs with explicit assembly set avctx->alignment to the
>   required value;
> - have all codecs that use generic DSP code init it through a function
>   that sets the alignment accordingly.

If you want to fix these problems, you should rather white-list codecs
which are known to be safe, instead of the other way around.

- Hendrik

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