[FFmpeg-devel] [PATCH v3 3/7] avcodec/mediacodecenc: use bsf to handle crop

[Tomas Härdin]

tis 2022-12-13 klockan 11:20 +0800 skrev zhilizhao(赵志立):
> 
> 
> > On Dec 12, 2022, at 23:27, Tomas Härdin <git at haerdin.se> wrote:
> > 
> > lör 2022-12-10 klockan 01:22 +0800 skrev Zhao Zhili:
> > > From: Zhao Zhili <zhilizhao at tencent.com>
> > > 
> > > It's well known that mediacodec encoder requires 16x16 alignment.
> > > Use our bsf to fix the crop info.
> > > ---
> > > v3: don't change the dimension for AV_PIX_FMT_MEDIACODEC. It can
> > > have
> > > side effect.
> > 
> > Looks like this silently crops? Is that really a good idea? We
> > usually
> > don't do stuff like that. For example codecs that require even
> > dimensions complain loudly then fail.
> 
> It’s reasonable to require even dimensions. Require dimensions
> aligned
> to 16 is uncommon. Everyone will complain why 1080x1920 doesn’t work.
> 
> A lot of apps just use aligned dimensions. Users have no control on
> these apps. It’s not the same with FFmpeg, users (developer or not)
> can specify the dimension directly.

Wait a sec, I think I was misunderstanding what the code is doing.
FFALIGN rounds *up*. Does this mean you insert fake data in the border
that then gets cropped away, meaning the original essence is still
"there"? That's a different thing and probably perfectly OK.

I think we might want something for this inside lavf somewhere, so that
encoders can signal dimension alignment requirements. Some containers
(MXF, MOV) support such cropping in a codec-agnostic manner.

> 
> If we don’t fix it, either:
> 
> 1. Reject and fail directly. User complain why.
> 2. Accept and keep going. Sometimes it works, sometimes don’t. It
> depends on the device and get into a confused situation.
> 
> I know there are getWidthAlignment()/getHeightAlignment() to get
> alignment info of codecs. The results are unreliable. The only
> reliable way I can find is don’t depends on those API and fix it
> by ourself.

Given how temperamental MC seems to be a "belt and braces" approach
might be appropriate when dealing with it. Tell users (ffmpeg.c is a
user here) that dimensions must be aligned by 16x16 and then
automagically doing the required padding and cropping somewhere (lavf
or ffmpeg.c) feels like a decent solution.

/Tomas