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

[Zhao Zhili <quinkblack@foxmail.com>]

On Thu, 2022-12-15 at 01:37 +0800, Zhao Zhili wrote:
> On Wed, 2022-12-14 at 18:08 +0100, Tomas Härdin wrote:
> > tis 2022-12-13 klockan 11:20 +0800 skrev zhilizhao(赵志立):
> > > > On Dec 12, 2022, at 23:27, Tomas Härdin <git@haerdin.se> wrote:
> > > > 
> > > > lör 2022-12-10 klockan 01:22 +0800 skrev Zhao Zhili:
> > > > > From: Zhao Zhili <zhilizhao@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.
> 
> Yes, the dimension passed to MC is rounding up, then use bsf to
> remove
> that border. It depends on our AVFrame data has been aligned
> properly.

Actually there's no such dependent.

> This job is supposed to be done by any decent encoder, not by a
> wrapper.
> 
> > 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.
> 
> From my own experience, dimension mismatch between codec and
> container
> makes a lot of trouble. ISO base format specification specified how
> to
> crop/scale after decoding clear, however, I don't think it has been
> widely supported, including FFmpeg. We can fix that inside of FFmpeg,
> but we should avoid such cases as much as we can.
> 
> > > 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
> > 
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