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

[Tomas Härdin]

tor 2022-12-15 klockan 01:37 +0800 skrev Zhao Zhili:
> On Wed, 2022-12-14 at 18:08 +0100, Tomas Härdin wrote:
> 
> > 
> > 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.

This is the difference between stored, sampled and display dimensions
in MXF. For example 1080i video has StoredHeight = 544, SampledHeight =
540 and DisplayHeight = 540 (see AS-10). When you add VBLANK and HBLANK
to the mix then all three dimensions are typically different.

Anyway specifying at the NAL level whenever the essence isn't a
multiple of 16x16 is obviously normal. The only complication I can
think of is 4:2:2 and 4:4:4. Does MC require 16x16 also in those cases?
I'd expect 16x8 and 8x8 respectively.

/Tomas