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

From: "\"zhilizhao(赵志立)\"" <quinkblack@foxmail.com>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v3 3/7] avcodec/mediacodecenc: use bsf to handle crop
Date: Wed, 21 Dec 2022 15:17:06 +0800
Message-ID: <tencent_6463B5CCC9042EA22545D301884EB3F0F305@qq.com> (raw)
In-Reply-To: <7fc5d621ee203d6059bebd47283df33f33152af8.camel@haerdin.se>

> On Dec 21, 2022, at 02:24, Tomas Härdin <git@haerdin.se> wrote:
> 
> 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.

It’s still 16x16. From H.264 specification:

macroblock: A 16x16 block of luma samples and two corresponding blocks of
chroma samples of a picture that has three sample arrays, or a 16x16 block
of samples of a monochrome picture or a picture that is coded using three
separate colour planes.

Macroblock has been replaced by coding tree unit with H.265, which can be
between 16×16 pixels and 64×64 pixels in size.

> 
> /Tomas
> 
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