Quoting James Almer (2021-01-21 14:29:22)
> On 1/21/2021 9:59 AM, Anton Khirnov wrote:
> > Quoting James Almer (2021-01-09 18:47:17)
> >> The st->codec values are updated based on the lowres factor by
> >> avformat_find_stream_info() when it runs an instance of the decoder
> >> internally,
> >> and the same thing happens in ffmpeg.c when we open ist->dec_ctx with
> >> avcodec_open2(), so these assignments are redundant.
> >>
> >> Signed-off-by: James Almer <jamr...@gmail.com>
> >> ---
> >> This chunk here is not properly wrapped with the relevant pre-processor
> >> check
> >> for AVStream->codec, and seeing it's ultimately redundant, i figured we
> >> might
> >> as well delete it now.
> >>
> >> For that matter, the deprecation of lowres in avcodec.h is in a very
> >> strange
> >> state (the field is not removed, its offset is changed instead). Once the
> >> value
> >> of FF_API_LOWRES is flipped, neither the field, the AVOption, or the usage
> >> within decoders will be removed, but some code in libavformat/utils.c will
> >> be
> >> disabled, and that may result in unexpected behavior.
> >
> > IMO it should just be made a codec-private option. And lavf has no
> > business treating it specially.
>
> Technically speaking, there's no reason for lavf to check for lowres to
> do what it's currently doing.
>
> The code
>
> > int orig_w = st->codecpar->width;
> > int orig_h = st->codecpar->height;
> > ret = avcodec_parameters_from_context(st->codecpar,
> > st->internal->avctx);
> > if (ret < 0)
> > goto find_stream_info_err;
> > ret = add_coded_side_data(st, st->internal->avctx);
> > if (ret < 0)
> > goto find_stream_info_err;
> > #if FF_API_LOWRES
> > // The decoder might reduce the video size by the lowres factor.
> > if (st->internal->avctx->lowres && orig_w) {
> > st->codecpar->width = orig_w;
> > st->codecpar->height = orig_h;
> > }
> > #endif
>
> Could be simplified to unconditionally set st->codecpar dimensions to
> the backed up ones. If you agree, i can send a patch.
I'm not sure that won't have other side effects - the decoder might have
different ideas about dimensions than the demuxer, which might change
something in some obscure cases. I guess try and see if FATE passes?
--
Anton Khirnov
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