[FFmpeg-devel] [PATCH 1/2] avcodec/libx264: add support for setting chroma sample location

[Jan Ekström]

On Fri, Aug 27, 2021 at 12:58 AM Jan Ekström <jeebjp at gmail.com> wrote:
>
> On Tue, Aug 24, 2021 at 12:47 AM Jan Ekström <jeebjp at gmail.com> wrote:
> >
> > ---
> >  libavcodec/libx264.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
> > index d48e142e41..379c167e6f 100644
> > --- a/libavcodec/libx264.c
> > +++ b/libavcodec/libx264.c
> > @@ -870,6 +870,8 @@ static av_cold int X264_init(AVCodecContext *avctx)
> >          x4->params.vui.i_colorprim = avctx->color_primaries;
> >      if (avctx->color_trc != AVCOL_TRC_UNSPECIFIED)
> >          x4->params.vui.i_transfer  = avctx->color_trc;
> > +    if (avctx->chroma_sample_location != AVCHROMA_LOC_UNSPECIFIED)
> > +        x4->params.vui.i_chroma_loc = avctx->chroma_sample_location - 1;
> >
> >      if (avctx->flags & AV_CODEC_FLAG_GLOBAL_HEADER)
> >          x4->params.b_repeat_headers = 0;
> > --
> > 2.31.1
> >
>
> I have re-tested these changes (for both x264 and x265), and BT.2100
> content that was marked with topleft chroma location now gets that
> information passed onto the output (whereas previously the content
> would get interpreted as with the implicit "left" value). Thus, unless
> there are objections, I will pull this set in soon.
>
> The only difference in default would be with unlabeled H.264/HEVC with
> streams that get that default value ("left") interpreted. Previously,
> as there was no support for writing the data, those streams would stay
> without any metadata on the output side, while now, as libavcodec
> doesn't differentiate between implicit and explicit values, the value
> gets written into the output, making an implicit value explicit. As
> far as whether this is a problem, I would consider it a "no", as the
> decoding result should be the same (due to the implicit default).

Applied the x264 patch as 2f0113be3ffb566f1bb7f3140f038318c447da9f
after checking that x264 ignores chroma sample location for
non-CHROMA_420 pix_fmts.

Will be posting a v2 of the x265 patch as it requires this logic to be
in the API client.

Jan