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

[Jan Ekström]

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).

Best regards,
Jan