[FFmpeg-devel] Re: [PATCH] avcodec/libvpxenc: fix sRGB colorspace for non-RGB pixel formats

[James Zern via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Hi,

On Mon, Feb 23, 2026 at 11:22 AM Guangyu Sun via ffmpeg-devel
<ffmpeg-devel@ffmpeg.org> wrote:
>
> When encoding VP9 with a YUV pixel format (e.g. yuv420p) and
> AVCOL_SPC_RGB colorspace metadata, libvpxenc unconditionally set
> VPX_CS_SRGB. This produced a spec-violating bitstream: Profile 0
> (4:2:0) with sRGB colorspace, which is only valid for Profile 1/3
> (4:4:4). The resulting file is undecodable.
>
> RGB pixel formats (GBRP, GBRP10, GBRP12) already correctly set
> ctx->vpx_cs to VPX_CS_SRGB in set_pix_fmt() and bypass the
> colorspace switch in set_colorspace(). So reaching the AVCOL_SPC_RGB
> case means the pixel format is YUV with incorrect RGB metadata.
> Log a warning and fall back to VPX_CS_BT_709.
>
> To reproduce:
>
>   # generate a bad source
>   ffmpeg -f lavfi -i testsrc=s=64x64:d=1:r=1 -pix_fmt yuv420p -colorspace rgb bad.mp4
>   # transcode with default parameters
>   ffmpeg -i bad.mp4 bad.webm
>   # check decoding
>   ffmpeg -i bad.webm -f null -
>   # -> 0 frames decoded, error
>
> Signed-off-by: Guangyu Sun <gsun@roblox.com>
> ---
>  libavcodec/libvpxenc.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/libvpxenc.c b/libavcodec/libvpxenc.c
> index 88c058c403..be731a014f 100644
> --- a/libavcodec/libvpxenc.c
> +++ b/libavcodec/libvpxenc.c
> @@ -902,7 +902,17 @@ static void set_colorspace(AVCodecContext *avctx)
>          vpx_cs = ctx->vpx_cs;
>      } else {
>          switch (avctx->colorspace) {
> -        case AVCOL_SPC_RGB:         vpx_cs = VPX_CS_SRGB;      break;
> +        case AVCOL_SPC_RGB:
> +            // RGB pixel formats (GBRP, GBRP10, GBRP12) set ctx->vpx_cs
> +            // to VPX_CS_SRGB in set_pix_fmt() and take the branch above,
> +            // so reaching here means the pixel format is YUV with
> +            // incorrect RGB colorspace metadata.

What if the input is yuv444? In that case it could be holding rgb
data. Profile 1 and 3 assume no subsampling when decoding sRGB
content.

> +            av_log(avctx, AV_LOG_WARNING,
> +                   "RGB colorspace is not compatible with pixel format %s, "
> +                   "using BT.709 instead.\n",
> +                   av_get_pix_fmt_name(avctx->pix_fmt));
> +            vpx_cs = VPX_CS_BT_709;
> +            break;
>          case AVCOL_SPC_BT709:       vpx_cs = VPX_CS_BT_709;    break;
>          case AVCOL_SPC_UNSPECIFIED: vpx_cs = VPX_CS_UNKNOWN;   break;
>          case AVCOL_SPC_RESERVED:    vpx_cs = VPX_CS_RESERVED;  break;
> --
> 2.52.0
>
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