[FFmpeg-devel] [PATCH] avcodec/libjxldec: fix gamma22 and gamma28 recognization

[Jan Ekström]

On Wed, Jan 18, 2023 at 4:24 PM Leo Izen <leo.izen at gmail.com> wrote:
>
> Gamma 2.2 and Gamma 2.8 are tagged in the file as 0.45455 and 0.35714,
> respectively (i.e. 1/2.2 and 1/2.8). Trying to identify them as 2.2 and
> 2.8 instead of these values will cause the transfer function to not
> properly be recognized. This patch fixes this.

Agreed upon reading the spec, which notes that the value is saved as
an unsigned 24bit integer consisting of the numerator part of a
rational number, with a 10^7 denumerator and a valid range of (0, 1].

Only nit: recognization -> recognition

> ---
>  libavcodec/libjxldec.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/libjxldec.c b/libavcodec/libjxldec.c
> index 55c7366f61..0009af9615 100644
> --- a/libavcodec/libjxldec.c
> +++ b/libavcodec/libjxldec.c
> @@ -167,9 +167,9 @@ static enum AVColorTransferCharacteristic libjxl_get_trc(void *avctx, const JxlC
>      case JXL_TRANSFER_FUNCTION_DCI: return AVCOL_TRC_SMPTE428;
>      case JXL_TRANSFER_FUNCTION_HLG: return AVCOL_TRC_ARIB_STD_B67;
>      case JXL_TRANSFER_FUNCTION_GAMMA:
> -        if (jxl_color->gamma > 2.199 && jxl_color->gamma < 2.201)
> +        if (jxl_color->gamma > 0.45355 && jxl_color->gamma < 0.45555)
>              return AVCOL_TRC_GAMMA22;
> -        else if (jxl_color->gamma > 2.799 && jxl_color->gamma < 2.801)
> +        else if (jxl_color->gamma > 0.35614 && jxl_color->gamma < 0.35814)

These values leave ~0.001 of leeway which seems like it was before.

LGTM