Re: [FFmpeg-devel] [PATCH] avcodec/dovi_rpudec - correctly read el_bit_depth_minus8 when ext_mapping_idc is non-zero

[Niklas Haas <ffmpeg@haasn.xyz>]

On Tue, 21 May 2024 01:17:32 +0000 Cosmin Stejerean via ffmpeg-devel <ffmpeg-devel@ffmpeg.org> wrote:
> From: Cosmin Stejerean <cosmin@cosmin.at>
> 
> It looks like the el_bitdepth_minus8 value in the header can also encode
> ext_mapping_idc in the upper 8 bits.
> 
> Samples having a non-zero ext_mapping_idc fail validation currently because the
> value returned is out of range. This bypasses this by currently ignoring the
> ext_mapping_idc and using only the lower 8 bits for el_bitdepth_minus8.

What is ext_mapping_idc? If it's signalled data that can't be
reconstructed, we need to store it somewhere into AVDOVIMetadata and
then re-synthesize it during encoding. Otherwise the RPU transcode will
be lossy.

> 
> ---
>  libavcodec/dovi_rpudec.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/dovi_rpudec.c b/libavcodec/dovi_rpudec.c
> index 7c7eda9d09..1b11ad3640 100644
> --- a/libavcodec/dovi_rpudec.c
> +++ b/libavcodec/dovi_rpudec.c
> @@ -411,7 +411,9 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size,
>  
>          if ((hdr->rpu_format & 0x700) == 0) {
>              int bl_bit_depth_minus8 = get_ue_golomb_31(gb);
> -            int el_bit_depth_minus8 = get_ue_golomb_31(gb);
> +            // this can encode a two byte value, with higher byte being ext_mapping_idc
> +            // use only the lower byte for el_bit_depth_minus8
> +            uint8_t el_bit_depth_minus8 = get_ue_golomb_long(gb) & 0xFF;
>              int vdr_bit_depth_minus8 = get_ue_golomb_31(gb);
>              VALIDATE(bl_bit_depth_minus8, 0, 8);
>              VALIDATE(el_bit_depth_minus8, 0, 8);
> -- 
> 2.42.1
> 
> 
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