[FFmpeg-devel] [PATCH 13/18] h264_sei: parse the picture timing SEIs correctly

[James Almer]

On 3/13/2020 7:28 AM, Anton Khirnov wrote:
> Those SEIs refer to the currently active SPS. However, since the SEI
> NALUs precede the coded picture data in the bitstream, the active SPS is
> in general not known when we are decoding the SEI.
> 
> Therefore, store the content of the picture timing SEIs and actually
> parse it when the active SPS is known.
> ---
>  libavcodec/h264_parser.c |  9 +++++
>  libavcodec/h264_sei.c    | 86 ++++++++++++++++++++++++----------------
>  libavcodec/h264_sei.h    | 11 +++++
>  libavcodec/h264_slice.c  | 10 +++++
>  4 files changed, 82 insertions(+), 34 deletions(-)
> 

[...]

> +static int decode_picture_timing(H264SEIPictureTiming *h, GetBitContext *gb,
> +                                 void *logctx)
> +{
> +    PutBitContext pb;
> +    int n;
> +
> +    if (get_bits_left(gb) > sizeof(h->payload) * 8) {
> +        av_log(logctx, AV_LOG_ERROR, "Picture timing SEI payload too large\n");
> +        return AVERROR_INVALIDDATA;
> +    }
> +
> +    init_put_bits(&pb, h->payload, sizeof(h->payload));
> +
> +    while (get_bits_left(gb) >= 32)
> +        put_bits(&pb, 32, get_bits_long(gb, 32));
> +
> +    n = get_bits_left(gb);
> +    if (n)
> +        put_bits(&pb, n, get_bits_long(gb, n));

I think you can do something like avpriv_copy_bits(&pb, gb->buffer,
get_bits_left(gb)) instead.
No need to skip the bits in gb afterwards since the GetBitContext struct
is exclusive for this SEI starting with the previous patch.

> +
> +    h->payload_size_bits = put_bits_count(&pb);
> +    flush_put_bits(&pb);
> +
>      h->present = 1;
>      return 0;
>  }