Re: [FFmpeg-devel] [PATCH 5/5] avcodec/mpeg4videodec: duplicate the last decoded frame when the last coded frame was skipped

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> This ensures the video stream duration is not lost after decoding.
> 
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
>  libavcodec/h263dec.c   | 13 +++++++++++++
>  libavcodec/mpegvideo.h |  1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/libavcodec/h263dec.c b/libavcodec/h263dec.c
> index ac7a8521e5..0a2d7487a8 100644
> --- a/libavcodec/h263dec.c
> +++ b/libavcodec/h263dec.c
> @@ -430,6 +430,18 @@ int ff_h263_decode_frame(AVCodecContext *avctx, AVFrame *pict,
>                  return ret;
>              s->next_picture_ptr = NULL;
>  
> +            *got_frame = 1;
> +        } else if (s->skipped_last_frame && s->current_picture_ptr) {
> +            /* Output the last picture we decoded again if the stream ended with
> +             * an NVOP */
> +            if ((ret = av_frame_ref(pict, s->current_picture_ptr->f)) < 0)
> +                return ret;
> +            /* Copy props from the last input packet. Otherwise, props from the last
> +             * returned picture would be reused */
> +            if ((ret = ff_decode_frame_props(avctx, pict)) < 0)
> +                return ret;
> +            s->current_picture_ptr = NULL;
> +
>              *got_frame = 1;
>          }
>  
> @@ -500,6 +512,7 @@ retry:
>              s->extradata_parsed = 1;
>          }
>          ret = ff_mpeg4_decode_picture_header(avctx->priv_data, &s->gb, 0, 0);
> +        s->skipped_last_frame = (ret == FRAME_SKIPPED);
>      } else if (CONFIG_H263I_DECODER && s->codec_id == AV_CODEC_ID_H263I) {
>          ret = ff_intel_h263_decode_picture_header(s);
>      } else if (CONFIG_FLV_DECODER && s->h263_flv) {
> diff --git a/libavcodec/mpegvideo.h b/libavcodec/mpegvideo.h
> index 6440b906b1..42275953b9 100644
> --- a/libavcodec/mpegvideo.h
> +++ b/libavcodec/mpegvideo.h
> @@ -175,6 +175,7 @@ typedef struct MpegEncContext {
>      Picture *last_picture_ptr;     ///< pointer to the previous picture.
>      Picture *next_picture_ptr;     ///< pointer to the next picture (for bidir pred)
>      Picture *current_picture_ptr;  ///< pointer to the current picture
> +    int skipped_last_frame;
>      int last_dc[3];                ///< last DC values for MPEG-1
>      int16_t *dc_val_base;
>      int16_t *dc_val[3];            ///< used for MPEG-4 DC prediction, all 3 arrays must be continuous

Can you give an example where this matters? And what does the spec say
about this? Is "output the last frame again" really the appropriate
response upon encountering a NVOP?

- Andreas

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