[FFmpeg-devel] [PATCH] libavcodec/h261dec: Fix keyframe markup and frame skipping.

[Andrey Semashev]

On 2019-10-26 15:49, Carl Eugen Hoyos wrote:
> Am Sa., 26. Okt. 2019 um 13:12 Uhr schrieb Andrey Semashev
> <andrey.semashev at gmail.com>:
>>
>> The decoder never marks pictures as I-frames, which results in no
>> keyframe indication and incorrect frame skipping, in cases when
>> keyframes should be decoded.
>>
>> This commit works around this decoder limitation and marks I-frames
>> and keyframes based on "freeze picture release" bit in h261 picture
>> header. This reflects h261enc behavior.
> 
>> diff --git a/libavcodec/h261dec.c b/libavcodec/h261dec.c
>> index 14a874c45d..3b1711a21d 100644
>> --- a/libavcodec/h261dec.c
>> +++ b/libavcodec/h261dec.c
>> @@ -502,9 +502,9 @@ static int h261_decode_picture_header(H261Context *h)
>>       s->avctx->framerate = (AVRational) { 30000, 1001 };
>>
>>       /* PTYPE starts here */
>> -    skip_bits1(&s->gb); /* split screen off */
>> -    skip_bits1(&s->gb); /* camera  off */
>> -    skip_bits1(&s->gb); /* freeze picture release off */
>> +    skip_bits1(&s->gb); /* split screen indicator */
>> +    skip_bits1(&s->gb); /* document camera indicator */
>> +    h->freeze_picture_release = get_bits1(&s->gb); /* freeze picture release */
>>
>>       format = get_bits1(&s->gb);
>>
>> @@ -532,7 +532,8 @@ static int h261_decode_picture_header(H261Context *h)
>>
>>       /* H.261 has no I-frames, but if we pass AV_PICTURE_TYPE_I for the first
>>        * frame, the codec crashes if it does not contain all I-blocks
>> -     * (e.g. when a packet is lost). */
>> +     * (e.g. when a packet is lost). We will fix the picture type in the
>> +     * output frame based on h->freeze_picture_release later. */
>>       s->pict_type = AV_PICTURE_TYPE_P;
> 
> Why can't you use freeze_picture_release here?

The comment says the decoder may crash on some input. I don't know if 
this is true and I don't have any test files not produced by ffmpeg 
itself to test.