keyframe is not a keyframe? AV_PKT_FLAG_KEY does not decode to AV_PICTURE_TYPE_I

Question

After decoding a packet containing AV_PKT_FLAG_KEY in the flags, I was expecting to get I-frames, but instead I got P-frames:

After a call to:

avcodec_decode_video2(codecCtx, frame, &frameFinished, &packet); // mpeg2 video

I print out the following as a sanity check:

    printf("packet flags: %d picture type: %c\n", packet.flags,
            av_get_picture_type_char(frame->pict_type));

Returns the output:

    packet flags: 1 picture type: P

When I was expecting:

    packet flags: 1 picture type: I  

Where '1' == AV_PKT_FLAG_KEY. and 'I' refers to AV_PICTURE_TYPE_I,

Is this behavior correct? (I'm decoding MPEG2 video).

Answer 1

I got a response from the ffmpeg bugtracker:

https://ffmpeg.org/trac/ffmpeg/ticket/2074

I suspect your are missing the codec delay. The video frame you get out usually isn't based on the packet you just sent in at all. This is especially true with multithreaded decoding, but even without it's not generally possible due to how B-frames work.

And, indeed I print out the ->pict_type "picture type" 3 frames later, there is an I-frame that pops up a few frames later with a "delay" of sorts exactly a 3 frames later. In my case, I have a quad-core computer, so perhaps there's a thread being allocated for each core on my computer to do the encoding.