On 10/30/23 14:16, Nicolas Gaullier wrote: >>> On 07-10-2023 17:20, Ulf Zibis wrote: >>>> Hi, >>>> >>>> what is the difference between i-frames and key frames? >>>> >>>> And how many other frames are between i-frames or key frames in mp4 >>>> files, created with ffmpeg and with default settings? >>>> >>>> -Ulf >>> In mp4 containers I-frames are key frames.> >> >> That is not correct. Via ffprobe, I have seen I-frames that were not marked >> 'key_frame=1'. > > Firstly, the definition of what a key frame is may depend on contexts and on > peoples, > it is often a source of headaches. > If you take a look at > https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/h264_parser.c > You can see that properly signaled recovery points are tagged as keyframes, > which means you don't need an I-frame to make a keyframe (some low latency > encoders make use of this). > And you can also see that kind of things: > // heuristic to detect non marked keyframes > if (p->ps.sps->ref_frame_count <= 1 && p->ps.pps->ref_count[0] <= 1 && > s->pict_type == AV_PICTURE_TYPE_I) > s->key_frame = 1; > as indeed, an I frame is not always a keyframe (long term references may be > required later for further processing). > > And, at the end, sorry for not answering your question ;) > > Nicolas
Thank you, Nicolas. Note that I code, but not 'C'. Regarding 'https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/h264_parser.c' line# / 273 s->pict_type=AV_PICTURE_TYPE_I; 274 s->key_frame=0; 338 switch(nal.type) { 349 case H264_NAL_IDR_SLICE: 350 s->key_frame=1; 357 case H264_NAL_SLICE: 361 if(p->sei.recovery_point.recovery_frame_cnt>=0)s->key_frame=1; 382 if ( p->ps.sps->ref_frame_count<=1 &&p->ps.pps->ref_count[0]<=1 &&s->pict_type==AV_PICTURE_TYPE_I )s->key_frame=1; 571 } What is 's'? What is 'p'? What does '->' mean?/do? [note 1] What is 'ps'? What is 'ps.sps'? What is H264_NAL_IDR_SLICE? [note 2] What is H264_NAL_SLICE? What is recovery_point? I think I know (guess) what everything else is/means. [note 1] Yes, I think that's a pointer. Please don't waste your time explaining pointers. [note 2] I've never seen an explanation what 'NAL' means. I think what's meant is 'network adaption layer', though I don't know what it has to do with networking, so I'm probably wrong. I must say that I don't care for the code. For example, 'pict_type' is initialized to 'AV_PICTURE_TYPE_I' and there's nothing between that, and the 'switch', and the 'case' that changes it, yet the case seems to test 'pict_type==AV_PICTURE_TYPE_I'. Without knowing 'C', I nonetheless have taught 'C' codesmiths how to build state machines that have 100% coverage and are 100% testable. In my experience, almost all bugs come from 'accidental' state machines using 'unrecognized' state variables implemented by threaded ('spaghetti') code. --Mark. _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
