Difference between i-frames and key frames
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
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. The amount of inter-frames (P and B) between intra-frames can vary, depending on codec settings and source material. Most commonly used with mp4 is h264 nowadays, but multiple other codecs are supported by this container. When using closed GOP, there is a defined number of frames between I-frames, unless de encoder senses the need for an extra I-frame like on scene changes. When used in open GOP, there can be very many B and P
On 07-10-2023 17:20, Ulf Zibis wrote: frames in between I-frames, depending on the amount of changes in the source. Open GOP will be more efficient in some cases, like streaming media because generally it uses less bandwidth and has better compression as I-frames only occur on scene changes when the encoder deems this necessary. When you want media to be searchable (fast forward/reverse) with fixed intervals, you need closed GOP. I do not know what the default for ffmpeg is.
On 10/7/23 16:28, Ferdi Scholten 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'.
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
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.
participants (5)
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Ferdi Scholten -
Mark Filipak -
markfilipak.imdb＠gmail.com -
Nicolas Gaullier -
Ulf Zibis