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23rd February 2018, 02:58 | #2 | Link | |
Retried Guesser
Join Date: Jun 2012
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'K' is from the original Vdub code, 'I' ('Intra') is from ffmpeg-based code. Probably.
https://github.com/FFMS/ffms2/blob/m...s2-avisynth.md Quote:
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23rd February 2018, 04:09 | #3 | Link |
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Here's a 5s clip (29 MB) MP4 from a Panasonic HC-V770 camcorder. It's nominally 50Mbps 1080p50. I've losslessly cut it on key-frames using ADM:
https://dl.dropboxusercontent.com/s/...p50%20clip.mp4 The GOP length appears to be 24 frames. The latest Vdub Filtermod still reports the frame-type sequence as: KBIBBPBBPBBPBBPBBPBBPBBP I'm hoping someone can confirm this sequence and explain why the 3rd frame in this sequence is showing as (I)? Dan. Last edited by dosdan; 23rd February 2018 at 05:23. |
23rd February 2018, 04:47 | #4 | Link |
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coded order (how it's stored) can differ from display order (what you see)
In display order, the first frame, frame zero, is a "B" frame . I don't know why it uses "K" notation for this . The I is correct for display order (it's frame 2 starting counting from zero) . In coded order the I-frame is zero. |
23rd February 2018, 06:26 | #5 | Link |
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Vdub FilterMod offers two ways of opening a file:
Caching Input Driver FFMPEG The program was using the caching input driver to open the clip. When I switched it to FFMPEG the frame-types changed to: BBIBBPBBPBBPBBPBBPBBPBBP So it appears to be coming from the caching input driver. I'll start a ticket to see if I can find out what's going on. Dan. Last edited by dosdan; 23rd February 2018 at 06:30. |
23rd February 2018, 13:24 | #6 | Link |
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K identifies keyframe. AFAIK the real keyframe is supposed to be IDR but here it is offset for some reason.
I noticed this behavior long ago: when the stream begins with B-frames the "keyframe" flags are offset. In theory it should be BBKBBP... Unfortunately dealing with keyframes in FFmpeg is unsupported/loose. What I have is best guess effort.
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VirtualDub2 |
5th March 2018, 20:04 | #7 | Link |
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Location: Poland
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ffmpeg should distinguish IDR frames which are key frames and "normal" I frames.
Not every I frame has to be an IDR frame. This is an explanation from x264 thread: IDR-Frame clears the reference picture buffer, i.e. no frame after IDR can reference any frame before IDR. That's not the case with "normal" I-Frame, i.e. P- or B-Frame after I-Frame can use as reference frames before I-Frame. In other words: You are always "cleanly" start decoding at an IDR-Frame, which is not (in general) possible with a "normal" I-Frame. Last edited by kolak; 5th March 2018 at 20:06. |
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