BtbN changed the topic of #ffmpeg to: Welcome to the FFmpeg USER support channel | Development channel: #ffmpeg-devel | Bug reports: https://ffmpeg.org/bugreports.html | Wiki: https://trac.ffmpeg.org/ | This channel is publically logged | FFmpeg 7.0 is released
<lmat>
I have a video that's a bit messed up. The audio is all fine, and the video starts fine, but then it loops back like 10 seconds.
<lmat>
I would like to create a new video with the audio from the original, and the first 15 seconds or so of the original, then skip maybe 10 seconds of video, then of course I'm missing some video, so a still image or all black frame for the rest of the video is fine.
<lmat>
How do I piece it to gether like that?
<lmat>
I've done a little video editing to match up an audio recording using higher-end equipment with some video footage, but it seems different now since there are pieces of video.
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<Cracki>
in the context of H.264, are "I-frames" an actual thing in the bitstream, or is that merely a description of a frame that "happens" to contain only intra slices?
<JEEB>
see table 7-1 - NAL unit type codes, syntax element categories and NAL unit type classes
<Cracki>
thx, looking it up
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<JEEB>
but yea, H.264 is really barebones with regards to VCL NAL units
<Cracki>
hate to be like this but "some dude said" that "I frame" refers to "inter" (not intra), and when I questioned that, he said "The one's I dealt with use slice groups and/or macroblocks from previous and future frames". IDK what to make of that.
<Cracki>
so far I gather that the term "I frame" is a colloquialism rather than a term used in specs
<furq>
if he said the first thing you can probably just ignore everything else he says
<Cracki>
thx, I had that impression of that particular individual from other matters that I am competent in
<JEEB>
well "I slice" is at least clearly defined, and given that frames are just VCL NAL units that are either IDR or not.
<furq>
i frame is a real term for other codecs but it doesn't cleanly apply to h264
<furq>
usually when people say it they mean idr frame
<JEEB>
so if you look at H.264 you can look up "I slice", and I'd say if someone is talking about "I frame" that then comes out of that
<Cracki>
my working definition (rough, hardly refined) of "I frame" is that it refers to a frame made up of I-slices exclusively, with or without IDR flag
<furq>
it can mean that as well
<JEEB>
dunno, I think FFmpeg flags intra only as intra only, and then the random access point flag is separate
<Cracki>
hmmm
<furq>
i was actually about to ask what ffprobe declares to be an i frame
<JEEB>
since you can't necessarily decode further than a normal intra only sliced frame
<furq>
i think i have some scripts that assume it means idr frame
<JEEB>
flags K for random access point in ffprobe
<Cracki>
do I understand this right, that a frame could be flagged as IDR but consist of P/B slices? that sounds interesting
<furq>
it can't contain B slices
<JEEB>
So H.264 uses the definitions "IDR access unit" and "IDR picture"
<Cracki>
... right, because B needs a reference on both sides
<JEEB>
IdrPicFlag needs to be 1 and I guess that then limits what slices there can be in that picture
<Cracki>
grepping T-REC-H.264-whatever.pdf, its first use of "access unit" is not preceded by a defintion of it. where would I want to look for a def of "access unit"?
<Cracki>
nvm, couple pages down
<Cracki>
to recap, and I'm learning to use those terms on the fly... an IDR access unit _can_ contain P-slices... but what can the reference, if it's flagged IDR? there are no decoded pictures... some sentence I read somewhere implied that P-slices can reference the picture that's currently being decoded
<Cracki>
*can they
<Cracki>
uhhhh what am I reading here? An IDR picture causes the decoding process to mark all reference pictures as "unused for reference" immediately after the decoding of the IDR picture.
<Cracki>
AFTER
<JEEB>
yea, that is a funky definition indeed :)
<JEEB>
but yup, was going to point you towards that definition xD
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<Cracki>
reads to me like it can use whatever's in the reference pics...
<JEEB>
and then it specifically talks about the stuff that follows it in decoding order
<Cracki>
o.o
<Cracki>
that sounds like IDR is the flag to put on the last picture of a GOP, not the first
<Cracki>
still reading/thinking
<Cracki>
ok yes I can kinda see how it conforms to my previous understanding (pics following an IDR pic can't reference to before it), but I didn't expect an IDR picture to be allowed to use references
<Cracki>
ok great, so IDR pics can use P-slices, but not B-slices, which is good to know. I think I'm starting to see where "that dude" was coming from
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<Cracki>
maybe just a mixup of terms. I can excuse that. girls are singing in the backyard. the cats join in.
<Cracki>
thx for the lesson in video things
<Cracki>
they sound drunk. not the cats. anyway. I think I can imagine an IDR picture that does use B-slices, if the next frame was already decoded
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<furq>
i've lost confidence in what i said about b slices
<furq>
but i'm not reading all 836 pages of this to find out
<Cracki>
understandable :D
<Cracki>
I tried claude 3 yesterday. it refused to choke on a mere 50 pages (a fraction of) a data sheet of some accelerometer chip
<Cracki>
free tier but whatever. I'm waiting for the day when these things will be able to handle such "context" in smarter ways
<furq>
i hope you've got a good calendar
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<Cracki>
I think the 2024 one will suffice
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