I have an MP4 file, and a 16kb excerpt from the file. I would like to know what frame(s) the excerpt matches. I can find the offset of the excerpt, but I believe MP4 encoding is variable-length (so I can't just say "this excerpt is from frame #(offset/framesize)") That also means that my excerpt might not be on a valid frame boundary, so I can't just rename it to "excerpt.mp4" and play it.
What's a good way to figure out what frames correspond to the excerpt?
Run
ffprobe -show_entries frame=pkt_pos,pkt_size -select_streams v -of csv=p=0:nk=1 -v 0 in.mp4
Its output will be
48|33720
54534|76
53159|974
54920|381
34070|19089
68405|520
67315|740
69279|397
55627|11356
84445|534
83571|536
85314|979
where the first column is the starting byte offset of a frame and the 2nd column is the size. Frame n appears on line # n. You can use whatever shell tools..etc to check, for which line, your discovered offset value is between position and position+size. The position series may not be monotonic, so simply checking the first column isn't reliable.