I was thinking progressive MP4, with sample table in the moov. But yes, cmaf and other fragmented MP4 profiles have ftyp and moov at the front, too. Rather th

I was thinking progressive MP4, with sample table in the moov. But yes, cmaf and other fragmented MP4 profiles have ftyp and moov at the front, too. Rather than putting the media in a contiguous blob, CMAF interleaves it with moofs that hold the sample byte ranges and timing. Moreover, while this interleaving allows most of the CMAF file to be progressively streamed to disk as the media is created, it has the same CATCH22 problem as the "progressive" MP4 file in that the index (sidx, in case of CMAF) cannot be written at the start of the file unless all the media it indexes has been processed. When writing CMAF, ffmpeg will usually omit the segment index which makes fast search painful. To insert the `sidx` (after ftyp+moov but before the moof+mdat s) you need to repackage (but not re-encode). Same problem, same solution more or less.