without reencoding) without unexpected adjustments to output video stream start time. I understand that this issue may be dependent on the codecs used. I used only one file above for ease of reproduction.
FFMPEG CONCAT DEMUXER SEEK MP4
Note that this issue occurs when concat demuxer is used to concatenate multiple mp4 files too. In the image linked below, the left is what we see when first opening before.mp4 the right is what we see when first opening concat.mp4: This causes the processed video, when opened using QuickTime player on Mac, to initially display a blank frame, although the rest of the video plays normally. Run command ffprobe concat.mp4 -show_streams and observe that video stream start_time and start_pts are now non-zero (in my tests, the magnitude of this shift is small, on the scale of 0 -> 0.022969).
![ffmpeg concat demuxer seek ffmpeg concat demuxer seek](https://media.vlpt.us/images/tlatldms/post/330b1fb0-84ef-4283-ab1c-2836b5d0ba4e/image.png)
![ffmpeg concat demuxer seek ffmpeg concat demuxer seek](https://i.stack.imgur.com/SX6WT.gif)
Run command ffmpeg -f concat -i concat.txt -c copy concat.mp4 Given concat demuxer input file concat.txt with contents: file before.mp4 Given an before.mp4 containing video and audio streams both with start_time=0 and start_pts=0. Ffmpeg concat demuxer shifts video stream start time when invoked with -c copy option.