How to make video play with correct aspect ratio

Question

I have the following issue: I have a video file (h264) that plays with the correct aspect ratio in mplayer. However, if I load it into Intel's Computer Vision Annotation Tool, or if I re-encode the video using openCV, the aspect ratio is no longer correct.

"ffprobe -i" on the original file gives

1440x1080 [SAR 4:3 DAR 16:9]

whereas for the re-encoded file I get

1440x1080 [SAR 1:1 DAR 4:3]

I have other video files with

1920x1080 [SAR 1:1 DAR 16:9]

that do not cause this issue.

Do I just have to modify the SAR in the header or is the problem more complicated? I would like to preserve the codec if possible.

Answer 1

For a H264 stream, use the h264_metadata bitstream filter

ffmpeg -i in.mp4 -c copy -bsf:v "h264_metadata=sample_aspect_ratio=4/3" out.mp4

Answer 2

If you want to change the Sample Aspect Ratio (aka Pixel Aspect Ratio) without reencoding use mkvmerge or mp4box:

mkvmerge

mkvmerge -o "out.mp4" --aspect-ratio-factor 0:4/3 "in.mp4"

mp4box

mp4box -add in.mp4 -par 1=4:3 out.mp4

In ffmpeg you cannot combine a filter with -c copy ...

ffmpeg -i in.mp4 -vf setsar=sar=4/3 -map 0 -c copy out.mp4

because

Filtering and streamcopy cannot be used together.