Problem in vidstabtransform

[Michael Koch]

It seems in the "vidstabtransform" filter the option "crop=black" doesn't work. Can be reproduced as follows:

rem Make a video with vertical jitter:
ffmpeg -f lavfi -i testsrc2=d=10 -lavfi sendcmd=f=cmd.txt,rgbashift=rv=0:gv=0:bv=0 -y shaky.mp4

The file cmd.txt contains these 3 lines:
0 [expr] rgbashift rv 'mod(3*N,10)';
0 [expr] rgbashift gv 'mod(3*N,10)';
0 [expr] rgbashift bv 'mod(3*N,10)';

rem Stabilize the video:
ffmpeg -i shaky.mp4 -lavfi vidstabdetect -y dummy.mp4
ffmpeg -i shaky.mp4 -lavfi vidstabtransform=crop=black:optzoom=0 -y out.mp4

I would expect sometimes to see black borders.
Either these areas are filled with other colors, or a zoom is active, which shouldn't be the case with optzoom=0.

[arifd]

Hi Michael,

Whilst I cannot explain why we don't get a black border with your input video. If we stabilize the attached video with crop=black:optzoom=0, then we do get the results as expected. In fact, we get the same results as the source of the algorithm: ​http://public.hronopik.de/vid.stab/features.php?lang=en

Also, if you set optzoom=1 on your input video, then a zoom does occur.

As such, I think we can safely assume there isn't a bug here.

Regards,
Arif Driessen

[Michael Koch]

In my opinion it's not the expected result. I would expect to see black borders, if "crop=black" is used. If this option isn't doing anything, then it can be removed.

[Michael Koch]

I saw that with the ski.mkv there are indeed black borders visible after stabilizing. Why aren't the black borders visible with my test video?

[Michael Koch]

With the ski.mkv video the no-data borders are also filled with the color of the border of the data area. That can be shown by drawing a red border around the frame, before applying vidstabtransform=crop=black. Black borders appear only when the shift is very large.

ffmpeg -i ski.mkv -lavfi vidstabdetect -y dummy.mp4
ffmpeg -i ski.mkv -lavfi drawbox=w=iw:h=ih:c=red,vidstabtransform=crop=black:optzoom=0 -y out.mp4