[FFmpeg-user] Differing behaviours by fade and afade filters

[Gyan]

The video fade filter has a start_frame option which starts the fade effect
when the ordinal count of the processed frame reaches the start_frame value
e.g.

ffmpeg -f lavfi -i testsrc2=d=5 -vf trim=2,fade=s=25:d=1
vfade-works-by-frame-count.mp4

In the command above, the video output fades in starting at pts_time 3.00

whereas the command,

ffmpeg -f lavfi -i testsrc2=d=5 -vf trim=2,fade=st=1:d=1
vfade-fails-by-timestamp.mp4

fails because 'st' references timestamps and trim does not reset them.

With the afade filter, the command

ffmpeg -f lavfi -i sine=d=5 -af atrim=2,afade=ss=44100:d=1
afade-fails-by-sample-count.mp4

fails. Audio remains at native volume throughout.

But the command

ffmpeg -f lavfi -i sine=d=5 -af atrim=2,afade=ss=88200:d=1
afade-works-if-sample-count-is-pts.mp4

works. Audio fades in from pts_time 2.00 to 3.00. Which indicates that 'ss'
refers to the original sample PTS rather than the ordinal count of
processed audio samples.

Is this intended? If yes, can one emulate the behaviour of the video fade
filter. If no, should I file a bug report?

All commands executed with N-85917 git-cc25a887c5

Thanks,
Gyan