[FFmpeg-user] xfade timing ambiguity

[Arif Driessen]

Consider this script:

---
#!/bin/bash

ffmpeg \
    -v error \
    -filter_complex \
    "
        color=blue,
        trim=0:3
        [blue];

        color=red,
        trim=0:5
        [red];

        [blue][red]
        xfade
    " \
    -vcodec libx264 \
    -f matroska out.mkv && \
    ffprobe -v error \
    -select_streams v \
    -of csv=print_section=0 \
    -show_entries format=duration \
    out.mkv

exit 0
---

It will report the duration of the resultant mkv file is 5 seconds.
Intuition would tell me to expect it to be 10 seconds long.

Change the duration of [blue] to 3 seconds, and the resultant duration is
still 5 seconds. Aha! So xfade blends the first input INTO the second. I
don't believe the documentation is clear on this:

> Apply cross fade from one input video stream to another input video
stream. The cross fade is applied for specified duration

Out of curiosity, what would happen if we keep the duration of [blue] to 3,
but set the xfade=offset to 2 seconds, since xfade=duration by default is
1... Now the resultant mkv file is 7 seconds long!? Can you explain the
rationale?

Even weirder, now let's set the duration of [blue] to 3 seconds and make
xfade=offset 3 seconds also... Now the resultant mkv file is 3 seconds
long!! Is this just undefined behaviour because we have attempted illegal
behaviour? (offset + duration >= input_duration)

Should we update the documentation to be clearer on this? (I am happy to do
the work). It was the source of a frustating timing issue that took me all
day to resolve. (back when I was emailing here about concating and fading
sources).

Btw, acrossfade, works intuitively, and thus (for me) without fault. This
concern/ambiguity is also expressed in Michael Koch's FFmpeg book.

Thanks,
Arif Driessen