The tpad filter is problematic on the variable-framerate webcam files,
and the result can end up being hangs (or, at least, very slow
processing) in the compositing.
Move the tpad filter to the compositing process where it can run after
the fps filter has converted the video to constant framerate. It still
needs to run before the start trimming, so switch to using the trim
filter rather than the fps filter's start_pts feature.
Even with the filter changes made, there's still some cases where
filter chain hangs can result from filter reconfigurations. To solve the
issue completely, I have split out pre-processing video files to
separate ffmpeg processes, so that the filter chain for compositing will
not ever be reconfigured.
Each input video now has a separate ffmpeg process run for it which
does the scaling, padding, and video extending steps. To avoid issues
with disk space usage and extra cpu usage or quality loss, the output
from these separate processes is sent to the compositing ffmpeg process
as uncompressed video in a pipe. To simplify the setup, named pipes
(special files) are used rather than setting up pipes in the ruby code
programmatically.
The extra ffmpeg processes are configured to log to files, and when
complete their log output is copied to the recording processing log.
Processes are joined to ensure zombie processes are not created, and
the return codes of all the processes are checked so errors can be
detected.
Due to the overhead of transferring video through pipes, this might
be a bit slower than the 2.4 recording processing - but on the other
hand, some of the video decoding and scaling happens in parallel, so it
might balance out.
Because the input videos for BigBlueButton recording processing switch
resolution and aspect ratio, the filter chain gets re-initialized, and
any state in the filters is lost. This causes problems with the
following filters:
`color`: Timestamps restart from 0, rather than continuing at the point
where they left off.
`fps=start_time=12.345`: After reset, the fps filter thinks it's at the
start of the file again, so the next frame it sees gets duplicated
output for timestamps from the `start_time` until it catches back up.
`setpts=PTS-STARTPTS`: The 'STARTPTS' is re-read as the first pts the
filter sees after reinitialization, so timestamp of the next frame is
reset to 0. (In practise, this didn't cause any problems because the
duplicate frames created by the fps filter had the original start time.)
The end result of all of these issues is that a lot of duplicate frames
were created with invalid timestamps, which eventually get discarded
by ffmpeg. But a lot of time is wasted, causing recordings to sometimes
take hours to process when they should be ready in minutes.
The fixes are as follows:
* The `color` filters are used to generate the background and
substitutes for missing videos. Move them out to separate filter
chains by using the 'lavfi' input format, which lets you use a filter
as if it was an input file.
* Rather than use the `fps` filter's `start_time` feature, use the
`trim` filter to remove early frames.
* The actual start pts is already known by the script, so replace
`setpts=PTS-STARTPTS` with `setpts=PTS-12.345/TB`, substituting in the
absolute time.
