Implement stdout message format argument for vmaf command

[stderr-to-devnull]

When running ab-av1 vmaf, I want to keep the normal output (progress) but there should be an option for VMAF score to be printed to stdout. Basically the --stdout-format argument behavior for sample-encode command.

In Linux, I tried a workaround by attempting to store the normal output in a variable but also print it to terminal with:

vmaf_output=$(ab-av1 vmaf [...] |& tee /dev/tty)

[2025-01-11T03:41:39Z INFO  ab_av1::command::vmaf] 2%, 147 fps, eta 16 minutes
[2025-01-11T03:41:55Z INFO  ab_av1::command::vmaf] 3%, 145 fps, eta 15 minutes

Unfortunately this causes ab-av1 to change its output because it is now piped, as can be seen above. I do not find any usefulness to this behavior, especially because it prints data at a very slow interval - every 16 seconds - for whatever reason.

With an option like --stdout-format, I can at least redirect stdout to a file and read the score from there and also have the normal progress output in the terminal.

Alternatively, the --enc argument could be made available to the vmaf command and then I could use it to pass ffmpeg's -progress argument like this: --enc progress=/path/to/file.

[alexheretic]

• The change in output behaviour is useful to me at least, it doesn't make sense to output bars when stderr is not a tty better to have logged progress. This is nice when using the tool with a backgroud task manager like pueue. So I think the default is ok, but we can make this behaviour configurable.
• Yes vmaf command should support --stdout-format, it's just that no-one has asked for that before and I haven't needed it.
• --enc is for encoding rather than vmaf, but yes it would be nice to be able to pass ffmpeg args to the vmaf call here and in other commands.

[stderr-to-devnull]

TL;DR version: implementing an option to not change ab-av1 output when piped would pretty much resolve everything

Longer answer

• I understand the current rationale, though in my scenario
  • I want to keep the visual progress feedback while still storing the resulting VMAF score; the tty output has the progress bar and also the elapsed, eta and fps stats which are quite useful to get a human sense of the progress
  • this is made difficult by the non-configurable behavior when output is piped / not to a tty
  • I found a wonky workaround of tricking ab-av1 to think it is outputting to tty via a function (faketty) which I call and provide the ab-av1 command as its argument: https://stackoverflow.com/a/20401674/3130850
• implementing --stdout-format would fix the issue with storing the VMAF score separately from the captured output, though I would still need to use the faketty function to prevent ab-av1 from changing its output format and update frequency
• the --enc proposal is to leverage the current implementation of ffmpeg's -progress option
  • to exemplify and elaborate, this is how I currently implemented a custom encoding progress bar with some stats around it, by running ffmpeg in background and in a detached state, using -progress /path/to/stats_file option, then reading the encoding progress from the stats file and displaying it in the form I want
  • hopefully, being able to pass -progress /path/to/stats_file to ffmpeg via --enc, would enable me to implement a custom progress bar in a similar manner (as described above with the encoding scenario) and also store the resulting VMAF score, without the need to trick ab-av1 so I can display its own progress bar
  • this ffmpeg feature would actually be very nice to implement for ab-av1 as well (progress stats to stdout or to a file) as it would make sense in the context of its mission statement and would resolve all issues with the output; currently ab-av1 does do this in a manner of speaking but the update interval is very slow at 16s, compared to ffmpeg's default 1s (which is configurable to be even shorter if desired)