Simple stream selection?

[whichken]

I'm trying to do some simple stream selection. Using ffmpeg directly, I'd use the map option. However, when trying to do this using ffmpeg-python, I can't seem to get it correct. The map argument is getting put into a bracket, which doesn't work.

>>> import ffmpeg
>>> infile = ffmpeg.input('test.mkv')
>>> stream = infile['1']
>>> outfile = ffmpeg.output(stream, 'test-out.mkv')
>>> ffmpeg.compile(outfile)
['ffmpeg', '-i', 'test.mkv', '-map', '[0:1]', 'test-out.mkv']

This fails with the error Output with label '0:1' does not exist in any defined filter graph, or was already used elsewhere.

What is the proper way to do stream selection when you don't need complex video graphs?

[alphabetaxz]

what you need may be: infile[0:'v']

[whichken]

That throws an exception.

>>> stream = infile[0:'v']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/project/env/lib/python3.5/site-packages/ffmpeg/nodes.py", line 63, in __getitem__
    raise TypeError("Expected string index (e.g. 'a'); got {!r}".format(index))
TypeError: Expected string index (e.g. 'a'); got slice(0, 'v', None)

[alphabetaxz]

sorry, i misunderstand what you mean, if you only want to select some streams from source to output without any filters ,i think you can use "map" option of ffmpeg, eg:

infile = ffmpeg.input('test.mkv')
outfile = ffmpeg.output(infile, 'test-out.mkv', map="0:v")
ffmpeg.run(outfile)

[whichken]

Thanks @alphabetaxz. We're getting real close. While that worked for a single stream, I can't use it for multiple streams since you can't duplicate kwargs.

So I can't do the equivalent of ffmpeg -i test.mkv -map 0:1 -map 0:2 out.mkv

Since the kwargs get passed verbatim, I tried to get clever and do this outfile = ffmpeg.output(infile, 'test-out.mkv', map="0:1 -map 0:2") but the space doesn't compile right and everything after the 0:1 is ignored.

Any ideas?

[alphabetaxz]

sorry , about this

[kkroening]

@whichken What's the raw ffmpeg command you're trying to emulate with ffmpeg-python?

(edit): Oh I see:

ffmpeg -i test.mkv -map 0:1 -map 0:2 out.mkv

Ok yeah, we need to figure out a way to support this. AFAICT there's no way to do this currently, or at least not in any reasonable manner.

[kkroening]

@whichken I think your example syntax in the first comment makes the most sense:

import ffmpeg
infile = ffmpeg.input('test.mkv')
outfile = ffmpeg.output(infile['1'], 'test-out.mkv')

And similarly for multiple streams:

infile = ffmpeg.input('test.mkv')
outfile = ffmpeg.output(infile['1'], infile['2'], 'test-out.mkv')

I consider it a bug that -map [0:1] is produced instead of -map 0:1. Will try to make a fix soon.

[kkroening]

Also, the [] operator should probably be able to accept integers (in addition to strings, as currently supported).

[kkroening]

Or perhaps eventually this shorthand would make things simpler:

infile = ffmpeg.input('test'.mk4')
outfile = ffmpeg.output(infile[[1, 2]], 'test-out.mkv')

or fluently:

(
    ffmpeg
    .input('test.mk4')
    [[1, 2]]
    .output('test-out.mkv')
    .run()
)

The [] operator would basically take either a single identifier (e.g. ['v']) or a list of identifiers (e.g. [[1, 2]]). This is similar to how a library like Pandas operates.

[whichken]

All those options look great.

I was going to suggest that the produced -map [0:1] was a bug, but I'm not familiar enough with ffmpeg to know if that was some other format that I was unfamiliar with.

Let me know if I can help out at all.

[kkroening]

#103 should fix this.

@whichken If you get a chance, let me know if #103 fixes this for you.

Regardless, I'll merge it in the next day or so if I don't hear back.

[whichken]

@kkroening I can confirm that this fixes my issue. Thanks for the quick turn around!