Pass-throughs should not use `-map`

[kkroening]

This currently produces the wrong ffmpeg args:

>>> ' '.join(ffmpeg
...     .merge_outputs(
...         (ffmpeg
...             .input('in1.mp4')
...             .output('out1.mp4')
...         ),
...         (ffmpeg
...             .input('in2.mp4')
...             .output('out2.mp4')
...         )
...     )
...     .get_args()
... )
u'-i in1.mp4 -i in2.mp4 out1.mp4 -map [1] out2.mp4'

The -map should not be there, since no -filter_complex param is needed.

Should be fairly easy to fix by omitting -map for passthroughs and generating output params in the correct order.

See test_multi_passthrough in feature/17 (or master once #17 is merged).

[kkroening]

Hmm, actually it might be a little more non-trivial. Take the following for example:

ffmpeg.view([
        (ffmpeg
            .input('in1.mp4')
            .output('out1.mp4')
        ),
        (ffmpeg
            .input('in2.mp4')
            .output('out2.mp4')
        ),
        (ffmpeg
            .input('in3.mp4')
            .hflip()
            .output('out3.mp4')
        ),
        (ffmpeg
            .input('in3.mp4')
            .vflip()
            .output('out4.mp4')
        ),
        (ffmpeg
            .input('in5.mp4')
            .hflip()
            .split()[0]
            .output('out5.mp4')
        ),
        (ffmpeg
            .input('in5.mp4')
            .hflip()
            .split()[1]
            .vflip()
            .output('out6.mp4')
        ),
])

This has four connected components, two of which are passthroughs and two of which use filters.

For fully connected graphs, this is a non-issue, so it seems to be an issue if one or more passthroughs is used and multiple connected components are present.

There are two solutions that I know of:

• Inject a dummy filter (e.g. split=n=1) into passthroughs so that they become part of the -filter_complex and thus get a stream number assigned so that -map can be used. Probably the simplest way to go.

• Detect the individual connected components and output each passthrough individually, and combine any that use filters. The picture above would be broken up as follows:

e.g. (bash)

$ (ffmpeg
    # first connected component (passthrough - no filters)
    -i in1.mp4 out1.mp4

    # second connected component (passthrough - no filters)
    -i in2.mp4 out2.mp4

    # third and fourth connected components using filter_complex
    -i in3.mp4
    -i in5.mp4
    -filter_complex '
        [2]vflip[s1];
        [s1]split[s2][s3];
        [3]hflip[s4];
        [s4]split=n=2[s5][s6];
        [s5]vflip[s7]
    '
    -map s2 out3.mp4
    -map s3 out4.mp4
    -map s5 out5.mp4
    -map s6 out6.mp4
)

The two passthroughs are highlighted in orange and the two filter-based components are combined in the purple box.

This is certainly more complicated but it gets rid of the need for an extra dummy filter. I'm not sure if a dummy filter affects performance in any way, so it's almost certainly best to do it the simple way first and then optimize later. Good to know that there's a way to do it in a more ideal way if needed though.

[kkroening]

(oops, forgot a split in the in3 -> out3 +out4 component; oh well...)

[kkroening]

Fixed by #103.