Support concat a/v params

Author: kkroening

examples/README.md

@@ -57,6 +57,21 @@ out, _ = (ffmpeg
 )
 ```
 
+## Process audio and video simultaneously
+```python
+in1 = ffmpeg.input('in1.mp4')
+in2 = ffmpeg.input('in2.mp4')
+v1 = in1['v'].hflip()
+a1 = in1['a']
+v2 = in2['v'].filter_('reverse').filter_('hue', s=0)
+a2 = in2['a'].filter_('areverse').filter_('aphaser')
+joined = ffmpeg.concat(v1, a1, v2, a2, v=1, a=1).node
+v3 = joined[0]
+a3 = joined[1].filter_('volume', 0.8)
+out = ffmpeg.output(v3, a3, 'out.mp4')
+out.run()
+```
+
 ## [Jupyter Frame Viewer](https://github.com/kkroening/ffmpeg-python/blob/master/examples/ffmpeg-numpy.ipynb)
 
 <img src="https://raw.githubusercontent.com/kkroening/ffmpeg-python/master/doc/jupyter-screenshot.png" alt="jupyter screenshot" width="75%" />

ffmpeg/_filters.py

@@ -372,7 +372,14 @@ def concat(*streams, **kwargs):
 
     Official documentation: `concat <https://ffmpeg.org/ffmpeg-filters.html#concat>`__
     """
-    kwargs['n'] = len(streams)
+    video_stream_count = kwargs.get('v', 1)
+    audio_stream_count = kwargs.get('a', 0)
+    stream_count = video_stream_count + audio_stream_count
+    if len(streams) % stream_count != 0:
+        raise ValueError(
+            'Expected concat input streams to have length multiple of {} (v={}, a={}); got {}'
+                .format(stream_count, video_stream_count, audio_stream_count, len(streams)))
+    kwargs['n'] = int(len(streams) / stream_count)
     return FilterNode(streams, concat.__name__, kwargs=kwargs, max_inputs=None).stream()
 
 

ffmpeg/tests/test_ffmpeg.py

@@ -221,6 +221,83 @@ def _get_complex_filter_asplit_example():
     )
 
 
+def test_filter_concat__video_only():
+    in1 = ffmpeg.input('in1.mp4')
+    in2 = ffmpeg.input('in2.mp4')
+    args = (
+        ffmpeg
+        .concat(in1, in2)
+        .output('out.mp4')
+        .get_args()
+    )
+    assert args == [
+        '-i',
+        'in1.mp4',
+        '-i',
+        'in2.mp4',
+        '-filter_complex',
+        '[0][1]concat=n=2[s0]',
+        '-map',
+        '[s0]',
+        'out.mp4',
+    ]
+
+
+def test_filter_concat__audio_only():
+    in1 = ffmpeg.input('in1.mp4')
+    in2 = ffmpeg.input('in2.mp4')
+    args = (
+        ffmpeg
+        .concat(in1, in2, v=0, a=1)
+        .output('out.mp4')
+        .get_args()
+    )
+    assert args == [
+        '-i',
+        'in1.mp4',
+        '-i',
+        'in2.mp4',
+        '-filter_complex',
+        '[0][1]concat=a=1:n=2:v=0[s0]',
+        '-map',
+        '[s0]',
+        'out.mp4'
+    ]
+
+
+def test_filter_concat__audio_video():
+    in1 = ffmpeg.input('in1.mp4')
+    in2 = ffmpeg.input('in2.mp4')
+    joined = ffmpeg.concat(in1['v'], in1['a'], in2.hflip(), in2['a'], v=1, a=1).node
+    args = (
+        ffmpeg
+        .output(joined[0], joined[1], 'out.mp4')
+        .get_args()
+    )
+    assert args == [
+        '-i',
+        'in1.mp4',
+        '-i',
+        'in2.mp4',
+        '-filter_complex',
+        '[1]hflip[s0];[0:v][0:a][s0][1:a]concat=a=1:n=2:v=1[s1][s2]',
+        '-map',
+        '[s1]',
+        '-map',
+        '[s2]',
+        'out.mp4',
+    ]
+
+
+def test_filter_concat__wrong_stream_count():
+    in1 = ffmpeg.input('in1.mp4')
+    in2 = ffmpeg.input('in2.mp4')
+    with pytest.raises(ValueError) as excinfo:
+        ffmpeg.concat(in1['v'], in1['a'], in2.hflip(), v=1, a=1).node
+    assert str(excinfo.value) == \
+        'Expected concat input streams to have length multiple of 2 (v=1, a=1); got 3'
+
+
 def test_filter_asplit():
     out = _get_complex_filter_asplit_example()
     args = out.get_args()