add ffmpeg audio filter support enable -af flag pass through

Signed-off-by: Stephen Grable  <stephengrable@gmail.com>

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased (on "next" branch)
 
 ### Added
+* FfmpegOutput support custom audio filter
 
 ### Changed
 

examples/ffmpeg_audio_filter.py

@@ -0,0 +1,26 @@
+#!/usr/bin/python3
+import time
+
+from picamera2 import Picamera2
+from picamera2.encoders import H264Encoder
+from picamera2.outputs import FfmpegOutput
+
+picam2 = Picamera2()
+video_config = picam2.create_video_configuration()
+picam2.configure(video_config)
+encoder = H264Encoder(10000000)
+
+# audio filter takes the left channel and copies it to the right channel
+# below example copies c0 (left channel) to c1 (right channel) - convert mono to stereo
+
+# or add audio delay on left channel like this: audio_filter="pan=stereo|adelay=1500|0"
+# source for more examples: https://ffmpeg.org/ffmpeg-filters.html#Examples-2
+output = FfmpegOutput(
+    'ffmpeg_audio_filter_test.mp4',
+    audio=True,
+    audio_filter="pan=stereo|c0=c0|c1=c0"
+)
+
+picam2.start_recording(encoder, output)
+time.sleep(10)
+picam2.stop_recording()

picamera2/outputs/ffmpegoutput.py

@@ -33,12 +33,13 @@ class FfmpegOutput(Output):
     """
 
     def __init__(self, output_filename, audio=False, audio_device="default", audio_sync=-0.3,
-                 audio_samplerate=48000, audio_codec="aac", audio_bitrate=128000, pts=None):
+                 audio_samplerate=48000, audio_codec="aac", audio_bitrate=128000, audio_filter='pan=stereo', pts=None):
         super().__init__(pts=pts)
         self.ffmpeg = None
         self.output_filename = output_filename
         self.audio = audio
         self.audio_device = audio_device
+        self.audio_filter = audio_filter
         self.audio_sync = audio_sync
         self.audio_samplerate = audio_samplerate
         self.audio_codec = audio_codec
@@ -70,7 +71,8 @@ def start(self):
                            '-thread_queue_size', '1024',  # necessary to prevent warnings
                            '-i', self.audio_device]
             audio_codec = ['-b:a', str(self.audio_bitrate),
-                           '-c:a', self.audio_codec]
+                           '-c:a', self.audio_codec,
+                           -'af', self.audio_filter]
 
         command = ['ffmpeg'] + general_options + audio_input + video_input + \
             audio_codec + video_codec + self.output_filename.split()