filter/ffmpeg: detect the output sample format

NEWS

 ver 0.22.4 (not yet released)
 * decoder
   - dsdiff: apply padding to odd-sized chunks
+* filter
+  - ffmpeg: detect the output sample format
 * output
   - moveoutput: fix always_on and tag lost on move
 

src/filter/plugins/FfmpegFilterPlugin.cxx

@@ -23,6 +23,7 @@
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
 #include "lib/ffmpeg/Filter.hxx"
+#include "lib/ffmpeg/DetectFilterFormat.hxx"
 #include "config/Block.hxx"
 
 class PreparedFfmpegFilter final : public PreparedFilter {
@@ -60,7 +61,9 @@ PreparedFfmpegFilter::Open(AudioFormat &in_audio_format)
 
 	graph.CheckAndConfigure();
 
-	auto out_audio_format = in_audio_format; // TODO
+	const auto out_audio_format =
+		Ffmpeg::DetectFilterOutputFormat(in_audio_format, *buffer_src,
+						 *buffer_sink);
 
 	return std::make_unique<FfmpegFilter>(in_audio_format,
 					      out_audio_format,

src/lib/ffmpeg/DetectFilterFormat.cxx

+/*
+ * Copyright 2003-2020 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "DetectFilterFormat.hxx"
+#include "Frame.hxx"
+#include "SampleFormat.hxx"
+#include "pcm/Silence.hxx"
+#include "pcm/CheckAudioFormat.hxx"
+#include "util/WritableBuffer.hxx"
+
+extern "C" {
+#include <libavfilter/buffersrc.h>
+#include <libavfilter/buffersink.h>
+}
+
+#include <cassert>
+
+namespace Ffmpeg {
+
+AudioFormat
+DetectFilterOutputFormat(const AudioFormat &in_audio_format,
+			 AVFilterContext &buffer_src,
+			 AVFilterContext &buffer_sink)
+{
+	uint_least64_t silence[MAX_CHANNELS];
+	const size_t silence_size = in_audio_format.GetFrameSize();
+	assert(sizeof(silence) >= silence_size);
+
+	PcmSilence(WritableBuffer<void>{&silence, silence_size},
+		   in_audio_format.format);
+
+	Frame frame;
+	frame->format = ToFfmpegSampleFormat(in_audio_format.format);
+	frame->sample_rate = in_audio_format.sample_rate;
+	frame->channels = in_audio_format.channels;
+	frame->nb_samples = 1;
+
+	frame.GetBuffer();
+
+	memcpy(frame.GetData(0), silence, silence_size);
+
+	int err = av_buffersrc_add_frame(&buffer_src, frame.get());
+	if (err < 0)
+		throw MakeFfmpegError(err, "av_buffersrc_add_frame() failed");
+
+	frame.Unref();
+
+	err = av_buffersink_get_frame(&buffer_sink, frame.get());
+	if (err < 0)
+		throw MakeFfmpegError(err, "av_buffersink_get_frame() failed");
+
+	const SampleFormat sample_format = FromFfmpegSampleFormat(AVSampleFormat(frame->format));
+	if (sample_format == SampleFormat::UNDEFINED)
+		throw std::runtime_error("Unsupported FFmpeg sample format");
+
+	return CheckAudioFormat(frame->sample_rate, sample_format,
+				frame->channels);
+}
+
+} // namespace Ffmpeg

src/lib/ffmpeg/DetectFilterFormat.hxx

+/*
+ * Copyright 2003-2020 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_FFMPEG_DETECT_FILTER_FORMAT_HXX
+#define MPD_FFMPEG_DETECT_FILTER_FORMAT_HXX
+
+struct AVFilterContext;
+struct AudioFormat;
+
+namespace Ffmpeg {
+
+/**
+ * Attempt to detect the output format of the given FFmpeg filter by
+ * sending one frame of silence and checking what format comes back
+ * from the filter.
+ *
+ * This is a kludge because MPD needs to know the output format of a
+ * filter while initializing and cannot cope with format changes in
+ * between.
+ *
+ * This function can throw if the FFmpeg filter fails.
+ */
+AudioFormat
+DetectFilterOutputFormat(const AudioFormat &in_audio_format,
+			 AVFilterContext &buffer_src,
+			 AVFilterContext &buffer_sink);
+
+} // namespace Ffmpeg
+
+#endif

src/lib/ffmpeg/meson.build

@@ -20,7 +20,10 @@ endif
 
 ffmpeg_sources = []
 if libavfilter_dep.found()
-  ffmpeg_sources += 'Filter.cxx'
+  ffmpeg_sources += [
+    'Filter.cxx',
+    'DetectFilterFormat.cxx',
+  ]
 endif
 
 ffmpeg = static_library(