ffmpeg: add -map_channel option.

Based on an initial work by Baptiste Coudurier.

15 files changed, 228 insertions, 22 deletions

diff --git a/Changelog b/Changelog
index 849e85f3d6..ceeead5af7 100644
--- a/Changelog
+++ b/Changelog
@@ -72,6 +72,7 @@ easier to use. The changes are:
 - Prores encoder
 - Video Decoder Acceleration (VDA) HWAccel module.
 - replacement Indeo 3 decoder
+- new ffmpeg option: -map_channel
 
 version 0.8:
 
diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
index 529f830fbc..06ef004226 100644
--- a/doc/ffmpeg.texi
+++ b/doc/ffmpeg.texi
@@ -721,6 +721,44 @@ ffmpeg -i INPUT -map 0 -map -0:a:1 OUTPUT
 
 Note that using this option disables the default mappings for this output file.
 
+@item -map_channel [@var{input_file_id}.@var{stream_specifier}.@var{channel_id}|-1][:@var{output_file_id}.@var{stream_specifier}]
+Map an audio channel from a given input to an output. If
+@var{output_file_id}.@var{stream_specifier} are not set, the audio channel will
+be mapped on all the audio streams.
+
+Using "-1" instead of
+@var{input_file_id}.@var{stream_specifier}.@var{channel_id} will map a muted
+channel.
+
+For example, assuming @var{INPUT} is a stereo audio file, you can switch the
+two audio channels with the following command:
+@example
+ffmpeg -i INPUT -map_channel 0.0.1 -map_channel 0.0.0 OUTPUT
+@end example
+
+If you want to mute the first channel and keep the second:
+@example
+ffmpeg -i INPUT -map_channel -1 -map_channel 0.0.1 OUTPUT
+@end example
+
+The order of the "-map_channel" option specifies the order of the channels in
+the output stream. The output channel layout is guessed from the number of
+channels mapped (mono if one "-map_channel", stereo if two, etc.). Using "-ac"
+in combination of "-map_channel" makes the channel gain levels to be updated if
+channel layouts don't match (for instance two "-map_channel" options and "-ac
+6").
+
+You can also extract each channel of an @var{INPUT} to specific outputs; the
+following command extract each channel of the audio stream (file 0, stream 0)
+to the respective @var{OUTPUT_CH0} and @var{OUTPUT_CH1}:
+@example
+ffmpeg -i INPUT -map_channel 0.0.0 OUTPUT_CH0 -map_channel 0.0.1 OUTPUT_CH1
+@end example
+
+Note that "-map_channel" is currently limited to the scope of one input for
+each output; you can't for example use it to pick multiple input audio files
+and mix them into one single output.
+
 @item -map_metadata[:@var{metadata_type}][:@var{index}] @var{infile}[:@var{metadata_type}][:@var{index}] (@emph{output,per-metadata})
 Set metadata information of the next output file from @var{infile}. Note that
 those are file indices (zero-based), not filenames.
diff --git a/ffmpeg.c b/ffmpeg.c
index ca63bef904..ace4d70ce2 100644
--- a/ffmpeg.c
+++ b/ffmpeg.c
@@ -99,6 +99,11 @@ typedef struct StreamMap {
     int sync_stream_index;
 } StreamMap;
 
+typedef struct {
+    int  file_idx,  stream_idx,  channel_idx; // input
+    int ofile_idx, ostream_idx;               // output
+} AudioChannelMap;
+
 /**
  * select an input file for an output file
  */
@@ -231,6 +236,8 @@ typedef struct OutputStream {
 
     /* audio only */
     int audio_resample;
+    int audio_channels_map[SWR_CH_MAX];  ///< list of the channels id to pick from the source stream
+    int audio_channels_mapped;           ///< number of channels in audio_channels_map
     int resample_sample_fmt;
     int resample_channels;
     int resample_sample_rate;
@@ -313,6 +320,8 @@ typedef struct OptionsContext {
     /* output options */
     StreamMap *stream_maps;
     int     nb_stream_maps;
+    AudioChannelMap *audio_channel_maps; ///< one info entry per -map_channel
+    int           nb_audio_channel_maps; ///< number of (valid) -map_channel settings
     /* first item specifies output metadata, second is input */
     MetadataMap (*meta_data_maps)[2];
     int nb_meta_data_maps;
@@ -409,6 +418,7 @@ static void reset_options(OptionsContext *o, int is_input)
     }
 
     av_freep(&o->stream_maps);
+    av_freep(&o->audio_channel_maps);
     av_freep(&o->meta_data_maps);
     av_freep(&o->streamid_map);
 
@@ -857,7 +867,7 @@ need_realloc:
                        ost->resample_channels    != dec->channels   ||
                        ost->resample_sample_rate != dec->sample_rate;
 
-    if ((ost->audio_resample && !ost->swr) || resample_changed) {
+    if ((ost->audio_resample && !ost->swr) || resample_changed || ost->audio_channels_mapped) {
         if (resample_changed) {
             av_log(NULL, AV_LOG_INFO, "Input stream #%d.%d frame changed from rate:%d fmt:%s ch:%d to rate:%d fmt:%s ch:%d\n",
                    ist->file_index, ist->st->index,
@@ -869,7 +879,7 @@ need_realloc:
             swr_free(&ost->swr);
         }
         /* if audio_sync_method is >1 the resampler is needed for audio drift compensation */
-        if (audio_sync_method <= 1 &&
+        if (audio_sync_method <= 1 && !ost->audio_channels_mapped &&
             ost->resample_sample_fmt  == enc->sample_fmt &&
             ost->resample_channels    == enc->channels   &&
             ost->resample_sample_rate == enc->sample_rate) {
@@ -879,8 +889,13 @@ need_realloc:
             ost->swr = swr_alloc2(ost->swr,
                                   enc->channel_layout, enc->sample_fmt, enc->sample_rate,
                                   dec->channel_layout, dec->sample_fmt, dec->sample_rate,
+                                  ost->audio_channels_mapped ? ost->audio_channels_map : NULL,
                                   0, NULL);
             av_set_double(ost->swr, "rmvol", ost->rematrix_volume);
+            if (ost->audio_channels_mapped) {
+                av_set_int(ost->swr, "icl", av_get_default_channel_layout(ost->audio_channels_mapped));
+                av_set_int(ost->swr, "uch", ost->audio_channels_mapped);
+            }
             av_set_int(ost->swr, "ich", dec->channels);
             av_set_int(ost->swr, "och", enc->channels);
             if(audio_sync_method>1) av_set_int(ost->swr, "flags", SWR_FLAG_RESAMPLE);
@@ -2176,7 +2191,23 @@ static int transcode_init(OutputFile *output_files, int nb_output_files,
                 if (codec->sample_fmt == AV_SAMPLE_FMT_NONE)
                     codec->sample_fmt = icodec->sample_fmt;
                 choose_sample_fmt(ost->st, ost->enc);
-                if (!codec->channels) {
+                if (ost->audio_channels_mapped) {
+                    /* the requested output channel is set to the number of
+                     * -map_channel only if no -ac are specified */
+                    if (!codec->channels) {
+                        codec->channels       = ost->audio_channels_mapped;
+                        codec->channel_layout = av_get_default_channel_layout(codec->channels);
+                        if (!codec->channel_layout) {
+                            av_log(NULL, AV_LOG_FATAL, "Unable to find an appropriate channel layout for requested number of channel\n");
+                            exit_program(1);
+                        }
+                    }
+                    /* fill unused channel mapping with -1 (which means a muted
+                     * channel in case the number of output channels is bigger
+                     * than the number of mapped channel) */
+                    for (j = ost->audio_channels_mapped; j < FF_ARRAY_ELEMS(ost->audio_channels_map); j++)
+                        ost->audio_channels_map[j] = -1;
+                } else if (!codec->channels) {
                     codec->channels = icodec->channels;
                     codec->channel_layout = icodec->channel_layout;
                 }
@@ -2388,6 +2419,15 @@ static int transcode_init(OutputFile *output_files, int nb_output_files,
                input_streams[ost->source_index].st->index,
                ost->file_index,
                ost->index);
+        if (ost->audio_channels_mapped) {
+            av_log(NULL, AV_LOG_INFO, " [ch:");
+            for (j = 0; j < ost->audio_channels_mapped; j++)
+                if (ost->audio_channels_map[j] == -1)
+                    av_log(NULL, AV_LOG_INFO, " M");
+                else
+                    av_log(NULL, AV_LOG_INFO, " %d", ost->audio_channels_map[j]);
+            av_log(NULL, AV_LOG_INFO, "]");
+        }
         if (ost->sync_ist != &input_streams[ost->source_index])
             av_log(NULL, AV_LOG_INFO, " [sync #%d.%d]",
                    ost->sync_ist->file_index,
@@ -2899,6 +2939,66 @@ static int opt_attach(OptionsContext *o, const char *opt, const char *arg)
     return 0;
 }
 
+static int opt_map_channel(OptionsContext *o, const char *opt, const char *arg)
+{
+    int n;
+    AVStream *st;
+    AudioChannelMap *m;
+
+    o->audio_channel_maps =
+        grow_array(o->audio_channel_maps, sizeof(*o->audio_channel_maps),
+                   &o->nb_audio_channel_maps, o->nb_audio_channel_maps + 1);
+    m = &o->audio_channel_maps[o->nb_audio_channel_maps - 1];
+
+    /* muted channel syntax */
+    n = sscanf(arg, "%d:%d.%d", &m->channel_idx, &m->ofile_idx, &m->ostream_idx);
+    if ((n == 1 || n == 3) && m->channel_idx == -1) {
+        m->file_idx = m->stream_idx = -1;
+        if (n == 1)
+            m->ofile_idx = m->ostream_idx = -1;
+        return 0;
+    }
+
+    /* normal syntax */
+    n = sscanf(arg, "%d.%d.%d:%d.%d",
+               &m->file_idx,  &m->stream_idx, &m->channel_idx,
+               &m->ofile_idx, &m->ostream_idx);
+
+    if (n != 3 && n != 5) {
+        av_log(NULL, AV_LOG_FATAL, "Syntax error, mapchan usage: "
+               "[file.stream.channel|-1][:syncfile:syncstream]\n");
+        exit_program(1);
+    }
+
+    if (n != 5) // only file.stream.channel specified
+        m->ofile_idx = m->ostream_idx = -1;
+
+    /* check input */
+    if (m->file_idx < 0 || m->file_idx >= nb_input_files) {
+        av_log(NULL, AV_LOG_FATAL, "mapchan: invalid input file index: %d\n",
+               m->file_idx);
+        exit_program(1);
+    }
+    if (m->stream_idx < 0 ||
+        m->stream_idx >= input_files[m->file_idx].nb_streams) {
+        av_log(NULL, AV_LOG_FATAL, "mapchan: invalid input file stream index #%d.%d\n",
+               m->file_idx, m->stream_idx);
+        exit_program(1);
+    }
+    st = input_files[m->file_idx].ctx->streams[m->stream_idx];
+    if (st->codec->codec_type != AVMEDIA_TYPE_AUDIO) {
+        av_log(NULL, AV_LOG_FATAL, "mapchan: stream #%d.%d is not an audio stream.\n",
+               m->file_idx, m->stream_idx);
+        exit_program(1);
+    }
+    if (m->channel_idx < 0 || m->channel_idx >= st->codec->channels) {
+        av_log(NULL, AV_LOG_FATAL, "mapchan: invalid audio channel #%d.%d.%d\n",
+               m->file_idx, m->stream_idx, m->channel_idx);
+        exit_program(1);
+    }
+    return 0;
+}
+
 static void parse_meta_type(char *arg, char *type, int *index)
 {
     if (*arg) {
@@ -3588,6 +3688,7 @@ static OutputStream *new_video_stream(OptionsContext *o, AVFormatContext *oc)
 
 static OutputStream *new_audio_stream(OptionsContext *o, AVFormatContext *oc)
 {
+    int n;
     AVStream *st;
     OutputStream *ost;
     AVCodecContext *audio_enc;
@@ -3616,6 +3717,21 @@ static OutputStream *new_audio_stream(OptionsContext *o, AVFormatContext *oc)
         MATCH_PER_STREAM_OPT(rematrix_volume, f, ost->rematrix_volume, oc, st);
     }
 
+    /* check for channel mapping for this audio stream */
+    for (n = 0; n < o->nb_audio_channel_maps; n++) {
+        AudioChannelMap *map = &o->audio_channel_maps[n];
+        InputStream *ist = &input_streams[ost->source_index];
+        if ((map->channel_idx == -1 || (ist->file_index == map->file_idx && ist->st->index == map->stream_idx)) &&
+            (map->ofile_idx   == -1 || ost->file_index == map->ofile_idx) &&
+            (map->ostream_idx == -1 || ost->st->index  == map->ostream_idx)) {
+            if (ost->audio_channels_mapped < FF_ARRAY_ELEMS(ost->audio_channels_map))
+                ost->audio_channels_map[ost->audio_channels_mapped++] = map->channel_idx;
+            else
+                av_log(NULL, AV_LOG_FATAL, "Max channel mapping for output %d.%d reached\n",
+                       ost->file_index, ost->st->index);
+        }
+    }
+
     return ost;
 }
 
@@ -4430,6 +4546,7 @@ static const OptionDef options[] = {
     { "codec", HAS_ARG | OPT_STRING | OPT_SPEC, {.off = OFFSET(codec_names)}, "codec name", "codec" },
     { "pre", HAS_ARG | OPT_STRING | OPT_SPEC, {.off = OFFSET(presets)}, "preset name", "preset" },
     { "map", HAS_ARG | OPT_EXPERT | OPT_FUNC2, {(void*)opt_map}, "set input stream mapping", "file.stream[:syncfile.syncstream]" },
+    { "map_channel", HAS_ARG | OPT_EXPERT | OPT_FUNC2, {(void*)opt_map_channel}, "map an audio channel from one stream to another", "file.stream.channel[:syncfile.syncstream]" },
     { "map_meta_data", HAS_ARG | OPT_EXPERT | OPT_FUNC2, {(void*)opt_map_meta_data}, "DEPRECATED set meta data information of outfile from infile",
       "outfile[,metadata]:infile[,metadata]" },
     { "map_metadata", HAS_ARG | OPT_EXPERT | OPT_FUNC2, {(void*)opt_map_metadata}, "set metadata information of outfile from infile",
diff --git a/ffplay.c b/ffplay.c
index ae9db33ffe..f58fabc13c 100644
--- a/ffplay.c
+++ b/ffplay.c
@@ -2077,7 +2077,7 @@ static int audio_decode_frame(VideoState *is, double *pts_ptr)
                     swr_free(&is->swr_ctx);
                 is->swr_ctx = swr_alloc2(NULL, is->audio_tgt_channel_layout, is->audio_tgt_fmt, is->audio_tgt_freq,
                                                dec_channel_layout,          dec->sample_fmt,   dec->sample_rate,
-                                               0, NULL);
+                                               NULL, 0, NULL);
                 if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) {
                     fprintf(stderr, "Cannot create sample rate converter for conversion of %d Hz %s %d channels to %d Hz %s %d channels!\n",
                         dec->sample_rate,
diff --git a/libswresample/audioconvert.c b/libswresample/audioconvert.c
index a1fa3eb10c..ba90348b89 100644
--- a/libswresample/audioconvert.c
+++ b/libswresample/audioconvert.c
@@ -35,11 +35,13 @@
 struct AVAudioConvert {
     int channels;
     int fmt_pair;
+    const int *ch_map;
 };
 
 AVAudioConvert *swr_audio_convert_alloc(enum AVSampleFormat out_fmt,
                                         enum AVSampleFormat in_fmt,
-                                        int channels, int flags)
+                                        int channels, const int *ch_map,
+                                        int flags)
 {
     AVAudioConvert *ctx;
     ctx = av_malloc(sizeof(AVAudioConvert));
@@ -47,6 +49,7 @@ AVAudioConvert *swr_audio_convert_alloc(enum AVSampleFormat out_fmt,
         return NULL;
     ctx->channels = channels;
     ctx->fmt_pair = out_fmt + AV_SAMPLE_FMT_NB*in_fmt;
+    ctx->ch_map   = ch_map;
     return ctx;
 }
 
@@ -58,15 +61,17 @@ void swr_audio_convert_free(AVAudioConvert **ctx)
 int swr_audio_convert(AVAudioConvert *ctx, AudioData *out, AudioData*in, int len)
 {
     int ch;
+    const uint8_t null_input[8] = {0};
 
     av_assert0(ctx->channels == out->ch_count);
 
     //FIXME optimize common cases
 
     for(ch=0; ch<ctx->channels; ch++){
-        const int is= (in ->planar ? 1 : in->ch_count) * in->bps;
+        const int ich= ctx->ch_map ? ctx->ch_map[ch] : ch;
+        const int is= ich < 0 ? 0 : (in->planar ? 1 : in->ch_count) * in->bps;
         const int os= (out->planar ? 1 :out->ch_count) *out->bps;
-        const uint8_t *pi= in ->ch[ch];
+        const uint8_t *pi= ich < 0 ? null_input : in->ch[ich];
         uint8_t       *po= out->ch[ch];
         uint8_t *end= po + os*len;
         if(!po)
diff --git a/libswresample/audioconvert.h b/libswresample/audioconvert.h
index e5fd4dfd80..ca98d549d9 100644
--- a/libswresample/audioconvert.h
+++ b/libswresample/audioconvert.h
@@ -42,11 +42,14 @@ typedef struct AVAudioConvert AVAudioConvert;
  * @param in_fmt Input sample format
  * @param channels Number of channels
  * @param flags See AV_CPU_FLAG_xx
+ * @param ch_map list of the channels id to pick from the source stream, NULL
+ *               if all channels must be selected
  * @return NULL on error
  */
 AVAudioConvert *swr_audio_convert_alloc(enum AVSampleFormat out_fmt,
                                         enum AVSampleFormat in_fmt,
-                                        int channels, int flags);
+                                        int channels, const int *ch_map,
+                                        int flags);
 
 /**
  * Free audio sample format converter context.
diff --git a/libswresample/swresample.c b/libswresample/swresample.c
index 7d864dfd48..e814cd2fa0 100644
--- a/libswresample/swresample.c
+++ b/libswresample/swresample.c
@@ -38,6 +38,7 @@
 static const AVOption options[]={
 {"ich",  "input channel count", OFFSET( in.ch_count   ), AV_OPT_TYPE_INT, {.dbl=2}, 1, SWR_CH_MAX, 0},
 {"och", "output channel count", OFFSET(out.ch_count   ), AV_OPT_TYPE_INT, {.dbl=2}, 1, SWR_CH_MAX, 0},
+{"uch",   "used channel count", OFFSET(used_ch_count  ), AV_OPT_TYPE_INT, {.dbl=2}, 1, SWR_CH_MAX, 0},
 {"isr",  "input sample rate"  , OFFSET( in_sample_rate), AV_OPT_TYPE_INT, {.dbl=48000}, 1, INT_MAX, 0},
 {"osr", "output sample rate"  , OFFSET(out_sample_rate), AV_OPT_TYPE_INT, {.dbl=48000}, 1, INT_MAX, 0},
 //{"ip" ,  "input planar"       , OFFSET( in.planar     ), AV_OPT_TYPE_INT, {.dbl=0},    0,       1, 0},
@@ -76,7 +77,7 @@ SwrContext *swr_alloc(void){
 
 SwrContext *swr_alloc2(struct SwrContext *s, int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate,
                        int64_t  in_ch_layout, enum AVSampleFormat  in_sample_fmt, int  in_sample_rate,
-                       int log_offset, void *log_ctx){
+                       const int *channel_map, int log_offset, void *log_ctx){
     if(!s) s= swr_alloc();
     if(!s) return NULL;
 
@@ -90,9 +91,11 @@ SwrContext *swr_alloc2(struct SwrContext *s, int64_t out_ch_layout, enum AVSampl
     av_set_int(s, "isf", in_sample_fmt);
     av_set_int(s, "isr", in_sample_rate);
 
+    s->channel_map = channel_map;
     s-> in.ch_count= av_get_channel_layout_nb_channels(s-> in_ch_layout);
     s->out.ch_count= av_get_channel_layout_nb_channels(s->out_ch_layout);
     s->int_sample_fmt = AV_SAMPLE_FMT_S16;
+    s->used_ch_count= s-> in.ch_count;
 
     return s;
 }
@@ -167,13 +170,16 @@ int swr_init(SwrContext *s){
         return -1;
     }
 
-    if(s-> in.ch_count && s-> in_ch_layout && s->in.ch_count != av_get_channel_layout_nb_channels(s-> in_ch_layout)){
-        av_log(s, AV_LOG_WARNING, "Input channel layout has a different number of channels than there actually is, ignoring layout\n");
+    if(!s->used_ch_count)
+        s->used_ch_count= s->in.ch_count;
+
+    if(s->used_ch_count && s-> in_ch_layout && s->used_ch_count != av_get_channel_layout_nb_channels(s-> in_ch_layout)){
+        av_log(s, AV_LOG_WARNING, "Input channel layout has a different number of channels than the number of used channels, ignoring layout\n");
         s-> in_ch_layout= 0;
     }
 
     if(!s-> in_ch_layout)
-        s-> in_ch_layout= av_get_default_channel_layout(s->in.ch_count);
+        s-> in_ch_layout= av_get_default_channel_layout(s->used_ch_count);
     if(!s->out_ch_layout)
         s->out_ch_layout= av_get_default_channel_layout(s->out.ch_count);
 
@@ -182,10 +188,13 @@ int swr_init(SwrContext *s){
 #define RSC 1 //FIXME finetune
     if(!s-> in.ch_count)
         s-> in.ch_count= av_get_channel_layout_nb_channels(s-> in_ch_layout);
+    if(!s->used_ch_count)
+        s->used_ch_count= s->in.ch_count;
     if(!s->out.ch_count)
         s->out.ch_count= av_get_channel_layout_nb_channels(s->out_ch_layout);
 
 av_assert0(s-> in.ch_count);
+av_assert0(s->used_ch_count);
 av_assert0(s->out.ch_count);
     s->resample_first= RSC*s->out.ch_count/s->in.ch_count - RSC < s->out_sample_rate/(float)s-> in_sample_rate - 1.0;
 
@@ -193,22 +202,27 @@ av_assert0(s->out.ch_count);
     s->int_bps= av_get_bits_per_sample_fmt(s->int_sample_fmt)/8;
     s->out.bps= av_get_bits_per_sample_fmt(s->out_sample_fmt)/8;
 
-    if(!s->resample && !s->rematrix){
+    if(!s->resample && !s->rematrix && !s->channel_map){
         s->full_convert = swr_audio_convert_alloc(s->out_sample_fmt,
-                                                  s-> in_sample_fmt, s-> in.ch_count, 0);
+                                                  s-> in_sample_fmt, s-> in.ch_count, NULL, 0);
         return 0;
     }
 
     s->in_convert = swr_audio_convert_alloc(s->int_sample_fmt,
-                                            s-> in_sample_fmt, s-> in.ch_count, 0);
+                                            s-> in_sample_fmt, s->used_ch_count, s->channel_map, 0);
     s->out_convert= swr_audio_convert_alloc(s->out_sample_fmt,
-                                            s->int_sample_fmt, s->out.ch_count, 0);
+                                            s->int_sample_fmt, s->out.ch_count, NULL, 0);
 
 
     s->postin= s->in;
     s->preout= s->out;
     s->midbuf= s->in;
     s->in_buffer= s->in;
+    if(s->channel_map){
+        s->postin.ch_count=
+        s->midbuf.ch_count=
+        s->in_buffer.ch_count= s->used_ch_count;
+    }
     if(!s->resample_first){
         s->midbuf.ch_count= s->out.ch_count;
         s->in_buffer.ch_count = s->out.ch_count;
@@ -325,7 +339,7 @@ int swr_convert(struct SwrContext *s, uint8_t *out_arg[SWR_CH_MAX], int out_coun
     if((ret=realloc_audio(&s->postin, in_count))<0)
         return ret;
     if(s->resample_first){
-        av_assert0(s->midbuf.ch_count ==  s-> in.ch_count);
+        av_assert0(s->midbuf.ch_count == s->used_ch_count);
         if((ret=realloc_audio(&s->midbuf, out_count))<0)
             return ret;
     }else{
diff --git a/libswresample/swresample.h b/libswresample/swresample.h
index d6aca219fe..f72715b152 100644
--- a/libswresample/swresample.h
+++ b/libswresample/swresample.h
@@ -25,7 +25,7 @@
 #include "libavutil/samplefmt.h"
 
 #define LIBSWRESAMPLE_VERSION_MAJOR 0
-#define LIBSWRESAMPLE_VERSION_MINOR 1
+#define LIBSWRESAMPLE_VERSION_MINOR 2
 #define LIBSWRESAMPLE_VERSION_MICRO 0
 
 #define SWR_CH_MAX 16
@@ -57,7 +57,7 @@ int swr_init(struct SwrContext *s);
  */
 struct SwrContext *swr_alloc2(struct SwrContext *s, int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate,
                               int64_t  in_ch_layout, enum AVSampleFormat  in_sample_fmt, int  in_sample_rate,
-                              int log_offset, void *log_ctx);
+                              const int *channel_map, int log_offset, void *log_ctx);
 
 /**
  * Free the given SwrContext.
diff --git a/libswresample/swresample_internal.h b/libswresample/swresample_internal.h
index 4678886bdb..4764ddfd6f 100644
--- a/libswresample/swresample_internal.h
+++ b/libswresample/swresample_internal.h
@@ -45,6 +45,8 @@ typedef struct SwrContext {          //FIXME find unused fields
     int     out_sample_rate;
     int flags;
     float slev, clev, rematrix_volume;
+    const int *channel_map;             ///< channel index (or -1 if muted channel) map
+    int used_ch_count;                  ///< number of used channels (mapped channel count if channel_map, otherwise in.ch_count)
 
     //below are private
     int int_bps;
diff --git a/libswresample/swresample_test.c b/libswresample/swresample_test.c
index 49723cdeac..1a37bcd5d2 100644
--- a/libswresample/swresample_test.c
+++ b/libswresample/swresample_test.c
@@ -131,9 +131,11 @@ int main(int argc, char **argv){
                                in_sample_rate, out_sample_rate,
                                av_get_sample_fmt_name(in_sample_fmt), av_get_sample_fmt_name(out_sample_fmt));
                         forw_ctx  = swr_alloc2(forw_ctx, out_ch_layout, out_sample_fmt+planar_out, out_sample_rate,
-                                                                  in_ch_layout,  in_sample_fmt+planar_in ,  in_sample_rate, 0, 0);
-                        backw_ctx = swr_alloc2(backw_ctx,in_ch_layout,  in_sample_fmt,  in_sample_rate,
-                                                                 out_ch_layout, out_sample_fmt+planar_out, out_sample_rate, 0, 0);
+                                                          in_ch_layout,  in_sample_fmt+planar_in ,  in_sample_rate,
+                                                          NULL, 0, 0);
+                        backw_ctx = swr_alloc2(backw_ctx,in_ch_layout,  in_sample_fmt,             in_sample_rate,
+                                                        out_ch_layout, out_sample_fmt+planar_out, out_sample_rate,
+                                                        NULL, 0, 0);
                         if(swr_init( forw_ctx) < 0)
                             fprintf(stderr, "swr_init(->) failed\n");
                         if(swr_init(backw_ctx) < 0)
diff --git a/tests/Makefile b/tests/Makefile
index 93f07a5bce..48e81faeb7 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -30,6 +30,14 @@ tests/data/asynth-16000-1.sw: tests/audiogen$(HOSTEXESUF)
 	@mkdir -p tests/data
 	$(M)./$< $@ 16000 1
 
+tests/data/mapchan-6ch.sw: tests/audiogen$(HOSTEXESUF)
+	@mkdir -p tests/data
+	$(M)./$< $@ 22050 6
+
+tests/data/mapchan-mono.sw: tests/audiogen$(HOSTEXESUF)
+	@mkdir -p tests/data
+	$(M)./$< $@ 22050 1
+
 tests/data/asynth%.sw tests/vsynth%/00.pgm: TAG = GEN
 
 include $(SRC_PATH)/tests/fate.mak
diff --git a/tests/fate2.mak b/tests/fate2.mak
index 376a0196b6..ad565b79e7 100644
--- a/tests/fate2.mak
+++ b/tests/fate2.mak
@@ -137,6 +137,18 @@ FATE_TESTS += fate-g722enc
 fate-g722enc: tests/data/asynth-16000-1.sw
 fate-g722enc: CMD = md5 -ar 16000 -ac 1 -f s16le -i $(TARGET_PATH)/tests/data/asynth-16000-1.sw -acodec g722 -ac 1 -f g722
 
+FATE_TESTS += fate-mapchan-6ch-extract-2
+fate-mapchan-6ch-extract-2: tests/data/mapchan-6ch.sw
+fate-mapchan-6ch-extract-2: CMD = avconv -ar 22050 -ac 6 -i $(TARGET_PATH)/tests/data/mapchan-6ch.sw -map_channel 0.0.0 -f wav md5: -map_channel 0.0.1 -f wav md5:
+
+FATE_TESTS += fate-mapchan-6ch-extract-2-downmix-mono
+fate-mapchan-6ch-extract-2-downmix-mono: tests/data/mapchan-6ch.sw
+fate-mapchan-6ch-extract-2-downmix-mono: CMD = md5 -ar 22050 -ac 6 -i $(TARGET_PATH)/tests/data/mapchan-6ch.sw -map_channel 0.0.1 -map_channel 0.0.0 -ac 1 -f wav
+
+FATE_TESTS += fate-mapchan-silent-mono
+fate-mapchan-silent-mono: tests/data/mapchan-mono.sw
+fate-mapchan-silent-mono: CMD = md5 -ar 22050 -ac 1 -i $(TARGET_PATH)/tests/data/mapchan-mono.sw -map_channel -1 -map_channel 0.0.0 -f wav
+
 FATE_TESTS += fate-msmpeg4v1
 fate-msmpeg4v1: CMD = framecrc -flags +bitexact -dct fastint -idct simple -i $(SAMPLES)/msmpeg4v1/mpg4.avi -an
 
diff --git a/tests/ref/fate/mapchan-6ch-extract-2 b/tests/ref/fate/mapchan-6ch-extract-2
new file mode 100644
index 0000000000..98c854069c
--- /dev/null
+++ b/tests/ref/fate/mapchan-6ch-extract-2
@@ -0,0 +1,2 @@
+6f091fe8c0be88c75921731dc9f74314
+5c2d162b9024329eb367295d37b8ca0a
diff --git a/tests/ref/fate/mapchan-6ch-extract-2-downmix-mono b/tests/ref/fate/mapchan-6ch-extract-2-downmix-mono
new file mode 100644
index 0000000000..f42de0c954
--- /dev/null
+++ b/tests/ref/fate/mapchan-6ch-extract-2-downmix-mono
@@ -0,0 +1 @@
+959645ed73e6d08d8f1e947eac5d0b92
diff --git a/tests/ref/fate/mapchan-silent-mono b/tests/ref/fate/mapchan-silent-mono
new file mode 100644
index 0000000000..a867e3b432
--- /dev/null
+++ b/tests/ref/fate/mapchan-silent-mono
@@ -0,0 +1 @@
+4f5148f08587a4b9794aa52aec7852ac