@@ -996,7 +996,7 @@ such streams is attempted.

 Allow input streams with unknown type to be copied instead of failing if copying

 such streams is attempted.

-@item -map_channel [@var{input_file_id}.@var{stream_specifier}.@var{channel_id}|-1][:@var{output_file_id}.@var{stream_specifier}]

+@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} is not set, the audio channel will

 be mapped on all the audio streams.

@@ -1005,6 +1005,10 @@ Using "-1" instead of

 @var{input_file_id}.@var{stream_specifier}.@var{channel_id} will map a muted

 channel.

+A trailing @code{?} will allow the map_channel to be

+optional: if the map_channel matches no channel the map_channel will be ignored instead

+of failing.

+

 For example, assuming @var{INPUT} is a stereo audio file, you can switch the

 two audio channels with the following command:

 @example

@@ -1052,6 +1056,13 @@ video stream), you can use the following command:

 ffmpeg -i input.mkv -filter_complex "[0:1] [0:2] amerge" -c:a pcm_s16le -c:v copy output.mkv

 @end example

+To map the first two audio channels from the first input, and using the

+trailing @code{?}, ignore the audio channel mapping if the first input is

+mono instead of stereo:

+@example

+ffmpeg -i INPUT -map_channel 0.0.0 -map_channel 0.0.1? OUTPUT

+@end example

+

 @item -map_metadata[:@var{metadata_spec_out}] @var{infile}[:@var{metadata_spec_in}] (@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.

@@ -405,6 +405,11 @@ static int opt_map_channel(void *optctx, const char *opt, const char *arg)

     int n;

     AVStream *st;

     AudioChannelMap *m;

+    char *allow_unused;

+    char *mapchan;

+    mapchan = av_strdup(arg);

+    if (!mapchan)

+        return AVERROR(ENOMEM);

     GROW_ARRAY(o->audio_channel_maps, o->nb_audio_channel_maps);

     m = &o->audio_channel_maps[o->nb_audio_channel_maps - 1];

@@ -415,6 +420,7 @@ static int opt_map_channel(void *optctx, const char *opt, const char *arg)

         m->file_idx = m->stream_idx = -1;

         if (n == 1)

             m->ofile_idx = m->ostream_idx = -1;

+        av_free(mapchan);

         return 0;

     }

@@ -450,11 +456,22 @@ static int opt_map_channel(void *optctx, const char *opt, const char *arg)

                m->file_idx, m->stream_idx);

         exit_program(1);

     }

+    /* allow trailing ? to map_channel */

+    if (allow_unused = strchr(mapchan, '?'))

+        *allow_unused = 0;

     if (m->channel_idx < 0 || m->channel_idx >= st->codecpar->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);

+        if (allow_unused) {

+            av_log(NULL, AV_LOG_VERBOSE, "mapchan: invalid audio channel #%d.%d.%d\n",

+                    m->file_idx, m->stream_idx, m->channel_idx);

+        } else {

+            av_log(NULL, AV_LOG_FATAL,  "mapchan: invalid audio channel #%d.%d.%d\n"

+                    "To ignore this, add a trailing '?' to the map_channel.\n",

+                    m->file_idx, m->stream_idx, m->channel_idx);

+            exit_program(1);

+        }

+

     }

+    av_free(mapchan);

     return 0;

 }

@@ -10,6 +10,14 @@ FATE_MAPCHAN-$(CONFIG_CHANNELMAP_FILTER) += fate-mapchan-silent-mono

 fate-mapchan-silent-mono: tests/data/asynth-22050-1.wav

 fate-mapchan-silent-mono: CMD = md5 -i $(TARGET_PATH)/tests/data/asynth-22050-1.wav -map_channel -1 -map_channel 0.0.0 -fflags +bitexact -f wav

+FATE_MAPCHAN-$(CONFIG_CHANNELMAP_FILTER) += fate-mapchan-2ch-extract-ch0-ch2-trailing

+fate-mapchan-2ch-extract-ch0-ch2-trailing: tests/data/asynth-44100-2.wav

+fate-mapchan-2ch-extract-ch0-ch2-trailing: CMD = md5 -i $(TARGET_PATH)/tests/data/asynth-44100-2.wav -map_channel 0.0.0 -map_channel 0.0.2? -fflags +bitexact -f wav

+

+FATE_MAPCHAN-$(CONFIG_CHANNELMAP_FILTER) += fate-mapchan-3ch-extract-ch0-ch2-trailing

+fate-mapchan-3ch-extract-ch0-ch2-trailing: tests/data/asynth-44100-3.wav

+fate-mapchan-3ch-extract-ch0-ch2-trailing: CMD = md5 -i $(TARGET_PATH)/tests/data/asynth-44100-3.wav -map_channel 0.0.0 -map_channel 0.0.2? -fflags +bitexact -f wav

+

 FATE_MAPCHAN = $(FATE_MAPCHAN-yes)

 FATE_FFMPEG += $(FATE_MAPCHAN)

new file mode 100644

@@ -0,0 +1 @@

+b6e1d142b4e484221562e7b66b9bbbdc

new file mode 100644

@@ -0,0 +1 @@

+ae533985186cab287309c04f6b3e866c