avformat/movenc: don't use mono layout when a front center label is expected

On output streams where a multichannel stream needs to be stored as one track
per channel, each track will have a channel layout describing the position of
the channel they contain. For the track with front center, the mov muxer was
using the mov layout "mono" instead of the label for the front center position.

Since our channel layout API considers front center == mono, we need to do some
heuristics. To achieve this, we make sure all audio tracks contain streams with
a single channel, and only one of them is front center. In that case, we write
the front center label instead of signaling mono layout.

Fixes the last part of ticket #2865

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/movenc.c

@@ -887,6 +887,17 @@ static int mov_write_chan_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
         return ret;
     }
 
+    if (layout_tag == MOV_CH_LAYOUT_MONO && track->mono_as_fc > 0) {
+        av_assert0(!channel_desc);
+        channel_desc = av_malloc(sizeof(*channel_desc));
+        if (!channel_desc)
+            return AVERROR(ENOMEM);
+
+        layout_tag = 0;
+        bitmap = 0;
+        *channel_desc = 3; // channel label "Center"
+    }
+
     num_desc = layout_tag ? 0 : track->par->ch_layout.nb_channels;
 
     avio_wb32(pb, 0);           // Size
@@ -6970,6 +6981,20 @@ static int mov_write_header(AVFormatContext *s)
             if (j == i)
                 continue;
 
+            if (stj->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
+                (trackj->par->ch_layout.nb_channels != 1 ||
+                 !av_channel_layout_compare(&trackj->par->ch_layout,
+                                            &(AVChannelLayout)AV_CHANNEL_LAYOUT_MONO))
+            )
+                track->mono_as_fc = -1;
+
+            if (stj->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
+                av_channel_layout_compare(&trackj->par->ch_layout,
+                                          &(AVChannelLayout)AV_CHANNEL_LAYOUT_MONO) &&
+                trackj->par->ch_layout.nb_channels == 1 && track->mono_as_fc >= 0
+            )
+                track->mono_as_fc++;
+
             if (stj->codecpar->codec_type != AVMEDIA_TYPE_AUDIO ||
                 av_channel_layout_compare(&trackj->par->ch_layout,
                                           &(AVChannelLayout)AV_CHANNEL_LAYOUT_MONO) ||

libavformat/movenc.h

@@ -107,6 +107,7 @@ typedef struct MOVTrack {
     int         tag; ///< stsd fourcc
     AVStream        *st;
     AVCodecParameters *par;
+    int mono_as_fc;
     int multichannel_as_mono;
 
     int         vos_len;