avformat/mxfenc: set/force channelcount in MXF D-10

There are interoperability issues with D-10 related to the channelcount property in the generic sound essence descriptor.

On one side, SMPTE 386M requires channel count to be 4 or 8, other values being prohibited.
The most widespread value is 8, which seems straightforward as it is the actual size of the allocated structure/disk space.
At the end, it appears that some vendors or workflows do require this descriptor to be 8, and otherwise just "fail".

On the other side, at least AVID and ffmpeg do write/set the channel count to the exact number of channels really "used",
usually 2 or 4, or any other value. And on the decoding side, ffmpeg (for example) make use of the channel count for probing
and only expose this limited number of audio streams
(which make sense but has strong impact on ffmpeg command line usage, output, and downstream workflow).

At the end, I find it pretty usefull to simply give ffmpeg the ability to force/set the channel count to any value the user wants.
(there are turnaround using complex filters, pans, amerge etc., but it is quite boring and requires the command line to be adapted to the input file properties)
Reviewed-by: Matthieu Bouron <matthieu.bouron@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfenc.c

@@ -294,6 +294,7 @@ typedef struct MXFContext {
     uint64_t body_offset;
     uint32_t instance_number;
     uint8_t umid[16];        ///< unique material identifier
+    int channel_count;
 } MXFContext;
 
 static const uint8_t uuid_base[]            = { 0xAD,0xAB,0x44,0x24,0x2f,0x25,0x4d,0xc7,0x92,0xff,0x29,0xbd };
@@ -996,6 +997,8 @@ static void mxf_write_mpegvideo_desc(AVFormatContext *s, AVStream *st)
 static void mxf_write_generic_sound_common(AVFormatContext *s, AVStream *st, const UID key, unsigned size)
 {
     AVIOContext *pb = s->pb;
+    MXFContext *mxf = s->priv_data;
+    int show_warnings = !mxf->footer_partition_offset;
 
     mxf_write_generic_desc(s, st, key, size+5+12+8+8);
 
@@ -1009,7 +1012,21 @@ static void mxf_write_generic_sound_common(AVFormatContext *s, AVStream *st, con
     avio_wb32(pb, 1);
 
     mxf_write_local_tag(pb, 4, 0x3D07);
-    avio_wb32(pb, st->codec->channels);
+    if (mxf->channel_count == -1) {
+        if (show_warnings && (s->oformat == &ff_mxf_d10_muxer) && (st->codec->channels != 4) && (st->codec->channels != 8))
+            av_log(s, AV_LOG_WARNING, "the number of audio channels shall be 4 or 8 : the output will not comply to MXF D-10 specs, use -mxf_channelcount to fix this\n");
+        avio_wb32(pb, st->codec->channels);
+    } else if (s->oformat == &ff_mxf_d10_muxer) {
+        if (show_warnings && (mxf->channel_count < st->codec->channels))
+            av_log(s, AV_LOG_WARNING, "mxf_channelcount < actual number of audio channels : some channels will be discarded\n");
+        if (show_warnings && (mxf->channel_count != 4) && (mxf->channel_count != 8))
+            av_log(s, AV_LOG_WARNING, "mxf_channelcount shall be set to 4 or 8 : the output will not comply to MXF D-10 specs\n");
+        avio_wb32(pb, mxf->channel_count);
+    } else {
+        if (show_warnings)
+            av_log(s, AV_LOG_ERROR, "-mxf_channelcount requires MXF D-10 and will be ignored\n");
+        avio_wb32(pb, st->codec->channels);
+    }
 
     mxf_write_local_tag(pb, 4, 0x3D01);
     avio_wb32(pb, av_get_bits_per_sample(st->codec->codec_id));
@@ -2156,6 +2173,19 @@ static int mxf_interleave(AVFormatContext *s, AVPacket *out, AVPacket *pkt, int
                                mxf_interleave_get_packet, mxf_compare_timestamps);
 }
 
+static const AVOption d10_options[] = {
+    { "mxf_channelcount", "Force/set channelcount in generic sound essence descriptor",
+      offsetof(MXFContext, channel_count), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 8, AV_OPT_FLAG_ENCODING_PARAM},
+    { NULL },
+};
+
+static const AVClass mxf_d10_muxer_class = {
+    .class_name     = "MXF-D10 muxer",
+    .item_name      = av_default_item_name,
+    .option         = d10_options,
+    .version        = LIBAVUTIL_VERSION_INT,
+};
+
 AVOutputFormat ff_mxf_muxer = {
     .name              = "mxf",
     .long_name         = NULL_IF_CONFIG_SMALL("MXF (Material eXchange Format)"),
@@ -2183,4 +2213,5 @@ AVOutputFormat ff_mxf_d10_muxer = {
     .write_trailer     = mxf_write_footer,
     .flags             = AVFMT_NOTIMESTAMPS,
     .interleave_packet = mxf_interleave,
+    .priv_class        = &mxf_d10_muxer_class,
 };