avcodev/libdcadec: implement request_channel_layout

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libdcadec.c

@@ -34,6 +34,7 @@ typedef struct DCADecContext {
     struct dcadec_context *ctx;
     uint8_t *buffer;
     int buffer_size;
+    int downmix_warned;
 } DCADecContext;
 
 static int dcadec_decode_frame(AVCodecContext *avctx, void *data,
@@ -132,6 +133,17 @@ static int dcadec_decode_frame(AVCodecContext *avctx, void *data,
     if (exss = dcadec_context_get_exss_info(s->ctx)) {
         enum AVMatrixEncoding matrix_encoding = AV_MATRIX_ENCODING_NONE;
 
+        if (!s->downmix_warned) {
+            uint64_t layout = avctx->request_channel_layout;
+
+            if (((layout == AV_CH_LAYOUT_STEREO_DOWNMIX || layout == AV_CH_LAYOUT_STEREO) && !exss->embedded_stereo) ||
+                ( layout == AV_CH_LAYOUT_5POINT1 && !exss->embedded_6ch))
+                av_log(avctx, AV_LOG_WARNING, "%s downmix was requested but no custom coefficients are available, "
+                                              "this may result in clipping\n",
+                                              layout == AV_CH_LAYOUT_5POINT1 ? "5.1" : "Stereo");
+            s->downmix_warned = 1;
+        }
+
         switch(exss->matrix_encoding) {
         case DCADEC_MATRIX_ENCODING_SURROUND:
             matrix_encoding = AV_MATRIX_ENCODING_DOLBY;
@@ -197,6 +209,27 @@ static av_cold int dcadec_init(AVCodecContext *avctx)
     if (avctx->flags & CODEC_FLAG_BITEXACT)
         flags |= DCADEC_FLAG_CORE_BIT_EXACT;
 
+    if (avctx->request_channel_layout > 0 && avctx->request_channel_layout != AV_CH_LAYOUT_NATIVE) {
+        switch (avctx->request_channel_layout) {
+        case AV_CH_LAYOUT_STEREO:
+        case AV_CH_LAYOUT_STEREO_DOWNMIX:
+            /* libdcadec ignores the 2ch flag if used alone when no custom downmix coefficients
+               are available, silently outputting a 5.1 downmix if possible instead.
+               Using both the 2ch and 6ch flags together forces a 2ch downmix using default
+               coefficients in such cases. This matches the behavior of the 6ch flag when used
+               alone, where a 5.1 downmix is generated if possible, regardless of custom
+               coefficients being available or not. */
+            flags |= DCADEC_FLAG_KEEP_DMIX_2CH | DCADEC_FLAG_KEEP_DMIX_6CH;
+            break;
+        case AV_CH_LAYOUT_5POINT1:
+            flags |= DCADEC_FLAG_KEEP_DMIX_6CH;
+            break;
+        default:
+            av_log(avctx, AV_LOG_WARNING, "Invalid request_channel_layout\n");
+            break;
+        }
+    }
+
     s->ctx = dcadec_context_create(flags);
     if (!s->ctx)
         return AVERROR(ENOMEM);