opusdec: Try harder to negotiate the upstream channels/rate preferences

It might be possible to fulfill those but not with the first caps
structure. Instead of just fixating the first caps structure, check if
the preference can be fulfilled by any of the structures as the first
step.

Without this the following pipeline negotiates to mono after the
decoder because opusenc only has a single channel in its first caps
structure.

    gst-launch-1.0 audiotestsrc ! audio/x-raw,channels=2 ! opusenc \
        ! queue ! opusdec ! queue ! opusenc ! fakesink

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3689>

diff --git a/subprojects/gst-plugins-base/ext/opus/gstopusdec.c b/subprojects/gst-plugins-base/ext/opus/gstopusdec.c
index 43bb1a4..60f8879 100644 (file)
--- a/subprojects/gst-plugins-base/ext/opus/gstopusdec.c
+++ b/subprojects/gst-plugins-base/ext/opus/gstopusdec.c
@@ -285,7 +285,7 @@ gst_opus_dec_negotiate (GstOpusDec * dec, const GstAudioChannelPosition * pos)
     gint rate = dec->sample_rate, channels = dec->n_channels;
     GstCaps *constraint, *inter;
 
-    constraint = gst_caps_from_string ("audio/x-raw");
+    constraint = gst_caps_new_empty_simple ("audio/x-raw");
     if (dec->n_channels <= 2) { /* including 0 */
       gst_caps_set_simple (constraint, "channels", GST_TYPE_INT_RANGE, 1, 2,
           NULL);
@@ -304,6 +304,41 @@ gst_opus_dec_negotiate (GstOpusDec * dec, const GstAudioChannelPosition * pos)
       return FALSE;
     }
 
+    /* If we have a channels preference (0 means we prefer 2), then check if
+     * we can passthrough that. The preferred channel count might not be in
+     * the first structure! */
+    if (dec->n_channels <= 2) {
+      GstCaps *preferred =
+          gst_caps_new_simple ("audio/x-raw", "channels", G_TYPE_INT,
+          dec->n_channels > 0 ? dec->n_channels : 2, NULL);
+      GstCaps *tmp;
+
+      tmp = gst_caps_intersect (inter, preferred);
+      if (!gst_caps_is_empty (tmp)) {
+        gst_caps_unref (inter);
+        inter = tmp;
+      }
+
+      gst_caps_unref (preferred);
+    }
+
+    /* If we have a rate preference, then check if we can passthrough that.
+     * The preferred rate might not be in the first structure! */
+    {
+      GstCaps *preferred =
+          gst_caps_new_simple ("audio/x-raw", "rate", G_TYPE_INT,
+          dec->sample_rate > 0 ? dec->sample_rate : 48000, NULL);
+      GstCaps *tmp;
+
+      tmp = gst_caps_intersect (inter, preferred);
+      if (!gst_caps_is_empty (tmp)) {
+        gst_caps_unref (inter);
+        inter = tmp;
+      }
+
+      gst_caps_unref (preferred);
+    }
+
     inter = gst_caps_truncate (inter);
     s = gst_caps_get_structure (inter, 0);
     rate = dec->sample_rate > 0 ? dec->sample_rate : 48000;