Adding easy channel layout extractors

libav.in.js

@@ -121,6 +121,31 @@
         return [dv.getInt32(0, true), dv.getInt32(4, true)];
     };
 
+    libavStatics.ff_channel_layout(frame) {
+        if (frame.channel_layout)
+            return frame.channel_layout;
+        else if (frame.channels && frame.channels !== 1)
+            return (1 << frame.channels) - 1;
+        else
+            return 4; // Mono
+    };
+
+    libavStatics.ff_channels(frame) {
+        if (frame.channels) {
+            return frame.channels;
+        } else if (frame.channel_layout) {
+            var channels = 0;
+            var cl = frame.channel_layout;
+            while (cl) {
+                channels += (cl & 1);
+                cl >>= 1;
+            }
+            return channels;
+        } else {
+            return 1;
+        }
+    };
+
     // Some enumerations lifted directly from FFmpeg
     function enume(vals, first) {
         if (typeof first === undefined)

libav.types.in.d.ts

@@ -407,6 +407,26 @@ declare namespace LibAV {
          */
         bigIntToi64(val: BigInt): [number, number];
 
+        /**
+         * Extract the channel layout from a frame (or any other source of
+         * channel layout). Unifies the various ways that channel layouts may
+         * be stored.
+         */
+        ff_channel_layout(frame: {
+            channel_layout?: number,
+            channels?: number
+        }): number;
+
+        /**
+         * Extract the channel count from a frame (or any other source of
+         * channel layout). Unifies the various ways that channel layouts may be
+         * stored.
+         */
+        ff_channels(frame: {
+            channel_layout?: number,
+            channels?: number
+        }): number;
+
         // Enumerations:
         AV_OPT_SEARCH_CHILDREN: number;
         AVMEDIA_TYPE_UNKNOWN: number;