Fix quality issues with SVC screenshares.

.changeset/wet-kids-unite.md

@@ -0,0 +1,5 @@
+---
+"livekit-client": patch
+---
+
+Fix quality issues with SVC screenshares

example/sample.ts

@@ -329,7 +329,11 @@ const appActions = {
     const enabled = currentRoom.localParticipant.isScreenShareEnabled;
     appendLog(`${enabled ? 'stopping' : 'starting'} screen share`);
     setButtonDisabled('share-screen-button', true);
-    await currentRoom.localParticipant.setScreenShareEnabled(!enabled, { audio: true });
+    try {
+      await currentRoom.localParticipant.setScreenShareEnabled(!enabled, { audio: true });
+    } catch (e) {
+      appendLog('error sharing screen', e);
+    }
     setButtonDisabled('share-screen-button', false);
     updateButtonsForPublishState();
   },

src/room/PCTransport.ts

@@ -286,31 +286,19 @@ export default class PCTransport extends EventEmitter {
             return true;
           }
 
-          let fmtpFound = false;
+          const startBitrate = Math.round(trackbr.maxbr * startBitrateForSVC);
+
           for (const fmtp of media.fmtp) {
             if (fmtp.payload === codecPayload) {
+              // if another track's fmtp already is set, we cannot override the bitrate
+              // this has the unfortunate consequence of being forced to use the
+              // initial track's bitrate for all tracks
               if (!fmtp.config.includes('x-google-start-bitrate')) {
-                fmtp.config += `;x-google-start-bitrate=${Math.round(
-                  trackbr.maxbr * startBitrateForSVC,
-                )}`;
-              }
-              if (!fmtp.config.includes('x-google-max-bitrate')) {
-                fmtp.config += `;x-google-max-bitrate=${trackbr.maxbr}`;
+                fmtp.config += `;x-google-start-bitrate=${startBitrate}`;
               }
-              fmtpFound = true;
               break;
             }
           }
-
-          if (!fmtpFound) {
-            media.fmtp.push({
-              payload: codecPayload,
-              config: `x-google-start-bitrate=${Math.round(
-                trackbr.maxbr * startBitrateForSVC,
-              )};x-google-max-bitrate=${trackbr.maxbr}`,
-            });
-          }
-
           return true;
         });
       }

src/room/participant/LocalParticipant.ts

@@ -732,21 +732,21 @@ export default class LocalParticipant extends Participant {
       // for svc codecs, disable simulcast and use vp8 for backup codec
       if (track instanceof LocalVideoTrack) {
         if (isSVCCodec(videoCodec)) {
-          // vp9 svc with screenshare has problem to encode, always use L1T3 here
-          if (track.source === Track.Source.ScreenShare && videoCodec === 'vp9') {
+          if (track.source === Track.Source.ScreenShare) {
+            // vp9 svc with screenshare cannot encode multiple spatial layers
+            // doing so reduces publish resolution to minimal resolution
+            opts.scalabilityMode = 'L1T3';
             // Chrome does not allow more than 5 fps with L1T3, and it has encoding bugs with L3T3
             // It has a different path for screenshare handling and it seems to be untested/buggy
             // As a workaround, we are setting contentHint to force it to go through the same
             // path as regular camera video. While this is not optimal, it delivers the performance
             // that we need
             if ('contentHint' in track.mediaStreamTrack) {
               track.mediaStreamTrack.contentHint = 'motion';
-              this.log.info('forcing contentHint to motion for screenshare with VP9', {
+              this.log.info('forcing contentHint to motion for screenshare with SVC codecs', {
                 ...this.logContext,
                 ...getLogContextFromTrack(track),
               });
-            } else {
-              opts.scalabilityMode = 'L1T3';
             }
           }
           // set scalabilityMode to 'L3T3_KEY' by default
@@ -881,7 +881,8 @@ export default class LocalParticipant extends Participant {
             maxbr: encodings[0]?.maxBitrate ? encodings[0].maxBitrate / 1000 : 0,
           });
         }
-      } else if (track.codec && isSVCCodec(track.codec) && encodings[0]?.maxBitrate) {
+      } else if (track.codec && track.codec == 'av1' && encodings[0]?.maxBitrate) {
+        // AV1 requires setting x-start-bitrate in SDP
         this.engine.pcManager.publisher.setTrackCodecBitrate({
           cid: req.cid,
           codec: track.codec,

src/room/participant/publishUtils.ts

@@ -125,26 +125,21 @@ export function computeVideoEncodings(
   );
 
   if (scalabilityMode && isSVCCodec(videoCodec)) {
-    log.debug(`using svc with scalabilityMode ${scalabilityMode}`);
-
     const sm = new ScalabilityMode(scalabilityMode);
 
     const encodings: RTCRtpEncodingParameters[] = [];
 
     if (sm.spatial > 3) {
       throw new Error(`unsupported scalabilityMode: ${scalabilityMode}`);
     }
-    for (let i = 0; i < sm.spatial; i += 1) {
-      encodings.push({
-        rid: videoRids[2 - i],
-        maxBitrate: videoEncoding.maxBitrate / 3 ** i,
-        /* @ts-ignore */
-        maxFramerate: original.encoding.maxFramerate,
-      });
-    }
-    /* @ts-ignore */
-    encodings[0].scalabilityMode = scalabilityMode;
-    log.debug('encodings', encodings);
+    encodings.push({
+      maxBitrate: videoEncoding.maxBitrate,
+      /* @ts-ignore */
+      maxFramerate: original.encoding.maxFramerate,
+      /* @ts-ignore */
+      scalabilityMode: scalabilityMode,
+    });
+    log.debug(`using svc encoding`, encodings[0]);
     return encodings;
   }