fix(RTC): In unified-plan mode, disable the low resolution streams for low fps SS.

In unified plan impl, it is not possible to enable/disable simulcast during the call since the same sender is re-used for all local video tracks. Therefore, disable the low resolution simulcast streams for low fps screensharing so that the bridge forwards only the highest resolution stream which is important for low fps screensharing.

modules/RTC/TraceablePeerConnection.js

@@ -1576,6 +1576,16 @@ TraceablePeerConnection.prototype._getSSRC = function(rtcId) {
     return this.localSSRCs.get(rtcId);
 };
 
+/**
+ * Checks if low fps screensharing is in progress.
+ *
+ * @private
+ * @returns {boolean} Returns true if 5 fps screensharing is in progress, false otherwise.
+ */
+TraceablePeerConnection.prototype._isSharingLowFpsScreen = function() {
+    return this._isSharingScreen() && this._capScreenshareBitrate;
+};
+
 /**
  * Checks if screensharing is in progress.
  *
@@ -1692,11 +1702,7 @@ TraceablePeerConnection.prototype.addTrack = function(track, isInitiator = false
             return Promise.reject(error);
         }
     } else {
-        // In all other cases, i.e., plan-b and unified plan bridge case, use addStream API to
-        // add the track to the peerconnection.
-        // TODO - addTransceiver doesn't generate a MSID for the stream, which is needed for signaling
-        // the ssrc to Jicofo. Switch to using UUID as MSID when addTransceiver is used in Unified plan
-        // JVB connection case as well.
+        // Use addStream API for the plan-b case.
         const webrtcStream = track.getOriginalStream();
 
         if (webrtcStream) {
@@ -1855,7 +1861,7 @@ TraceablePeerConnection.prototype.getConfiguredVideoCodec = function() {
 TraceablePeerConnection.prototype.setDesktopSharingFrameRate = function(maxFps) {
     const lowFps = maxFps <= SS_DEFAULT_FRAME_RATE;
 
-    this._capScreenshareBitrate = this.isSimulcastOn() && lowFps && !this._usesUnifiedPlan;
+    this._capScreenshareBitrate = this.isSimulcastOn() && lowFps;
 };
 
 /**
@@ -2311,15 +2317,13 @@ TraceablePeerConnection.prototype.setSenderVideoDegradationPreference = function
         return Promise.resolve();
     }
     const parameters = videoSender.getParameters();
-    const preference = localVideoTrack.videoType === VideoType.CAMERA
-        ? DEGRADATION_PREFERENCE_CAMERA
-        : this._capScreenshareBitrate && !this._usesUnifiedPlan
+    const preference = this._isSharingLowFpsScreen()
 
-            // Prefer resolution for low fps share.
-            ? DEGRADATION_PREFERENCE_DESKTOP
+        // Prefer resolution for low fps share.
+        ? DEGRADATION_PREFERENCE_DESKTOP
 
-            // Prefer frame-rate for high fps share.
-            : DEGRADATION_PREFERENCE_CAMERA;
+        // Prefer frame-rate for high fps share and camera.
+        : DEGRADATION_PREFERENCE_CAMERA;
 
     logger.info(`${this} Setting a degradation preference [preference=${preference},track=${localVideoTrack}`);
     parameters.degradationPreference = preference;
@@ -2349,16 +2353,14 @@ TraceablePeerConnection.prototype.setMaxBitRate = function() {
         return Promise.resolve();
     }
 
-    const videoType = localVideoTrack.videoType;
-    const planBScreenSharing = !this._usesUnifiedPlan && videoType === VideoType.DESKTOP;
+    const videoType = localVideoTrack.getVideoType();
 
     // Apply the maxbitrates on the video track when one of the conditions is met.
     // 1. Max. bitrates for video are specified through videoQuality settings in config.js
-    // 2. Track is a desktop track and bitrate is capped using capScreenshareBitrate option in plan-b mode.
-    // 3. The client is running in Unified plan mode.
-    if (!((this.options.videoQuality && this.options.videoQuality.maxBitratesVideo)
-        || (planBScreenSharing && this._capScreenshareBitrate)
-        || this._usesUnifiedPlan)) {
+    // 2. Track is a low fps desktop track.
+    // 3. The client is running in Unified plan mode (the same sender is re-used for different types
+    // of tracks so bitrates have to be configured whenever the local tracks are replaced).
+    if (!(this.options?.videoQuality?.maxBitratesVideo || this._isSharingLowFpsScreen() || this._usesUnifiedPlan)) {
         return Promise.resolve();
     }
 
@@ -2371,31 +2373,25 @@ TraceablePeerConnection.prototype.setMaxBitRate = function() {
     }
     const parameters = videoSender.getParameters();
 
-    if (!(parameters.encodings && parameters.encodings.length)) {
+    if (!parameters.encodings?.length) {
         return Promise.resolve();
     }
 
     if (this.isSimulcastOn()) {
         for (const encoding in parameters.encodings) {
             if (parameters.encodings.hasOwnProperty(encoding)) {
-                let bitrate;
+                const bitrate = this._isSharingLowFpsScreen()
 
-                if (planBScreenSharing) {
-                    // On chromium, set a max bitrate of 500 Kbps for screenshare when capScreenshareBitrate
-                    // is enabled through config.js and presenter is not turned on.
+                    // For low fps screensharing, set a max bitrate of 500 Kbps when presenter is not turned on.
                     // FIXME the top 'isSimulcastOn' condition is confusing for screensharing, because
-                    // if capScreenshareBitrate option is enabled then the simulcast is turned off
-                    bitrate = this._capScreenshareBitrate
-                        ? presenterEnabled ? HD_BITRATE : DESKTOP_SHARE_RATE
-
-                        // Remove the bitrate config if not capScreenshareBitrate:
-                        // When switching from camera to desktop and videoQuality.maxBitratesVideo were set,
-                        // then the 'maxBitrate' setting must be cleared, because if simulcast is enabled for screen
-                        // and maxBitrates are set then Chrome will not send the screen stream (plan B).
-                        : undefined;
-                } else {
-                    bitrate = this.tpcUtils.localStreamEncodingsConfig[encoding].maxBitrate;
-                }
+                    // if capScreenshareBitrate option is enabled then simulcast is turned off for the stream.
+                    ? presenterEnabled ? HD_BITRATE : DESKTOP_SHARE_RATE
+
+                    // For high fps screenshare, 'maxBitrate' setting must be cleared on Chrome, because if simulcast is
+                    // enabled for screen and maxBitrates are set then Chrome will not send the desktop stream.
+                    : videoType === VideoType.DESKTOP && browser.isChromiumBased()
+                        ? undefined
+                        : this.tpcUtils.localStreamEncodingsConfig[encoding].maxBitrate;
 
                 logger.info(`${this} Setting a max bitrate of ${bitrate} bps on layer `
                     + `${this.tpcUtils.localStreamEncodingsConfig[encoding].rid}`);
@@ -2546,7 +2542,7 @@ TraceablePeerConnection.prototype.setSenderVideoConstraint = function(frameHeigh
     }
     const parameters = videoSender.getParameters();
 
-    if (!parameters || !parameters.encodings || !parameters.encodings.length) {
+    if (!parameters?.encodings?.length) {
         return Promise.resolve();
     }
 
@@ -2565,6 +2561,18 @@ TraceablePeerConnection.prototype.setSenderVideoConstraint = function(frameHeigh
         if (newHeight > 0 && ldStreamIndex !== -1) {
             this.encodingsEnabledState[ldStreamIndex] = true;
         }
+
+        // Disable the lower spatial layers for screensharing in Unified plan when low fps screensharing is in progress
+        // There is no way to enable or disable simulcast during the call since we are re-using the same sender.
+        // Safari is an exception here since it does not send the desktop stream at all if only the high resolution
+        // stream is enabled.
+        if (this._isSharingLowFpsScreen() && this._usesUnifiedPlan && !browser.isWebKitBased()) {
+            const highResolutionEncoding = browser.isFirefox() ? 0 : this.encodingsEnabledState.length - 1;
+
+            this.encodingsEnabledState = this.encodingsEnabledState
+                .map((encoding, idx) => idx === highResolutionEncoding);
+        }
+
         for (const encoding in parameters.encodings) {
             if (parameters.encodings.hasOwnProperty(encoding)) {
                 parameters.encodings[encoding].active = this.encodingsEnabledState[encoding];
@@ -2572,9 +2580,7 @@ TraceablePeerConnection.prototype.setSenderVideoConstraint = function(frameHeigh
         }
         this.tpcUtils.updateEncodingsResolution(parameters);
     } else if (newHeight > 0) {
-        // Do not scale down the desktop tracks until SendVideoController is able to propagate the sender constraints
-        // only on the active media connection. Right now, the sender constraints received on the bridge channel
-        // are propagated on both the jvb and p2p connections in cases where they both are active at the same time.
+        // Do not scale down encodings for desktop tracks for non-simulcast case.
         parameters.encodings[0].scaleResolutionDownBy
             = localVideoTrack.videoType === VideoType.DESKTOP || localVideoTrack.resolution <= newHeight
                 ? 1
@@ -2793,11 +2799,11 @@ TraceablePeerConnection.prototype._createOfferOrAnswer = function(
                     dumpSDP(resultSdp));
             }
 
-            // Configure simulcast for camera tracks always and for desktop tracks only when
-            // the "capScreenshareBitrate" flag in config.js is disabled.
+            const localVideoTrack = this.getLocalVideoTrack();
+
+            // Configure simulcast for camera tracks and for desktop tracks that need simulcast.
             if (this.isSimulcastOn() && browser.usesSdpMungingForSimulcast()
-                && (!this._capScreenshareBitrate
-                || (this._capScreenshareBitrate && !this._isSharingScreen()))) {
+                && (localVideoTrack?.getVideoType() === VideoType.CAMERA || !this._isSharingLowFpsScreen())) {
                 // eslint-disable-next-line no-param-reassign
                 resultSdp = this.simulcast.mungeLocalDescription(resultSdp);
                 this.trace(
@@ -2997,11 +3003,9 @@ TraceablePeerConnection.prototype.generateNewStreamSSRCInfo = function(track) {
         logger.error(`${this} Overwriting local SSRCs for track id=${rtcId}`);
     }
 
-    // Configure simulcast for camera tracks always and for desktop tracks only when
-    // the "capScreenshareBitrate" flag in config.js is disabled.
+    // Configure simulcast for camera tracks and desktop tracks that need simulcast.
     if (this.isSimulcastOn()
-        && (!this._capScreenshareBitrate
-        || (this._capScreenshareBitrate && !this._isSharingScreen()))) {
+        && (track.getVideoType() === VideoType.CAMERA || !this._isSharingLowFpsScreen())) {
         ssrcInfo = {
             ssrcs: [],
             groups: []