Default to highest Simulcast layer

Better experience then just defaulting to first packet we see

Author: Sean-Der

internal/webrtc/sessions/whep/types.go

@@ -22,7 +22,8 @@ type (
 		PeerConnectionLock sync.RWMutex
 		PeerConnection     *webrtc.PeerConnection
 
-		// Protects VideoTrack, VideoTimestamp, VideoPacketsWritten, VideoSequenceNumber
+		// Protects VideoTrack, VideoTimestamp, VideoPacketsWritten, VideoSequenceNumber,
+		// and auto video layer selection state.
 		VideoLock               sync.RWMutex
 		VideoTrack              *codecs.TrackMultiCodec
 		VideoTimestamp          uint32
@@ -34,6 +35,8 @@ type (
 		VideoPacketsDropped     atomic.Uint64
 		VideoSequenceNumber     uint16
 		VideoLayerCurrent       atomic.Value
+		videoLayerPriority      int
+		videoLayerExplicit      bool
 
 		// Protects AudioTrack, AudioTimestamp, AudioPacketsWritten, AudioSequenceNumber
 		AudioLock           sync.RWMutex

internal/webrtc/sessions/whep/whep.go

@@ -113,7 +113,13 @@ func (w *WHEPSession) SetAudioLayer(encodingID string) {
 // Sets the requested video layer for this WHEP session.
 func (w *WHEPSession) SetVideoLayer(encodingID string) {
 	log.Println("Setting Video Layer")
+
+	w.VideoLock.Lock()
 	w.VideoLayerCurrent.Store(encodingID)
+	w.videoLayerPriority = 0
+	w.videoLayerExplicit = encodingID != ""
+	w.VideoLock.Unlock()
+
 	w.IsWaitingForKeyframe.Store(true)
 	w.SendPLI()
 }
@@ -147,16 +153,34 @@ func (w *WHEPSession) updateVideoBitrateLocked(now time.Time) {
 	w.videoBitrateWindowBytes = w.VideoBytesWritten
 }
 
-func (w *WHEPSession) GetVideoLayerOrDefault(defaultLayer string) string {
+func (w *WHEPSession) GetVideoLayerOrDefault(defaultLayer string, defaultPriority int) string {
 	w.VideoLock.Lock()
 	defer w.VideoLock.Unlock()
 
 	currentLayer, _ := w.VideoLayerCurrent.Load().(string)
-	if currentLayer != "" {
+	if w.videoLayerExplicit {
 		return currentLayer
 	}
 
-	w.VideoLayerCurrent.Store(defaultLayer)
-	w.IsWaitingForKeyframe.Store(true)
-	return defaultLayer
+	if currentLayer == "" {
+		w.VideoLayerCurrent.Store(defaultLayer)
+		w.videoLayerPriority = defaultPriority
+		w.IsWaitingForKeyframe.Store(true)
+		return defaultLayer
+	}
+
+	if currentLayer == defaultLayer {
+		w.videoLayerPriority = defaultPriority
+		return currentLayer
+	}
+
+	// Lower numeric priority value means a better simulcast layer.
+	if w.videoLayerPriority == 0 || defaultPriority < w.videoLayerPriority {
+		w.VideoLayerCurrent.Store(defaultLayer)
+		w.videoLayerPriority = defaultPriority
+		w.IsWaitingForKeyframe.Store(true)
+		return defaultLayer
+	}
+
+	return currentLayer
 }

internal/webrtc/sessions/whip/handlers.go

@@ -35,7 +35,7 @@ func (w *WHIPSession) onTrackHandler(peerConnection *webrtc.PeerConnection, stre
 
 		if strings.HasPrefix(remoteTrack.Codec().MimeType, "audio") {
 			// Handle audio stream
-			w.audioWriter(remoteTrack, streamKey, peerConnection)
+			w.audioWriter(remoteTrack, streamKey)
 		} else {
 			// Handle video stream
 			w.videoWriter(remoteTrack, streamKey, peerConnection)

internal/webrtc/sessions/whip/writers.go

@@ -17,7 +17,7 @@ import (
 	pionCodecs "github.com/pion/rtp/codecs"
 )
 
-func (w *WHIPSession) audioWriter(remoteTrack *webrtc.TrackRemote, streamKey string, peerConnection *webrtc.PeerConnection) {
+func (w *WHIPSession) audioWriter(remoteTrack *webrtc.TrackRemote, streamKey string) {
 	id := remoteTrack.RID()
 
 	if id == "" {
@@ -31,8 +31,6 @@ func (w *WHIPSession) audioWriter(remoteTrack *webrtc.TrackRemote, streamKey str
 		return
 	}
 
-	track.Priority = w.getPrioritizedStreamingLayer(id, peerConnection.CurrentRemoteDescription().SDP)
-
 	rtpPkt := &rtp.Packet{}
 	rtpBuf := make([]byte, 1500)
 	for {
@@ -183,7 +181,7 @@ func (w *WHIPSession) videoWriter(remoteTrack *webrtc.TrackRemote, streamKey str
 		}
 
 		for _, whepSession := range sessions {
-			if whepSession.GetVideoLayerOrDefault(id) != id {
+			if whepSession.GetVideoLayerOrDefault(id, track.Priority) != id {
 				continue
 			}