snapshot of https://github.com/pion/webrtc.git (main, 2026-09-15; blobs > 131072B omitted)

examples/swap-tracks/README.md

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+# swap-tracks
+swap-tracks demonstrates how to swap multiple incoming tracks on a single outgoing track.
+
+## Instructions
+### Download swap-tracks
+```
+go install github.com/pion/webrtc/v4/examples/swap-tracks@latest
+```
+
+### Open swap-tracks example page
+[jsfiddle.net](https://jsfiddle.net/1rx5on86/) you should see two text-areas and two buttons: `Copy browser SDP to clipboard`, `Start Session`.
+
+### Run swap-tracks, with your browsers SessionDescription as stdin
+In the jsfiddle the top textarea is your browser's Session Description. Press `Copy browser SDP to clipboard` or copy the base64 string manually.
+We will use this value in the next step.
+
+#### Linux/macOS
+Run `echo $BROWSER_SDP | swap-tracks`
+#### Windows
+1. Paste the SessionDescription into a file.
+1. Run `swap-tracks < my_file`
+
+### Input swap-tracks's SessionDescription into your browser
+Copy the text that `swap-tracks` just emitted and copy into second text area
+
+### Hit 'Start Session' in jsfiddle, enjoy your video!
+Your browser should send streams to Pion, and then a stream will be relayed back, changing every 5 seconds.
+
+Congrats, you have used Pion WebRTC! Now start building something cool

examples/swap-tracks/jsfiddle/demo.css

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+/*
+    SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+    SPDX-License-Identifier: MIT
+*/
+textarea {
+    width: 500px;
+    min-height: 75px;
+}

examples/swap-tracks/jsfiddle/demo.details

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+---
+# SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+# SPDX-License-Identifier: MIT
+
+name: swap-tracks
+description: Example of how to have Pion swap incoming tracks on a single outgoing track
+authors:
+  - Chad Retz

examples/swap-tracks/jsfiddle/demo.html

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+<!--
+	SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+	SPDX-License-Identifier: MIT
+-->
+Browser base64 Session Description<br />
+<textarea id="localSessionDescription" readonly="true"></textarea> <br />
+<button onclick="window.copySDP()">
+  Copy browser SDP to clipboard
+</button>
+<br />
+<br />
+
+Golang base64 Session Description<br />
+<textarea id="remoteSessionDescription"></textarea> <br/>
+<button onclick="window.startSession()"> Start Session </button><br />
+
+<br />
+
+<div style="display: flex">
+  <div>
+    Browser stream 1<br />
+    <canvas id="canvasOne" height="200" width="200"></canvas>
+  </div>
+  <div>
+    Browser stream 2<br />
+    <canvas id="canvasTwo" height="200" width="200"></canvas>
+  </div>
+  <div>
+    Browser stream 3<br />
+    <canvas id="canvasThree" height="200" width="200"></canvas>
+  </div>
+</div>
+
+Video from server<br />
+<video id="serverVideo" width="200" height="200" autoplay muted></video> <br />

examples/swap-tracks/jsfiddle/demo.js

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+/* eslint-env browser */
+
+// SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+// SPDX-License-Identifier: MIT
+
+// Create peer conn
+const pc = new RTCPeerConnection({
+  iceServers: [
+    {
+      urls: 'stun:stun.l.google.com:19302'
+    }
+  ]
+})
+
+pc.oniceconnectionstatechange = e => {
+  console.debug('connection state change', pc.iceConnectionState)
+}
+pc.onicecandidate = event => {
+  if (event.candidate === null) {
+    document.getElementById('localSessionDescription').value = btoa(JSON.stringify(pc.localDescription))
+  }
+}
+
+pc.onnegotiationneeded = e =>
+  pc.createOffer().then(d => pc.setLocalDescription(d)).catch(console.error)
+
+pc.ontrack = event => {
+  console.log('Got track event', event)
+  document.getElementById('serverVideo').srcObject = new MediaStream([event.track])
+}
+
+const canvases = [
+  document.getElementById('canvasOne'),
+  document.getElementById('canvasTwo'),
+  document.getElementById('canvasThree')
+]
+
+// Firefox requires getContext to be invoked on an HTML Canvas Element
+// prior to captureStream
+const canvasContexts = canvases.map(c => c.getContext('2d'))
+
+// Capture canvas streams and add to peer conn
+const streams = canvases.map(c => c.captureStream())
+streams.forEach(stream => stream.getVideoTracks().forEach(track => pc.addTrack(track, stream)))
+
+// Start circles
+requestAnimationFrame(() => drawCircle(canvasContexts[0], '#006699', 0))
+requestAnimationFrame(() => drawCircle(canvasContexts[1], '#cf635f', 0))
+requestAnimationFrame(() => drawCircle(canvasContexts[2], '#46c240', 0))
+
+function drawCircle (ctx, color, angle) {
+  // Background
+  ctx.clearRect(0, 0, 200, 200)
+  ctx.fillStyle = '#eeeeee'
+  ctx.fillRect(0, 0, 200, 200)
+  // Draw and fill in circle
+  ctx.beginPath()
+  const radius = 25 + 50 * Math.abs(Math.cos(angle))
+  ctx.arc(100, 100, radius, 0, Math.PI * 2, false)
+  ctx.closePath()
+  ctx.fillStyle = color
+  ctx.fill()
+  // Call again
+  requestAnimationFrame(() => drawCircle(ctx, color, angle + (Math.PI / 64)))
+}
+
+window.startSession = () => {
+  const sd = document.getElementById('remoteSessionDescription').value
+  if (sd === '') {
+    return alert('Session Description must not be empty')
+  }
+
+  try {
+    pc.setRemoteDescription(JSON.parse(atob(sd)))
+  } catch (e) {
+    alert(e)
+  }
+}
+
+window.copySDP = () => {
+  const browserSDP = document.getElementById('localSessionDescription')
+
+  browserSDP.focus()
+  browserSDP.select()
+
+  try {
+    const successful = document.execCommand('copy')
+    const msg = successful ? 'successful' : 'unsuccessful'
+    console.log('Copying SDP was ' + msg)
+  } catch (err) {
+    console.log('Unable to copy SDP ' + err)
+  }
+}

examples/swap-tracks/main.go

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+// SPDX-FileCopyrightText: 2026 The Pion community <https://pion.ly>
+// SPDX-License-Identifier: MIT
+
+//go:build !js
+
+// swap-tracks demonstrates how to swap multiple incoming tracks on a single outgoing track.
+package main
+
+import (
+	"bufio"
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/pion/rtcp"
+	"github.com/pion/rtp"
+	"github.com/pion/webrtc/v4"
+)
+
+// nolint: cyclop
+func main() { // nolint:gocognit
+	// Everything below is the Pion WebRTC API! Thanks for using it ❤️.
+
+	// Prepare the configuration
+	config := webrtc.Configuration{
+		ICEServers: []webrtc.ICEServer{
+			{
+				URLs: []string{"stun:stun.l.google.com:19302"},
+			},
+		},
+	}
+	// Create a new RTCPeerConnection
+	peerConnection, err := webrtc.NewPeerConnection(config)
+	if err != nil {
+		panic(err)
+	}
+	defer func() {
+		if cErr := peerConnection.Close(); cErr != nil {
+			fmt.Printf("cannot close peerConnection: %v\n", cErr)
+		}
+	}()
+
+	// Create Track that we send video back to browser on
+	outputTrack, err := webrtc.NewTrackLocalStaticRTP(webrtc.RTPCodecCapability{
+		MimeType: webrtc.MimeTypeVP8,
+	}, "video", "pion")
+	if err != nil {
+		panic(err)
+	}
+
+	// Add this newly created track to the PeerConnection
+	rtpSender, err := peerConnection.AddTrack(outputTrack)
+	if err != nil {
+		panic(err)
+	}
+
+	// Read incoming RTCP packets
+	// Before these packets are returned they are processed by interceptors. For things
+	// like NACK this needs to be called.
+	go func() {
+		rtcpBuf := make([]byte, 1500)
+		for {
+			if _, _, rtcpErr := rtpSender.Read(rtcpBuf); rtcpErr != nil {
+				return
+			}
+		}
+	}()
+
+	// Wait for the offer to be pasted
+	offer := webrtc.SessionDescription{}
+	decode(readUntilNewline(), &offer)
+
+	// Set the remote SessionDescription
+	err = peerConnection.SetRemoteDescription(offer)
+	if err != nil {
+		panic(err)
+	}
+
+	// Which track is currently being handled
+	currTrack := 0
+	// The total number of tracks
+	trackCount := 0
+	// The channel of packets with a bit of buffer
+	packets := make(chan *rtp.Packet, 60)
+
+	// Set a handler for when a new remote track starts
+	peerConnection.OnTrack(func(track *webrtc.TrackRemote, receiver *webrtc.RTPReceiver) { //nolint: revive
+		fmt.Printf("Track has started, of type %d: %s \n", track.PayloadType(), track.Codec().MimeType)
+		trackNum := trackCount
+		trackCount++
+		// The last timestamp so that we can change the packet to only be the delta
+		var lastTimestamp uint32
+
+		// Whether this track is the one currently sending to the channel (on change
+		// of this we send a PLI to have the entire picture updated)
+		var isCurrTrack bool
+		for {
+			// Read RTP packets being sent to Pion
+			rtp, _, readErr := track.ReadRTP()
+			if readErr != nil {
+				panic(readErr)
+			}
+
+			// Change the timestamp to only be the delta
+			oldTimestamp := rtp.Timestamp
+			if lastTimestamp == 0 {
+				rtp.Timestamp = 0
+			} else {
+				rtp.Timestamp -= lastTimestamp
+			}
+			lastTimestamp = oldTimestamp
+
+			// Check if this is the current track
+			if currTrack == trackNum { //nolint:nestif
+				// If just switched to this track, send PLI to get picture refresh
+				if !isCurrTrack {
+					isCurrTrack = true
+					if track.Kind() == webrtc.RTPCodecTypeVideo {
+						if writeErr := peerConnection.WriteRTCP([]rtcp.Packet{
+							&rtcp.PictureLossIndication{MediaSSRC: uint32(track.SSRC())},
+						}); writeErr != nil {
+							fmt.Println(writeErr)
+						}
+					}
+				}
+				packets <- rtp
+			} else {
+				isCurrTrack = false
+			}
+		}
+	})
+
+	ctx, done := context.WithCancel(context.Background())
+
+	// Set the handler for Peer connection state
+	// This will notify you when the peer has connected/disconnected
+	peerConnection.OnConnectionStateChange(func(state webrtc.PeerConnectionState) {
+		fmt.Printf("Peer Connection State has changed: %s\n", state.String())
+
+		if state == webrtc.PeerConnectionStateFailed {
+			// Wait until PeerConnection has had no network activity for 30 seconds or another failure.
+			// It may be reconnected using an ICE Restart.
+			// Use webrtc.PeerConnectionStateDisconnected if you are interested in detecting faster timeout.
+			// Note that the PeerConnection may come back from PeerConnectionStateDisconnected.
+			done()
+		}
+
+		if state == webrtc.PeerConnectionStateClosed {
+			// PeerConnection was explicitly closed. This usually happens from a DTLS CloseNotify
+			done()
+		}
+	})
+
+	// Create an answer
+	answer, err := peerConnection.CreateAnswer(nil)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create channel that is blocked until ICE Gathering is complete
+	gatherComplete := webrtc.GatheringCompletePromise(peerConnection)
+
+	// Sets the LocalDescription, and starts our UDP listeners
+	err = peerConnection.SetLocalDescription(answer)
+	if err != nil {
+		panic(err)
+	}
+
+	// Block until ICE Gathering is complete, disabling trickle ICE
+	// we do this because we only can exchange one signaling message
+	// in a production application you should exchange ICE Candidates via OnICECandidate
+	<-gatherComplete
+
+	fmt.Println(encode(peerConnection.LocalDescription()))
+
+	// Asynchronously take all packets in the channel and write them out to our
+	// track
+	go func() {
+		var currTimestamp uint32
+		for i := uint16(0); ; i++ {
+			packet := <-packets
+			// Timestamp on the packet is really a diff, so add it to current
+			currTimestamp += packet.Timestamp
+			packet.Timestamp = currTimestamp
+			// Keep an increasing sequence number
+			packet.SequenceNumber = i
+			// Write out the packet, ignoring closed pipe if nobody is listening
+			if err := outputTrack.WriteRTP(packet); err != nil {
+				if errors.Is(err, io.ErrClosedPipe) {
+					// The peerConnection has been closed.
+					return
+				}
+
+				panic(err)
+			}
+		}
+	}()
+
+	// Wait for connection, then rotate the track every 5s
+	fmt.Printf("Waiting for connection\n")
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		default:
+		}
+
+		// We haven't gotten any tracks yet
+		if trackCount == 0 {
+			continue
+		}
+
+		fmt.Printf("Waiting 5 seconds then changing...\n")
+		time.Sleep(5 * time.Second)
+		if currTrack == trackCount-1 {
+			currTrack = 0
+		} else {
+			currTrack++
+		}
+		fmt.Printf("Switched to track #%v\n", currTrack+1)
+	}
+}
+
+// Read from stdin until we get a newline.
+func readUntilNewline() (in string) {
+	var err error
+
+	r := bufio.NewReader(os.Stdin)
+	for {
+		in, err = r.ReadString('\n')
+		if err != nil && !errors.Is(err, io.EOF) {
+			panic(err)
+		}
+
+		if in = strings.TrimSpace(in); len(in) > 0 {
+			break
+		}
+	}
+
+	fmt.Println("")
+
+	return
+}
+
+// JSON encode + base64 a SessionDescription.
+func encode(obj *webrtc.SessionDescription) string {
+	b, err := json.Marshal(obj)
+	if err != nil {
+		panic(err)
+	}
+
+	return base64.StdEncoding.EncodeToString(b)
+}
+
+// Decode a base64 and unmarshal JSON into a SessionDescription.
+func decode(in string, obj *webrtc.SessionDescription) {
+	b, err := base64.StdEncoding.DecodeString(in)
+	if err != nil {
+		panic(err)
+	}
+
+	if err = json.Unmarshal(b, obj); err != nil {
+		panic(err)
+	}
+}