Switching video sources

A call can send the camera, a screen or window, or a video file, and the effects output when a background is on. Switching between them never renegotiates. The video sender stays the same and only what feeds it changes.

That matters for two reasons:

• E2EE. A frame cryptor is bound to one sender and one receiver. Keep the sender and the encryption keeps working.
• Stability. A renegotiation recreates the receive streams, and so the video decoders, on both sides. On Android the remote video froze after a few frames when that happened.

How each platform does it

Platform	Camera ↔ screen	Camera ↔ file
Web	replaceTrack()	replaceTrack()
iOS, macOS	Same RTCVideoSource, another capturer	Same RTCVideoSource, another capturer
Android	New video track, RtpSender.setTrack()	Same VideoSource, new capturer
Windows	New track on the same sender	New track on the same sender

Android gives the screen its own VideoSource because only a screencast source adapts by frame rate instead of resolution. The microphone track is never replaced anywhere, so muting keeps working while you share.

The web client, as an example

Each source has its own track. outgoingVideoTrack() in media.ts picks the one to send: the screen or file while sharing, else the effects canvas when an effect is on, else the camera. syncVideo() puts it on the sender with replaceTrack().

Sharing stops the camera, and stopping the share opens it again.

Starting without a camera

A call can start with no camera: another app has it (Windows gives a camera to one app at a time), none is plugged in, or access is blocked. The video sender exists anyway:

• The 1:1 offerer adds a sendrecv video transceiver without a track.
• The answerer turns the offer's recvonly video transceiver into sendrecv before answering.
• The group publish connection always has its sendonly video transceiver.
• Windows puts a placeholder track on the sender.

Turning the camera on later only needs to put a track on that sender. If it can't open, a toast says why: in use by another app, blocked, or none.

Screen sharing per platform

Platform	How
Web	getDisplayMedia()
Android	MediaProjection with a foreground service (ScreenCaptureService.kt). Rotating the phone resizes the virtual display.
iOS	A ReplayKit broadcast extension sends JPEG frames to the app over a Unix socket. See iOS.
macOS	ScreenCaptureKit (SCStream), with live thumbnails in the picker.
Windows	libwebrtc's desktop capturer, with live thumbnails in the picker.

Video files are read with <video>.captureStream() on the web, Mp4VideoCapturer on Android, RTCFileVideoCapturer on iOS, AVAssetReader on the Mac and Media Foundation on Windows. All of them loop.