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fix(call): do not downscale screenshare on Firefox #18747

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fix(call): do not downscale screenshare on Firefox #18747

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22 changes: 22 additions & 0 deletions src/utils/webrtc/simplewebrtc/peer.js
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Original file line number Diff line number Diff line change
Expand Up @@ -356,6 +356,22 @@ function mungeSdpForSimulcasting(sdp) {
}
/* eslint-enable */

/**
* Raises the "max-fs" (maximum frame size in macroblocks - 16x16 px)
* codec parameter in an SDP so high-resolution screens are not downscaled.
* Firefox defaults the VP8/VP9 "max-fs" to ~QHD (2560×1440px):
* - a=fmtp:120 max-fs=12288;max-fr=60
* Raising it lets Firefox encode up to ~4K (3840×2160px):
* - a=fmtp:120 max-fs=36864;max-fr=60
* Other browsers are not noticed to be affected with that limitation.
*
* @param {string} sdp the SDP string to munge
* @return {string} the updated SDP string
*/
function mungeSdpForScreenMaxFs(sdp) {
return sdp.replace(/max-fs=\d+/g, 'max-fs=' + 36864)
}

Peer.prototype.offer = function(options) {
const sendVideo = this.sendVideoIfAvailable && this.type !== 'screen'
if (sendVideo && this.enableSimulcast && adapter.browserDetails.browser === 'firefox') {
Expand Down Expand Up @@ -401,6 +417,12 @@ Peer.prototype.offer = function(options) {
}
}

// SDP munging only needed for Firefox screensharing (for 4K screens)
if (this.type === 'screen' && adapter.browserDetails.browser === 'firefox') {
console.debug('Raising VP8/9 max-fs for Firefox screensharing')
offer.sdp = mungeSdpForScreenMaxFs(offer.sdp)
}

this.pc.setLocalDescription(offer).then(function() {
if (this.parent.config.nick) {
// The offer is a RTCSessionDescription that only serializes
Expand Down
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