{"id":153,"date":"2016-10-19T14:36:41","date_gmt":"2016-10-19T14:36:41","guid":{"rendered":"https:\/\/testmozilla.wpengine.com\/webrtc\/?p=153"},"modified":"2018-03-23T21:14:47","modified_gmt":"2018-03-23T21:14:47","slug":"warm-up-with-replacetrack","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/webrtc\/warm-up-with-replacetrack\/","title":{"rendered":"Warm-up with dummy tracks and replaceTrack"},"content":{"rendered":"

If you’ve looked at the WebRTC spec<\/a> over the last year, you’ll find a gap between what it says and what browsers have implemented so far. The big difference is the spec talks about senders and receivers of tracks<\/a>, whereas most browsers still operate with streams.<\/p>\n

Firefox is the only browser so far to have pivoted to tracks, the stuff streams are made of. This opens up exciting possibilities like sender.replaceTrack()<\/code>. It lets you switch out an already-sending track without needing to renegotiate, and still see the change reflected remotely. We’re not fully done with senders and receivers, and we have more to do (like transports), but replaceTrack works.<\/p>\n

The spec, however, goes even further, defining transceivers, a pairing of a sender and a receiver. Even Firefox hasn’t implemented that yet. The spec shows an example of connection warm-up<\/a> using transceivers to set up a connection before media is ready. This means you can send media the instant it becomes available. Quite neat, and a benefit of the spec moving to objects that can exist before media is added.<\/p>\n

But it turns out we don’t have to wait for transceivers to do warm-up, which is what we’re going to show here!<\/p>\n

You can already read my other blog on how to renegotiate quickly over datachannels<\/a>, but that won’t cut it here. For parity with the spec’s warm-up example, we must be ready to send media without any additional negotiation at all.<\/p>\n

We’re going to (surprise!) use replaceTrack for this: Basically, we’ll set up and negotiate a connection using dummy tracks, and replace them with real media later once available. To do this, we first need dummy tracks.<\/p>\n

Dummy tracks<\/h3>\n

For all the bells and whistles of WebRTC, you’d think creating dummy audio and video tracks would be basic. Well, it’s not, but it’s doable. Without further ado, here’s what we have to do:<\/p>\n