Re: [webrtc-pc] scaleResolutionDownBy validation steps are inconsistent, browser interop issues (#2873)

> When creating a transceiver we do 4:2:1 but when setting parameters we do 1:1:1.

If I addTransceiver, default is 4:2:1 → getParameters is 4:2:1 → setParameters will set 4:2:1 unless I change it.
If I setRemoteDescription, default is 4:2:1 → getParameters is 4:2:1 → setParameters will set 4:2:1 unless I change it.

I would characterize that as consistent.

Are we only talking about an app that manipulates all layers to `undefined` on purpose? E.g.
```js
const params = sender.getParameters();
for (encoding of params.encodings) {
  delete encoding.scaleResolutionDownBy;
}
await sender.setParameters(params)
```
Why would anyone...
> In Chrome, whether or not you specify scaleResolutionDownBy affects whether you get VP9 simulcast or backwards-compat VP9 legacy SVC mode ([PSA](https://groups.google.com/g/discuss-webrtc/c/-QQ3pxrl-fw)), which may skew apps' preferences towards not specifying scaleResolutionDownBy in the medium term

I see.

What behavior are you proposing? That the code I show above produce 4:2:1 for three layers, and 2:1 for two?

If so, what should the following produce?
```js
const params = sender.getParameters();
delete params.encodings[0].scaleResolutionDownBy;
delete params.encodings[1].scaleResolutionDownBy;
await sender.setParameters(params);
```
Would it depend on how many encodings there are or what `encodings[2]` is? E.g. 1:1:1 if three, or 2:1 for two?

From my recollection of discussions on the existing behavior, we stopped short of applying magic defaults once parameters have been fiddled with, since they arguably stop being POLA at that point.

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Received on Thursday, 18 May 2023 21:19:08 UTC