RE: Screen sharing and device pixel ratio

________________________________________
From: Martin Thomson [martin.thomson@gmail.com]
Sent: Monday, December 14, 2015 12:39 PM
To: Harald Alvestrand
Cc: public-media-capture@w3.org
Subject: Re: Screen sharing and device pixel ratio

On 14 December 2015 at 20:58, Harald Alvestrand <harald@alvestrand.no> wrote:
> I see logical pixels as a hack that was introduced in order to avoid
> having to retrofit applications that made unwise assumptions about the
> relationship of pixels to screen size; I'd prefer that we not perpetuate
> the lunacy that they cause.

Unfortunately, it's not that simple.

[MH] And unfortunately, that ship has sailed a long time ago. All OSes expose some kind of logical screen size and scaling ratio. The Web Platform accepted this reality and partially exposes that through Window.devicePixelRatio. I'm not asking that we extend those concepts to video streams, on the contrary, I agree with Cullen that we should keep video pixels simple squares. However we have to acknowledge the source of the stream might have a different representation, and offer API surface for the app to decide what to do in those cases.

In an application-sharing case, there are potentially different
windows with different pixel density.  Compositing those into a single
track will result in windows with different sizes.

Also, it's not clear what the scale-down factor needs to be.  An
application doesn't know the pixel density of a given window.  Again,
if multiple windows are shared, the size discrepancy could be jarring.

[MH] And the density might actually change dynamically when a window is dragged between 2 monitors that have different pixel densities.