Screen sharing and device pixel ratio

Hi,

The screen sharing spec doesn't mention anything about how to handle sources with various device pixel ratios.

I believe current implementations simply capture the physical pixels without taking into consideration the physical to virtual pixel ratio of the source.

E.g. if a full screen source of 1440 x 900 has a ratio of 2 dppx (Mac retina display), the captured stream will have a size of 2880x1800.

As an application I might not want a size that large, especially when transmitting to another display with a different pixel ratio (zoom 200%).

Existing max width / height constraints are not adapted to dealing with the resolution in this case:
- The content is really meant to be scaled by 2, not a floating point number that would make it just fit in the constrained size
- A small application window might have a size below the max constraints

One workaround with the state of the current specs could be to:
- gDM
- Read width / height of stream
- use Window.devicePixelRatio API [1] to guess aspect ratio of content
- applyConstraints on stream to scale it by that ratio

Unfortunately this might fail on multi-monitor systems that have different pixel ratios (Windows 10), when the browser tab and content captured are on different monitors. Especially because windows can be dragged across monitors, dynamically changing pixel ratios!

What I would like is a sort of "constraint" on the gDM call to indicate to the browser I need a stream "scaled" to be in virtual pixels instead of physical pixels.

Additionally, I would also consider adding a way to "read" the current pixel ratio of the source. This is to know if the captured stream is currently scaled when I ask for a "virtual pixels" stream, or the factor by which to scale it when asking the "physical pixels" and I want to do that processing on my own (e.g. I want to compute a scaling factor adapted to the pixel ratio of the receiving display)

Would there be any issues adding these to the spec?

Mathieu

[1] https://drafts.csswg.org/cssom-view/#dom-window-devicepixelratio

Received on Thursday, 10 December 2015 04:41:09 UTC