[cssom-view] Expose video plane pixel ratio

[chcunningham]

Discussion started in #5044. The main idea is to expose the device pixel ratio for <video>, allowing authors to select an optimal content resolution. UAs on TV's and dongles will often have a distinct video "plane" with a higher device pixel ratio than reported by window.devicePixelRatio.

I propose adding deviceVideoPixelRatio to Screen. Authors can then determine the "video" pixels of anything, including screen (width, height), window (availWidth, availHeigh), as well as inline. For example:

full screen case

targetResolution = screen.width * window.deviceVideoPixelRatio;

inline case

targetResolution = element.clientWidth * window.deviceVideoPixelRatio;

[chcunningham]

This proposal was discussed in today's call. Full transcript here. Let me respond to some points:

smfr: imagine you have two screens, right hand is dedicated to video presentation
smfr: somehow magically when you present video, it shows up on the right
smfr: but window object is associated with the left hand screen
smfr: so would be weird if it reported characteristics of right hand screen
???: Is this possible today?
smfr: UA thing, UA could decide to send fullscreen video to a particular screen
...
florian: a browser could have two different videos on different screens, but that's a very different setup than here
smfr: I understand, I just don't want us to design a problem for foldables

I think this idea to relocate fullscreen video is problematic because most streaming sites actually fullscreen a div containing the video to allow for custom controls.

florian: This isn't necessarily about fullscreen video.
florian: if you have youtube playing in the middle of your web page, you would apply this ratio within the video
smfr: No, I don't think so, I think it only applies to fullscreen
...
smfr: I think we should ask experts for feedback

@jpiesing (TV expert) indicated that this is not just about full screen in #5044 (comment).

smfr: I think it would be very confusing for author

@smfr, can you say more about the confusion? Streaming sites are already be accustomed to listening for changes to the element size and/or fullscreen events as a trigger to adapt content resolution.

[smfr]

Do devices with a separate video plane use that for inline video?

[chrisn]

TV devices do that, yes. The hardware then arranges for the video plane to be composited (with appropriate scaling) with the graphics plane when to produce the output for the display.