Give CanvasRenderingContext2D a getContextAttributes method

[foolip]

WebGLRenderingContextBase has this:
https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.2

For CanvasRenderingContext2D, it would return a CanvasRenderingContext2DSettings dictionary, which currently has only an alpha member.

This was shipped in Chrome in 2013 based on https://wiki.whatwg.org/wiki/CanvasOpaque but in commit 14d9138 only the getContext('2d', {alpha:false}) bit was added.

There is now an Intent to Remove: CanvasRenderingContext2D#getContextAttributes but it will probably be needed again.

Tests: https://jsbin.com/zabifor/edit?html,output (the last test fails in Chrome, but passes if it's the only test, which seems like a bug)

[foolip]

Looking for implementer interest:

• Blink: @zakerinasab @fserb @junov?
• EdgeHTML: @jatindersmann @techtoons?
• Gecko: @wisniewskit @msreckovic @MichaelKohler?
• WebKit: @cdumez? (can't find anyone else in https://github.com/WebKit/webkit/commits/master/Source/WebCore/html/canvas/CanvasRenderingContext2D.idl on GitHub)

[fserb]

Yeah, Blink talked about this while implementing color space stuff. I think it would make sense to have it as a proper standard.

Apart from color space and alpha, do you see other attributes that could go here?

[foolip]

I imagine it would return a CanvasRenderingContext2DSettings dictionary reflecting what was passed to getContext, or possibly exposing if some settings were ignored/coerced if there are combinations of things that don't make sense and yet do not throw exceptions when used.

For the color space stuff, do you plan to us a partial dictionary CanvasRenderingContext2DSettings, and how will it integrate with https://html.spec.whatwg.org/multipage/scripting.html#2d-context-creation-algorithm? If you need a hook there to avoid monkey patching, we should add that.

[cdumez]

I'll defer to @grorg on this.

[junov]

The proposal for adding color space support re-introduces this API. The proposal thread is here:
https://discourse.wicg.io/t/canvas-color-spaces-wide-gamut-and-high-bit-depth-rendering/1505
And the current version of the proposal is here:
https://github.com/WICG/canvas-color-space/blob/master/CanvasColorSpaceProposal.md

TL;DR: The canvas color space feature has special feature detection requirement. For example, a UA may not support all possible bit depths. When user code requests 10 bits per component, the browser may fall back to 8 bits if it does not support the requested bit depth. getContextAttributes() provides a nice way to detect the actual (as opposed to requested) canvas parameters.

[yell0wd0g]

With this intent to ship this issue should be getting renovated attention since it's in the idl file.

[yell0wd0g]

I filed crbug.com/919118 for implementing this feature, blocked on WPT tests for it in crbug.com/919116. Other browsers issues/bugs should follow shortly.

[yell0wd0g]

Filed as well https://bugzilla.mozilla.org/show_bug.cgi?id=1517786 and https://bugs.webkit.org/show_bug.cgi?id=193144