631640 - add support for context options to 2D canvas context

Status: NEW

(Core :: Graphics: Canvas2D, defect)

Description

[Vladimir Vukicevic [:vlad] [:vladv] (needinfo me, slow to respond)]

We added the notion of context options for WebGL, but we can use them for 2D as well.  We have moz-opaque as an attribute, but we can also define it as a getContext option, e.g. { alpha: false }, just like WebGL.

An additional one to consider would be something like optimizeForPixelManipulation, which would optimize for getImageData/putImageData by creating an image surface instead of a d2d surface on windows.

Comment 1

[Jesse Ruderman]

This has happened. The second argument to getContext is declared in WebIDL as "any", but when the first argument is "2d", the second argument is treated by C++ as a "dictionary ContextAttributes2D".

Currently, the dictionary has willReadFrequently (added in bug 884226) and alpha (added in bug 982480).

The first argument is logically an enum; I don't know why it's declared as a string. The second argument is logically a "ContextAttributes2D or WebGLContextAttributes", but maybe it has to be "any" for implementation reasons.

Comment 2

[Florian Scholz (Open Web Docs)]

What Jesse mentions has happened indeed.

What about implementing context.getContextAttributes() to retrieve the context's attributes here?

interface Canvas2DRenderingContext {
    ...
    Canvas2DContextAttributes? getContextAttributes();
}

http://wiki.whatwg.org/wiki/CanvasOpaque
Inspired by WebGL: https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.2

Implemented and enabled in Chrome 32: https://code.google.com/p/chromium/issues/detail?id=234297
WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=112628