[whatwg] Challenging canvas.supportsContext

James Robinson jamesr at google.com
Wed Jun 19 15:39:06 PDT 2013

On Wed, Jun 19, 2013 at 3:24 PM, Kenneth Russell <kbr at google.com> wrote:

> On Wed, Jun 19, 2013 at 3:06 PM, James Robinson <jamesr at google.com> wrote:
> > On Wed, Jun 19, 2013 at 3:04 PM, Kenneth Russell <kbr at google.com> wrote:
> >>
> >> supportsContext() can give a much more accurate answer than
> >> !!window.WebGLRenderingContext. I can only speak for Chromium, but in
> >> that browser, it can take into account factors such as whether the GPU
> >> sub-process was able to start, whether WebGL is blacklisted on the
> >> current card, whether WebGL is disabled on the current domain due to
> >> previous GPU resets, and whether WebGL initialization succeeded on any
> >> other page. All of these checks can be done without the heavyweight
> >> operation of actually creating an OpenGL context.
> >
> >
> > That's true, but the answer still doesn't promise anything about what
> > getContext() will do.  It may still return null and code will have to
> check
> > for that.  What's the use case for calling supportsContext() without
> calling
> > getContext()?
> Any application which has a complex set of fallback paths. For example,
>   - Preference 1: supportsContext('webgl', { softwareRendered: true })
>   - Preference 2: supportsContext('2d', { gpuAccelerated: true })
>   - Preference 3: supportsContext('webgl', { softwareRendered: false })
>   - Fallback: 2D canvas

I'm assuming you have (1) and (3) flipped here and both supportsContext()
and getContext() support additional attributes to dictate whether a
software-provided context can be supplied.  In that case, in order to write
correct code I'd still have to attempt to fetch the contexts before using
them, i.e.:

var ctx = canvas.getContext('webgl', { 'allowSoftware': false});
if (ctx) {
ctx = canvas.getContext('2d', {'allowSoftware': false});
if (ctx) {
// etc

how could I simplify this code using supportsContext() ?

> I agree that ideally, if supportsContext returns true then -- without
> any other state changes that might affect supportsContext's result --
> getContext should return a valid rendering context.

It seems overwhelmingly likely that one of the state changes that might
affect the result will be attempting to instantiate a real context.

> It's simply
> impossible to guarantee this correspondence 100% of the time, but if
> supportsContext's spec were tightened somehow, and conformance tests
> were added which asserted consistent results between supportsContext
> and getContext, would that address your concern?

I don't see how supportsContext() could be as accurate as getContext()
without doing all of the work getContext() does.  If it's not 100%
accurate, when is it useful?

- James

> -Ken

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