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Re: [Public WebGL] getContext multiple context language



On 1/26/2010 1:04 PM, Vangelis Kokkevis wrote:


On Tue, Jan 26, 2010 at 12:41 PM, Kenneth Russell <kbr@google.com> wrote:
On Mon, Jan 25, 2010 at 4:00 PM, Vladimir Vukicevic
<vladimir@mozilla.com> wrote:
>
> Ok, here's some updated text... how's this look?
>
> object getContext(in DOMString contextId, in optional any attributes)
>
> 'contextId' must be a string naming a canvas rendering context to be
> returned.  For example, this specification defines the '2d' context, which,
> if requested, will return either a reference to an object implementing
> CanvasRenderingContext2D or null, if a 2D context cannot be created at this
> time.  Other specifications may define their own contexts, which would
> return different objects.
>
> If getContext() is called with a contextId that the implementation supports
> and such a context can be created, a reference to an object implementing the
> context API is returned and the new context is added to the list of active
> contexts for this canvas.  If contextId is not supported or it cannot be
> created, null is returned.  The optional 'attributes' parameter must be
> either null, or an object specific to the context being requested.  A null
> or unspecified value for the 'attributes' parameter is equivalent to
> requesting default attributes for the given context.  Any unknown attributes
> should be ignored by the context.
>
> If the canvas has at least one active context and getContext() is called
> with a contextId that names an already-active context, it must return the
> same resulting context object provided that the attributes are identical to
> all previous requests for that context ID.  If the attributes are not
> identical, getContext() must raise an INVALID_STATE_ERR exception.  (XXX is
> INVALID_STATE the right thing here? or is SYNTAX_ERR better?)  Deciding
> which attributes are identical is up to the specific context being
> requested, but should mean that the requested attributes and their values
> are the same as in all previous getContext() calls with this contextId.

I think it would be better if the attributes were ignored for the
second and subsequent requests for a given contextId. The context
creation attributes, at least in the case of WebGL, are a request,
rather than a requirement, of capabilities. If a given platform
happens to not support multisampling, then the return value of
WebGLRenderingContext.getContextAttributes() will contain "false" for
its "antialias" attribute, regardless of whether the user requested
antialiasing during the Canvas.getContext() call.

One thing that I have on my todo list (it's actually done, just need to make sure I don't screw up the svn line endings) is to change the webgl attributes object a bit -- we changed 'antialias' to 'antialiasHint', and made the other non-Hint attributes requirements... that is, if you request stencil, you either get a context with a stencil buffer, or null.  For the Hint attributes, you can get anything, but you indicate your preference.

One thing that I didn't do here but might be a good idea is to move getContextAttributes(contextId) to the canvas itself, as opposed to putting it on the context.  That seems to be better symmetry.

If we require that the attributes must be identical during all
requests, then exactly what version of the attributes need to be
passed? The originally requested ones, which might not have been able
to be honored, or the actual attributes that were satisfied by the
implementation?

For this reason I strongly think that it would be a mistake to pay
attention to the attributes on second and subsequent getContext()
calls. Here is suggested re-wording:

"If getContext() is called with a contextId that names an
already-active context, it must return the same resulting context
object. In this case, the attributes, if any, are ignored."

-Ken

I agree with Ken's suggestion.  If the point of being able to call getContext() multiple times is to avoid having to pass the context object around then requiring every call to use the same arguments seems to negate that benefit (you would have to pass the arguments around instead).  It also possibly creates an expectation that by passing a different set of arguments you could modify an existing context which we know it's not what happens.

It would be simpler, but the latter bit is exactly what worries me -- if the attributes are just silently ignored, then the user might think that a switch took place.

What about a third alternative: attributes are invalid on a getContext() call for an already-active context?  That is, any subsequent getContext call must not specify any attributes?

    - Vlad