[Public WebGL] getContext multiple context language

Vangelis Kokkevis [email protected]
Tue Jan 26 13:04:29 PST 2010


On Tue, Jan 26, 2010 at 12:41 PM, Kenneth Russell <[email protected]> wrote:

> On Mon, Jan 25, 2010 at 4:00 PM, Vladimir Vukicevic
> <[email protected]> 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.
>
> 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.

Vangelis






>
> > If the canvas has at least one active context and getContext() is called
> > with a contextId that does not name an already-active context, it may
> either
> > return a new canvas context that can be used simultaneously with all
> > previously obtained contexts, or return null to indicate that the newly
> > requested context cannot be used with the previously obtained contexts.
> If
> > a non-null value is returned, the new context is added to the list of
> active
> > contexts for this canvas.
> >
> > If multiple rendering contexts are active, they all render to the same
> > canvas bitmap; they are not layered or otherwise isolated.  Changes made
> to
> > the canvas bitmap with one context must be immediately visible to any
> other
> > active contexts on the canvas.  It is up to the implementation to manage
> > synchronization issues associated with rendering with different contexts
> to
> > the same canvas.
> >
> >
>
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