[Public WebGL] getContext multiple context language
Wed Jan 27 15:22:22 PST 2010
Ok, here's the compromise language... on subsequent calls to
getContext() for the same context, attrs must be either unspecified (in
which case you get what's there), or if given, must be compatible with
what's already active (or you get a hard error).
object getContext(in DOMString contextId, in optional any attributes)
A canvas may be rendered to using one or more contexts, each named
by a string context ID. For each canvas, there is a set of zero or
more active contexts. The getContext() method is used to obtain a
particular rendering context for the canvas.
'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 context ID that the implementation
does not support, it must return null.
For supported context IDs, the getContext() method behaves
differently depending on whether the requested context ID is already
an active context for the canvas.
If there are no active contexts for the canvas, the implementation
should attempt to create the specified context for the canvas.
If there are one or more active contexts and a context ID that is
not currently active is requested, it is up to the implementation to
determine whether the requested context can be used simultaneously
with all currently active canvas contexts. If simultaneous
rendering with the requested context is not possible, getContext()
must return null. Otherwise the implementation should attempt to
create the specified context for the canvas.
If a new context is to be created, the optional 'attributes'
parameter must be either unspecified or an object specific to the
context being requested. An unspecified value indicates a default
set of attributes, as defined by the context. Unknown attributes
should be ignored by the context.
If a new context is successfully 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 the canvas.
If a context ID that is already an active context for the canvas is
requested, then if the optional 'attributes' parameter is
unspecified, a reference to the existing context object must be
returned. If the 'attributes' parameter is specified, the context
must determine whether the given set of attributes is compatible
with the attributes set when the context was created. If the
attributes are compatible, the existing context object must be
returned. If the attributes are not compatible, getContext() must
raise an INVALID_STATE_ERR exception.
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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