[Public WebGL] getContext multiple context language

Vladimir Vukicevic [email protected]
Tue Jan 26 13:44:55 PST 2010

On 1/26/2010 1:26 PM, Kenneth Russell wrote:
> On Tue, Jan 26, 2010 at 1:09 PM, Vladimir Vukicevic
> <[email protected]>  wrote:
>> On 1/26/2010 1:04 PM, Vangelis Kokkevis wrote:
>> 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.
>> 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.
> I thought from earlier discussions that we were going to stay closer
> to WGL's (and Mac OS X's) approach to the matching of the attributes
> than that of GLX. In WGL, the pixel format best matching the requested
> attributes is returned, while in GLX, if any one of the requested
> attributes can't be satisfied, glXChooseVisual / glXChooseFBConfig
> fails. GLX's behavior is too stringent in my experience. If we do
> this, then it isn't necessary to change the naming of any of the
> attributes in WebGLContextAttributes.
> In particular, I don't like the idea that we are mandating the
> presence of the stencil buffer by default. This would mean that all
> current WebGL implementations are not spec compliant.

That's a good point; but I'd rather fix that by changing the defaults in 
the spec.  Things like stencil and depth buffer seem like hard 
requirements for an app to me; I guess if a stencil buffer isn't 
available an app could use a different algorithm in some cases, but if 
the app does have that option then it could just retry the getContext() 
call.  I realize that this gets into the somewhat ugly gl attribs loops 
that are found in C, but it's either that or requiring every app to 
ensure that, for example, it got a depth buffer.  Seems that the 
simplest code would be for an app to request some basic capabilities, 
such as depth and maybe stencil, and require that it gets those; 
antialiasing is the outlier here, since the app would run just fine 
without AA... thus the 'Hint' status.

>> 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.
> Perhaps. For convenience though it seems helpful to retain
> getContextAttributes() on the context object itself.
>>>> 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?
> I think this will lead to problems when composing multiple libraries
> that all use WebGL, so I still think the attributes should just be
> ignored on the second and subsequent getContext() calls. Well-written
> libraries will check the context attributes and ensure that they got
> all of the features they require; but the vast majority of libraries
> that don't will still work, and interoperate cleanly.

Hmm, that's exactly the problem that I was trying to solve by requiring 
the attributes to always be passed... that is, if library A requests 
some attributes, and library B requests other attributes, library B 
won't have any option but to fail if it inspects the attributes after 
the fact.. or it would have ignored them in the first place.

However, really, I want to strongly discourage people from using 
getContext(); I'd like authors to call getContext once and then pass 
around the context object.  The language that's in there currently about 
requiring the same attributes is an oblique way at encouraging that.  If 
we didn't already have code using getContext("2d") all over the place in 
rendering loops, I'd push for making it illegal to call getContext more 
than once with the same context name.  I don't think the point of being 
able to call getContext() multiple times is necessarily to avoid having 
to pass around the context... you'd still have to pass around the canvas 

I still think the safest option is to require that the same set of 
attributes is specified in each call -- silent-ignore behaviour for 
something like this seems wrong, and my suggested 
error-if-specified-at-all behaviour is really just a bandaid for that 

We can try to craft another alternative:

- First call to getContext -- unspecified attributes means defaults, 
specified as given.
- Subsequent calls to getContext -- unspecified attributes means "give 
me what's there", specified as given, error if specified isn't a subset 
of currently active attributes.

That doesn't discourage using getContext() multiple times, but it gets 
rid of any silent-ignore behaviour, and gives libraries a way to 
interoperate if they have no specific attribute requests.

     - Vlad

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