[whatwg] createImageData

[Vladimir Vukicevic]

On May 13, 2008, at 2:58 PM, Oliver Hunt wrote:
>
> On May 13, 2008, at 1:53 PM, Vladimir Vukicevic wrote:
>> The first argument to the method must be an ImageData object  
>> returned by createImageData(), getImageData(), or an object  
>> constructed with the necessary properties by the user.  If the  
>> object was constructed by the user, its width and height dimensions  
>> are specified in device pixels (which may not map directly to CSS  
>> pixels).  If null or any other object is given that does not  
>> present the ImageData interface, then the putImageData() method  
>> must raise a TYPE_MISMATCH_ERR exception.
>
> If we were to add that we should include a note to indicate that  
> using a custom object is not recommended -- Any code that uses a  
> custom created object will never benefit from improvements in  
> ImageData performance made by the UA.

I'm fine with adding that language (the first part, anyway); something  
like "Using a custom object is not recommended as the UA may be able  
to optimize operations using ImageData if they were created via  
createImageData() or getImageData()."

> That said I still don't believe custom objects should be allowed,  
> aside from the resolution (which may or may not be relevant) and  
> performance issues, a custom object with a generic JS array, rather  
> than an ImageData object will have different behaviour -- a proper  
> ImageData will clamp on assignment, and throw in cases that a custom  
> object won't.

That verification seems odd; doing those checks (clamping, conversion  
to number) on every single pixel assignment is going the wrong  
direction for performance -- you really want to validate everything at  
once.

     - Vlad