[whatwg] More random comments on the putImageData definition

Oliver Hunt oliver at apple.com
Sun Feb 10 14:37:04 PST 2008

On Feb 10, 2008, at 2:26 PM, Robert O'Callahan wrote:

> On Feb 10, 2008 10:07 PM, Oliver Hunt <oliver at apple.com> wrote:
> That said, basically what you're saying is that canvas should not  
> support hidpi.  At all. There is no need to request the dpi of a  
> canvas, but (and here's the critical bit) you can't have get/ 
> putImageData work at a different resolution from the backing buffer.
> Why not?

Because the purpose of get/putImageData *is* to get to the device  
pixels, that's their purpose, making them not do that is both bizarre,  
breaks the semantic that putImageData(getImageData(x,y,w,h), x, y)  
will leave the canvas unchanged, and the cost of sub-/super-sampling  
removes the whole "speed" thing that the API was originally added for.

> Their sole purpose is to be a 1:1 mapping to the canvas backing  
> store, so saying get/putImageData should work in canvas pixels and  
> not device pixels seems to defy the whole reason for this API  
> existing.
> It seems to me the API would still be useful even if it downsampled  
> the backing store.
> I guess we'll just cross our fingers and hope Web developers get it  
> right. We could implement a higher-resolution canvas backing store  
> for Firefox, but that wouldn't really help since most developers  
> won't test with a setup that would trigger it.

Yeah, unfortunately we all know the web authors tend to favour the "it  
works now, so must be correct" philosophy -- it's looking more and  
more like i will be forced to convert the canvas from hidpi to 1:1 the  
moment any of the imagedata/toDataURL routines are used.  *sigh*


> Rob
> -- 
> "He was pierced for our transgressions, he was crushed for our  
> iniquities; the punishment that brought us peace was upon him, and  
> by his wounds we are healed. We all, like sheep, have gone astray,  
> each of us has turned to his own way; and the LORD has laid on him  
> the iniquity of us all." [Isaiah 53:5-6]

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