[whatwg] Offscreen canvas (or canvas for web workers).

Maciej Stachowiak mjs at apple.com
Mon Mar 15 04:22:00 PDT 2010

On Mar 15, 2010, at 3:46 AM, Philip Taylor wrote:

> On Mon, Mar 15, 2010 at 7:05 AM, Maciej Stachowiak <mjs at apple.com>  
> wrote:
>> Copying from one canvas to another is much faster than copying to/ 
>> from
>> ImageData. To make copying to a Worker worthwhile as a responsiveness
>> improvement for rotations or downscales, in addition to the  
>> OffscreenCanvas
>> proposal we would need a faster way to copy image data to a Worker.  
>> One
>> possibility is to allow an OffscreenCanvas to be copied to and from a
>> background thread. It seems this would be much much faster than  
>> copying via
>> ImageData.
> Maybe this indicates that implementations of getImageData/putImageData
> ought to be optimised? e.g. do the expensive multiplications and
> divisions in the premultiplication code with SIMD. (A seemingly
> similar thing at http://bugzilla.openedhand.com/show_bug.cgi?id=1939
> suggests SSE2 makes things 3x as fast). That would avoid the need to
> invent new API, and would also benefit anyone who wants to use
> ImageData for other purposes.

It might be possible to make getImageData/putImageData faster than  
they are currently, certainly the browsers at the slower end of the  
ImageData performance spectrum must have a lot of headroom. But they  
probably also probably have room to optimize drawImage. (Looking back  
at my data I noticed that getImageData + putImageData in Safari is  
about as fast or faster than two drawImage calls in the other browsers  

In the end, though, I doubt that it's possible for getImageData or  
putImageData to be as fast as drawImage, since drawImage doesn't have  
to do any conversion of the pixel format.


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