[whatwg] getImageData/putImageData comments

[Boris Zbarsky]

I was just looking at the spec text on getImageData/putImageData, and I 
had a few comments.  Of the three, #3 is the most important one:

1)  It may be worth noting that if the canvas backing store is stored
     as premultiplied rgba, then getImageData right after a putImageData
     may well not return the values in the CanvasPixelArray that was
     put, due to rounding when converting to and from premultiplied
     colors.
2)  The description of putImageData says it "Paints the data from the
     given ImageData object onto the canvas".  It may be worth
     specifying that this uses the SOURCE operator, though this is
     clear later on when defining what the method _really_ does.
3)  It's not clear to me why imagedata actually exposes device pixels,
     nor is it clear to me how this is supposed to work if the same
     document is being rendered to multiple devices.  Is a UA allowed
     to have a higher internal resolution for a canvas (in device pixels)
     and then sample when painting to the device?  This might well be
     desirable if the UA expects the canvas to be scaled; it can well
     reduce scaling artifacts in that situation.  It doesn't seem
     reasonable, to me, to expose such super-sampling via imageData;
     it's entirely an optimization detail.

     Worse yet, the current setup means that a script that tries
     createImageData, fill in the pixels, and then paint it to the
     canvas, needs to fill different numbers of pixels depending on the
     output device.  I fully expect script authors to get this very very
     wrong, since it's such non-intuitive behavior.  It would make more
     sense to just have the script work entirely in CSS pixels; if it
     wishes to create a higher-resolution image it can create a canvas
     with bigger dimensions (and scale its actual display via setting
     its width and height CSS properties).

-Boris