[whatwg] proposed canvas 2d API additions

Vladimir Vukicevic vladimirv at gmail.com
Fri May 5 14:56:19 PDT 2006

Also... should the RGBA data be returned with premultiplied alpha or
not?  Premultiplied tends to be better for the math, non-premultiplied
tends to be easier to understand.  (That is, 50% opaque green is
(0,255,0,128) if non-premultiplied, or (0,128,0,128) if

    - Vlad

On 5/4/06, Vladimir Vukicevic <vladimirv at gmail.com> wrote:
> On 4/28/06, Vladimir Vukicevic <vladimirv at gmail.com> wrote:
> > interface ImageData {
> >   readonly attribute string format; /* only "rgba" is valid for now */
> >   readonly attribute long int width;
> >   readonly attribute long int height;
> >   readonly attribute Array data;
> > }
> Actually, let's step back a second; this may be massive
> overengineering.  What if we simply had:
>     readonly attribute float deviceScaling;
> on the 2D context, which would give the scaling factor between
> canvas-space pixels (that is, the space that the <canvas> width/height
> attributes are in) and device-space pixels (the pixels of the actual
> backing store).  So if <canvas width="200" height="200"/> was
> represented with a 300x300 backing store, deviceScaling would be 1.5;
> if 400x400, it would be 2.0.  (If necessary, we can have
> deviceScalingX, deviceScalingY.)
> Then getPixels is defined to take parameters in canvas pixel space,
> and returns the ARGB array in device space; if you ask for a 50x50
> region, you'll get back 100x100x4 samples, with a deviceScaling of
> 2.0.  putPixels would take coordinates in canvas pixel space again,
> but would take the appropriate device-pixel-sized ARGB array.  This
> becomes tricky with non-integer deviceScaling; that is, if a 2x2
> region becomes a 3x3 region with a deviceScaling of 1.5, what do you
> return when you're asked for x=1 y=1 w=1 h=1?  I'd say that you end up
> resampling and shifting over your 3x3 device space backing store by .5
> pixels so that the region would start on a device pixel boundary.
> This would obviously not be a clean round-trip, but the spec can
> inform authors how to ensure a clean round trip (only request regions
> where your x/y * deviceScaling are integers).
> This removes the need for a separate ImageData object and all the
> extra gunk necessary there, but still maintains full resolution
> independence.  Any thoughts on this?
>    - Vlad

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