[whatwg] "canvas" reading ???
ian at hixie.ch
Tue Jun 14 03:12:35 PDT 2005
On Mon, 13 Jun 2005, Charles Iliya Krempeaux wrote:
> I know people are going to want to read individual or groups of pixel
> values. Certain image processing algorithms require it. (For
> example, certain edge detection algorithms. Etc. There's alot of
> different ones.)
The problem is that it is quite easy for a single x,y coordinate of the
canvas to have either multiple pixels, or have a pixel that is shared
between multiple x,y coordinates.
That is to say, <canvas> is resolution-independent. The units of the API
calls are CSS pixels, which are a length, not an index into the bitmap
that implements the canvas.
When you say
<canvas width="20" height="40">
...the coorindate space for the drawing commands is a 20x40 space, but the
actual bitmap depends on the actual size of the canvas when it is
rendered. If CSS specifies that it is 100px x 50px, then the bitmap will
(probably) be a 100x50 pixel bitmap. Except if you are on a printer or a
high-res screen, in which case it might be more like a 600x300 dot bitmap.
So which pixel do you return when you get asked for a specific pixel?
Ian Hickson U+1047E )\._.,--....,'``. fL
http://ln.hixie.ch/ U+263A /, _.. \ _\ ;`._ ,.
Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.'
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