[whatwg] proposed canvas 2d API additions

David Hyatt hyatt at apple.com
Sat Apr 22 18:15:48 PDT 2006

The getPixels proposal seems broken to me in that it does not take  
into account high DPI, i.e., a situation where the canvas pixel space  
has been scaled and a more detailed image is possibly rendering  
within a region of the canvas.  In this situation, a "canvas pixel"  
could actually be a bunch of device pixels that contain different  
color information.

(hyatt at apple.com)

On Apr 21, 2006, at 12:10 PM, Vladimir Vukicevic wrote:

> Hi folks,
> I'd like to suggest extending the HTML canvas 2d context with a few
> additions.  These are variations on some of the methods added to
> Opera's "opera-2dgame" context.  The methods are intended to give
> content authors direct pixel access to the canvas, as well as provide
> some basic point-in-path testing functionality.
>     float [] getPixels (in integer x, in integer y, in integer width,
> in integer height);
> Returns an array of floats representing the color values in the region
> of pixels in the canvas whose upper left corner is at (x,y) and which
> extends for width,height pixels.  These coordinates are in canvas
> pixel space (that is, the same space that the canvas width and height
> attributes are specified in).  The color values for each pixel are
> returned as 4 floats, each in the range of 0.0 to 1.0, in R,G,B,A
> order.  That is, given the paramters (0,0,2,2), the returned array
> will be [R00 G00 B00 A00 R10 G10 B10 A10 R01 G01 B01 A01 R11 B11 G11
> A11].
> Note: we could return the pixels as integers in the range of 0..255,
> as 8-bit color is most likely what canvases will be dealing with.
> However, using floats allow us to easily extend into a 16-bit
> colorspace without any API changes.  In addition, any computation
> using these pixels is often done in normalized colors, so the division
> by 255 would need to happen anyway.
>     void putPixels (in float [] pixels, in integer x, in integer y, in
> integer width, in integer height);
> Does the opposite of getPixels; the given array must be exactly width
> * height * 4 elements in length.  The values are to be clamped to
> 0.0..1.0.
>     boolean pointInPathFill(in float x, in float y);
> pointInPathFill returns true if the given point would be inside the
> region filled by the current path, and false otherwise.  The x,y
> coordinates are in the current space of the canvas; that is, they are
> transformed by the CTM and do not necessarily map directly to pixels.
> I'd suggest that these three functions be added directly to the "2d"
> context; content authors can test for their presence by checking the
> function is not null on the 2d context object.  We might want a more
> comprehensive way of letting authors test whether particular features
> are supported, e.g. "shadows", "pixel-access", etc, but maybe it's not
> necessary.
> How's this sound?
>     - Vlad

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