[cairo] defining CLAMP extend mode
spitzak at d2.com
Mon Oct 23 15:50:28 PDT 2006
I might have the terms wrong. By "EXTEND_PAD" I mean the filtering is
done such that when requesting a pixel to be filtered, the pixel
coordinates are clamped to the source rectangle.
Another way of looking at it is imagining the image has the rows and
columns of pixels on the edge repeat to infinity, producing "stripes" at
right angles to the image edge, and the corners are filled to infinity
with the value of the corner pixel. This new source image is used for
This is entirely independent of figuring out the antialised coverage of
the resulting output pixel. This needs to be done by multiplying the
filtered image pixel by a correctly antialiased rectangle. Don't be
tempted to think that the clamping and the antialiasing use the same
code, they don't, because of math inaccuracy, there very well may be
pixels that are 100% opaque but have clamped samples, and pixels that
have no clamped samples but are partially transparent.
Bill Spitzak wrote:
> I'm quite certain the only working implementation is equivalent to doing
> EXTEND_PAD, after first clipping to a (antialiased) rectangle that is
> the source image boundary transformed to the destination. Don't think of
> it as clamping, the actual sampling *is* EXTEND_PAD and is calculated
> entirely seperately from the clip.
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