[cairo] Transparent pattern may be wrong

[Bill Spitzak]

In an anti-aliased system such as Cairo specifying patterns as having 
transparent edges may be the wrong thing to do. The problem is 
double-premultiply:

It would be very common for a user to draw a rectangle exactly the same 
size as the area of their source image. However if the transformation is 
such that the rectangle does not land on pixel boundaries, the edge 
pixels will be antialiased and thus the pattern pixels will be 
multiplied by this antialias value. But the pattern itself is also 
filtered, so if the edge is transparent, the pixel will already be 
multiplied by the fraction that is inside. Thus it will be multiplied by 
the coverage twice, producing a black border around the rectangle. The 
effect is subtle but can get really annoying fast. This is especially 
bad if various other errors cause the rectangle to not exactly line up 
with the image edge.

I do not think an operation with such unexpected side effects should be 
the default. In fact the only operations without unexpected effects is 
nearest-pixel and a "mirror" repeat of the image. I recommend 
nearest-pixel be the default. This also has the convient effect of being 
the expected value for gradients.

There is no need to support transparent edge. The transparent edge can 
be exactly emulated by making the pattern be 1 pixel larger on all 
sides, with a border of transparent pixels, and using nearest-pixel for 
the outside. Hardware mipmap implementation of transparent edge *has* to 
do this anyway, so it may be more efficient to move this up to the Cairo 
API because the pattern can be loaded directly into the texture. 
Transparent edge can also be emulated for most operators by making a 
clip region of the proper rectangle.