Bug#454550: Here is a conflict of our interests

[Alexander E. Patrakov]

Zack Weinberg wrote:
> The heuristic that comes to
> mind is to enable interpolation only for images that are not being
> scaled up by more than 10% or so of their natural size.

Here is a better approach that has correct limits and also handles non-uniform 
scaling and rotation.

Note that the source bitmap defines a function of two real (not integer!) 
variables (xdest and ydest) that has a constant value in each rectangle 
corresponding to a source pixel (and discontinuities at the edges of source 
pixels). The value of the destination pixel should be the integral from this 
(piecewise-constant) function over the area covered by this destination pixel.

-- 
Alexander E. Patrakov