[Digikam-devel] Patch: fix automatic correction of lens chromatic aberration

[Matthias Welwarsky]

Hello,

the attached patch fixes CA correction in the lens auto-correction tool. The fix 
consists of two parts:
1.) set Alpha component of corrected pixel to "fully opaque", otherwise 
corrected image is not visible (trivial)
2.) Implement sub-pixel interpolation in class DImg, necessary for CA and CI 
correction with lensfun.
There are two new pixel color accessor functions: getSubPixelColor() and 
getSubPixelColorFast(). The first one uses Lanczos interpolation to compute 
subpixel colors, the second one bilinear interpolation. Lanczos yields better 
results, but is a lot slower than bilinear. Before using getSubPixelColor(), 
prepareSubPixelAccess() needs to be called to precompute the Lanczos kernel.

Lanczos implementation was adapted from ufraw, Bilinear was implemented 
according to Wikipedia description.

I would be happy to hear comments on how to optimize the computation, it's 
probably not optimal yet.

regards,
	matthias 
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