this is a straightforward implementation of richardson lucy deconvolution. it's inspired by rawtherapee's "capture sharpening" and can help reduce some blur caused by optical low pass filters, blurry lenses, or soft demosaicing.
it should be run on linear data just after demosaicing.
sigma the parameter of the gaussian blur assumed to have deteriorated the imageiter number of iterations to run the algorithm for. runtime scales pretty much linearly in this number.