First proof of concept

I have a first working proof of concept ready, and the results look very promising. The source code can be found here.

By now, I also came across publication [5], a paper that could have saved me quite a bit of work.
;-) 

In the end, though, it was fun to do the according adaptations myself.

To recall from the first post: I am using the method of [3], [4] and [5] only for chroma. For luma and for establishing directionality, I use the open source Markesteijn algorithm. I found this to deliver superior results than using either of the two algorithms in its pure form.

This latter algorithm, as many other attempts to tackle this CFA, suffers from severe false colour artifacts in image areas where luminance has a high spatial frequency.

Sample images

Here are some sample images from such difficult image areas for comparison. Image pairs are first Markesteijn, followed by my combined approach using frequency domain chroma. Markesteijn is 3 pass. None of the output has further noise processing or median filtering.

Markesteijn 3 pass

Frequency Domain Chroma

Markesteijn 3 pass

Frequency Domain Chroma

Markesteijn 3 pass

Frequency Domain Chroma

Visual evaluation

The images resulting from demosaicking by the combined method employing frequency domain chroma look notably cleaner. The last image pair, for example, is part of a concrete flower pot, and should be grey, with only traces of greenish brown from moss. This is well achieved by the combined method, whereas the 3 pass Markesteijn algorithm alone struggles.

Next steps

Presently, the filters used are 31x31. This filter size is most certainly overkill. So the next challenge will be to reduce filter size whilst maintaining image quality. [4] suggests rectangular filters supporting the directionality, and I am intending to do the same here.

Ultimately, the aim would be to move from the initial proof of concept towards a more workable prototypic implementation.

References

[5] M. Rafinazari and E. Dubois, “Demosaicking algorithm for the Fujifilm X-TRANS color filter array,” in Proc. International Conference on Image Processing (ICIP2014), Paris, France, October 2014, pp. 660-663.