The new release candidate of Darktable (2.4-rc0) introduces a new demosaicing algorithm for xtrans sensors, called “Frequency domain chroma” (FDC). At first, as a Fujifilm X-T2 owner I got really excited. So I compiled Darktable form source, and started to compare the Markesteijn and FDC algorithms.
So far I only tested the algorithms on X-T2 images, but I found that the FDC to be marginally better in some cases, with the improvements vsible only when pixel peeping at 200%, while I also found some adverse effects (color fringing and moiré) more visible in some cases (100% zoom).
I was initially thinking to send a feature request to RT devs to add this new algorithm to RT, but now I’m not sure anymore if it is worth it. This post is just to discuss about the pros and cons of adding this new algorithm to RT. I have seen more cons than pros so far.
Below is an example of an X-T2 chart from DPReview (Markesteijn algo top, FDC algo bottom, in Darktable at 200%):
Differences can be seen at the center of the wheel (hard to say which algo is better, I personally prefer the Markesteijn result), and also in the smaller text (here, the FDC algo is better here). Faint differences, at 200% zoom level…
Now, another comparison usin an X-T2 resolution chart from Imaging Resources (left: Markesteijn 3-pass, right: FDC, in Darktable at 100%):
Here you can see some color moiré visible in the FDC-interpolated image, while the Markesteijn onoe is clean.
Last, a real-world shot of mine (X-T2) showing what happens in a transition zone to highlights (Markesteijn 3-pass (top) vs. FDC (bottom), zoomed in at 200% in Darktable):
The strong fringing introduced by the FDC algorithm seems quite problematic to me.
I’ll see how both algorithms compare for older xtrans sensors.