darkable VNG4 debayer algorithm produces banding-like artifacts
The RAW source is from EOS650D camera, in Av mode, ISO 100.
The picture was somewhat overexposed, so I need to apply -0.86 EV with Exposure filter. I also use Color Reconstruction filter with different methods and the threshold set to 1.000. The kind of error depends on the method, the behaviour changes, but in all cases darktable produces incorrect results with VNG4 and expected results with other debayer algorithms.
When I view the picture downscaled (say, 92%), it renders correctly with any debayer algorithm and all color reconstructuion settings. (screenshots 1, 2, 3)
If I zoom it to 100%, I see incorrect rendering in overlit parts of the picture.
If color reconstruction is disabled, I expect pink in overexposured areas, but it renders them in uniform gray. (screenshot 4)
If color reconstruction is set to "recreate in LCh", it looks the same, while I expect some details.
If color recostruction is set to "recreate color", it seems to correctly draw completely overlit areas, but the rest is uniform grey, while I expect details there (screenshot 5).
In all cases, the histogram is always same, I assume it is correct. The exported file always looks wrong like in 100% zoom, regardless of chosen file format, color format (png 8/16), export picture size in pixels and whether "high quality export" setting enabled or not.
I noticed the issue with 1.6.4, then installed version 1.6.8, but it did not change anything.
I can't attach source raw, it is available here: http://dwt.merlin-vrn.tk/IMG_4726.CR2