@LuisSanz, to add further detail and examples regarding how various demosaicing algorithms render grain structure. I’m attaching annotated crops magnified to 300% for easy quick viewing, and also the raw file. It is from a Sony A7R II at ISO 800. It was shot on a Star Adventurer tracker which I aimed blind at Polaris because I was standing next to a cliff so the tracking is not perfect. I think this astro image is a reasonably good test because there is a fair bit of noise, smooth areas where it’s easy to see any patterning artifacts, and fine star details which tend to produce a lot of false colour artifacts.
In terms of noise pattern, I like VNG4 and Adobe Camera Raw/Lightroom’s result the best. Note the bright star in the upper right. VNG4 produces a zipper artifact along that edge while Adobe does not, bearing in mind that the Adobe algorithm is prone to severe zipper artifacts as demonstrated in the earliest examples. In this case it did not trigger with Adobe but in VNG4 we see it. I find VNG4 to be very prone to this kind of zipper artifacts too.
The Raw Pedia page recommends LMMSE or IGV for noisy images, which I find to be a poor recommendation. They both enhance the noise contrast more than VNG4 does, and perhaps the color artifact suppression is a bit strong and robs away some star colors. There are also random 45 degree diagonal artifacts usually of three or four pixels in length where the pixels are too dark in smooth (!) areas, which is something I’ve not seen before. Sometimes it is two adjacent pixels, or two adjacent pixels separated by a lighter middle pixel.
AMaZE has slightly less of the mazing pattern in the noise, DCB has a little more and AHD has the most. These patterns are impossible to remove with typical noise reduction tools in the raw converter or third party apps like Topaz Denoise. @afre, I feel your pain. AMaZE tends to pick up and increase the contrast of a random pixel here and there a bit too much making it too dark, while DCB’s micro contrast is smoother (i.e. less standard deviation of brightness of pixels in the dark star-less sky areas which is not featured in this crop illustration).
VNG4 produces a lot of green fringing false color halos around the stars. Adobe color fringing errors tend towards red/magenta. LMMSE produces even less false color, and IGV has the lease false color and color noise is strongly suppressed, but star colors seem to be destroyed also. DCB seems to produce the better balance of false color/color fringing artifacts, followed closely by AMaZE, but one can find stars where either one produces a better result.
Overall it’s close between Adobe and VNG4 when it comes to dealing with noisy data. The Adobe result has less zipper artifacts in the high contrast stars, eliminates hot pixels slightly better and generally has less objectionable color fringing artifacts. The VNG4’s green halos on high contrast edges (stars) I find to be objectionable, which is not removed by the false color artifacts suppression slider, but can be dealt with using the Defringe tool. VNG4 produces better star shapes in some of the smaller, fainter stars, which I appreciate.
If RCD is able to avoid the color artifacts better, and incorporate VNG4’s method of treating the green channel pixels as separate etc. etc, I think we may have a winner.
VNG4 does rob quite a bit of resolution away, so my current favourite technique is to render the image twice, once with DCB and once again in VNG4, and mask out the stars which will come from the DCB image, and the smooth low contrast areas will be from VNG4. I agree that a modification to RCD to handle noise patterns better should be kept separate, the loss of resolution and detail is pretty significant. I’d much rather have the final control of manually blending the two together for best results.