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I am using Darktable as my RAW processing software. In darker pictures that have been taken with an ISO of 800 or more, I frequently encounter great issues with luminance noise.

The following two images (one of clouds and one of pure sky) are cuts from a recent photo that I took. There clearly is some noise, especially in the second picture. Clouds raw Sky raw

After doing some standard edits like changing exposure and adding contrast, the noise becomes more notable as can be seen in the next two images. Here, color denoising was already applied but nothing for luminance. Clouds in Darktable (no denoise) Sky in Darktable (no denoise)

And finally, this is what Darktable's denoising filter makes of this. There are heavy artifacts that are arguably even worse than the original noise. Clouds in Darktable (denoised) Sky in Darktable (denoised)

Those artifacts look similar to what a phone camera puts out when you take a picture too dark with it, so I assume it's not a completely obscure problem that I am encountering here.

When I tried to replicate something similar in Adobe Lightroom, the denoising was so much better (see next two images). So apparently, it is possible to implement a better denoising procedure in theory. Clouds in Lightroom Sky in Lightroom

I wonder, is there a step that I am missing in my editing workflow which causes these artifacts to be there, or is it simply a bad algorithm that is used by Darktable and there is nothing I can do to improve the image?

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  • Sorry, I should have made it clear that I in my title I was referring to the artifacts introduced by Darktable, not the white noise from the camera. – Andreas T Oct 30 '18 at 13:42
  • It's not clear what "artifacts" you are referring to. Can you describe them? – xiota Oct 30 '18 at 20:46
  • Those in the 5th and 6th picture, compared to the 7th and 8th. – Andreas T Oct 30 '18 at 21:15
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    Is it possible you have some raw images you could share so we could work through the same process with the same images? – user67208 Nov 13 '18 at 22:30
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Many of DarkTable's algorithms are significantly different from those typically used by other image processing programs. If you cannot get the results you want from DarkTable, consider trying other raw processing software, such as RawTherapee or UFRaw.

  • The images are consistent with the use of noise reduction filters with different "strengths", where the DarkTable filter is much stronger than the one from Lightroom. If there are user-adjustable parameters in DarkTable, try changing them to reduce the strength of the filter.

  • Different algorithms detect and treat noise differently. The DarkTable images have some characteristics that are consistent with median averaging.

  • Some processing steps, such as increasing contrast and sharpening, are known to increase the appearance of noise. The DarkTable images appear to at least have had a greater contrast boost than the Lightroom images.

  • I wouldn't say it's too strong or rather that that is not the only issue. Too much denoising in LR just makes the image blurry, it doesn't introduce new artifacts. I also tried "all" possible values for denoise in Darktable and it's either not strong enough (meaning there is still noise to be seen) or the artifacts appear. – Andreas T Oct 30 '18 at 13:39
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My guess is you're using denoise (profiled). If so, then you should read darktable's manual section on profiled denoise, where it is clearly stated that you should use a blend mode to avoid that effect you've shown in your examples that look smeared and painterly.

To summarize:

  • Use non-local means for luma noise, combined with blend mode ligthness or HSV lightness.
  • Use wavelet for chroma noise, blend mode color or HSV color.

I usually apply two instances of denoise-profiled, one to reduce luma noise the other to reduce chroma noise, with opacity of the blending ranging from 30 to 100%.

The results are equal and sometimes better than what I was used to achieve with Lightroom. Lightroom's NR is obviously easier to use.

  • What you described (two instances of profiled denoise with different blends) is exactly what I did as well. – Andreas T Jan 4 at 10:07
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Denoising basically replaces each pixel with a weighted average of itself plus the neighboring pixels.

There are multiple parameters, and in LightRoom, you can set them independantly: the size of the circle of pixels that are considered, the weighting of the pixels based on distance, and the aggressiveness / strength of the overall approach.
From my experience, the default settings are pretty well chosen in LR, so you simply move the slider until you like the result. I don't know DR well enough to compare, but probably it has similar or equal settings available somewhere.

  • The "basically" in your first sentence seems to be the key point, as Darktable slightly moved away from that already with non-local means and Lightroom probably even more so. Unfortunately, Lightroom is of course proprietary software, so there is no documentation about what kind of algorithm it uses. I could not replicate any similar effect for the denoise in Darktable by just playing around with the sliders. – Andreas T Oct 30 '18 at 13:41

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