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05-11-2018, 02:04 PM   #1
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Anyone using "selective Gaussian blur" for noise reduction?

Since my move away from Lightroom 6 on Win10 to Darktable on Linux Mint, there are very few things I miss so far, and lots of new things I like. But one of the areas I'm less impressed with is Darktable's luminance noise reduction... It's not bad at all, but doesn't produce the fine-grained results that Lightroom does, especially on high ISO files. Of course, that could very well be my lack of experience, but I've been playing with the various approaches quite extensively (including profiled denoise, which I'm really not keen on), and still haven't found one that works quite so well as Lightroom.

Anyway, today I came across an article on "selective Gaussian blur" for noise reduction, and found that GIMP - the image editor I'm using - has a filter for this, albeit with a very limited number of settings (I'm guessing, therefore, that commercial equivalents like Photoshop also have it?). I've been experimenting on a few ISO 1600 - 12,800 files from various cameras I own, and I'm really quite impressed by the results. It takes a while to apply the chosen level of selective blur to a full-size 24MP image, so it doesn't make for a fast workflow - but with careful adjustment, the end result is really rather impressive. I'm surprised at how much detail is retained, although admittedly I'm not one for heavy-handed noise reduction so my application of the filter has been relatively gentle.

So I'm wondering... who else here uses this technique for noise reduction, and what (I wonder) are the limitations? There's always a cost and/or limitations to these things... and I'd be interested to know if anyone has knowledge or advice they could impart.

Thanks in advance

05-11-2018, 02:52 PM   #2
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I've used gaussian blur for noise reduction, but I've always done it with frequency separation. I just applied the blur to a lower layer, with a high pass layer on top of it to retain the detail. Sometimes fine tuning with layer masks. I'm not sure I'd want to throw a readymade filter at the problem and sacrifice the level of control that frequency separation gives.
05-11-2018, 03:29 PM   #3
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QuoteOriginally posted by Dartmoor Dave Quote
I've used gaussian blur for noise reduction, but I've always done it with frequency separation. I just applied the blur to a lower layer, with a high pass layer on top of it to retain the detail. Sometimes fine tuning with layer masks. I'm not sure I'd want to throw a readymade filter at the problem and sacrifice the level of control that frequency separation gives.
Thanks, Dave

I believe that's the purpose of the selective gaussian blur I'm trying out, rather than just a straight gaussian blur on the entire image. As I understand it, selective gaussian blur looks for areas of contrast below a user defined threshold and blurs only those areas.

The limitation I've already found is that if the image contains low contrast detail - such as fine detail on a flower petal - it's not always possible to find a threshold that keeps those details whilst blurring noise. Still, I'm finding it quite useful on higher ISO photos where the finer details are already compromised...
05-11-2018, 04:50 PM   #4
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Try Gimp it has a lot of filters and plugins. The new 2.10 version supports floating point for color channels and parallel processing.

05-11-2018, 06:15 PM - 2 Likes   #5
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QuoteOriginally posted by BigMackCam Quote
I've been playing with the various approaches quite extensively (including profiled denoise, which I'm really not keen on), and still haven't found one that works quite so well as Lightroom.
Interesting that you aren't fond of the darktable's denoise (profiled).
To my eye it is their best denoise tool and typically for me (K1 and K-3 II) works really well, and much better than I have ever been able to achieve using Rawtherapee, but RT has always been a big mystery to me and I probably haven't been using it in the best way.
Darktable's denoise (profiled) controls can have a significant effect on the result.
I typically use the minimum patch size and strength often about 0.5-0.6 for ISO<=800 and maybe higher for higher ISO.
The other variable in this is that I might be accepting results that you wouldn't!

Cheers,
Terry
05-12-2018, 07:59 AM - 1 Like   #6
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Interesting idea, I'm going to try selective Gaussian blur next time I have some high-iso photos to process. In darktable, I think you could achieve the same with the low pass filter (under the effects group), which has options to soften with bi-lateral or gaussian. And of course you get the same parametric masks.


Another pseudo-nr option I've used for lower iso shots is amaze demosaic set to max number of color smoothing passes; it adds quite a bit of processing time.


QuoteOriginally posted by tduell Quote
To my eye it is their best denoise tool
I should give profiled denoise a shot again. In the past, in darktable I've had better success with "denoise (non-local means)" over profiled denoise, I start with chroma 100%, luma correction quite a bit lower than their defaults (0-30%), large number for path size (which... means smaller area, or at least seems to yield better sharpness?!) and adjust as needed. But I haven't tried profiled denoise in a while, perhaps it's been improved.

Last edited by aaacb; 05-12-2018 at 08:08 AM.
05-12-2018, 01:53 PM   #7
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QuoteOriginally posted by aaacb Quote
Interesting idea, I'm going to try selective Gaussian blur next time I have some high-iso photos to process. In darktable, I think you could achieve the same with the low pass filter (under the effects group), which has options to soften with bi-lateral or gaussian. And of course you get the same parametric masks.

Thank you... I'll take a look at the Lowpass module. I wasn't even aware of that


Since my original post, I've been playing with the Equalizer module, and I'm getting the best NR results I've achieved so far with Darktable. Chroma noise isn't so well controlled with this tool (at least, not with very high ISO images) - but a combination of Equalizer and gentle denoise (bilateral filter) sorts that out. For luminance noise, it's awesome.



Per your comment, yes - I've found that a good demosaicing algorithm is essential in dealing with colour noise. I typically use VNG with five times colour smoothing. I understand it's quite process-intensive compared to PPG, but I'm running a fairly quick laptop (i7-4700MQ quad-core @ 2.4GHz, with 12GB of RAM and GeForce GT740M GPU), so it doesn't make a big difference for me.
05-12-2018, 02:59 PM - 1 Like   #8
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Just a quick example of the results I'm finding possible with a combination of the equalizer and denoise (bilateral filter) modules...


This was a test shot I took some time ago with the K-5, at an extended ISO setting of 51,200. Processed in darktable for the first time today, and without a great deal of work. I'm sure I could improve on it. What impresses me is that the noise reduction has retained little details like the corrosion on the top of the guitar's machine head pillars. I'm impressed

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05-12-2018, 04:30 PM - 1 Like   #9
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QuoteOriginally posted by aaacb Quote
I should give profiled denoise a shot again. In the past, in darktable I've had better success with "denoise (non-local means)" over profiled denoise, I start with chroma 100%, luma correction quite a bit lower than their defaults (0-30%), large number for path size (which... means smaller area, or at least seems to yield better sharpness?!) and adjust as needed. But I haven't tried profiled denoise in a while, perhaps it's been improved.
'Denoise (profiled)' depends very much on the quality of the data in the 'noiseprofiles' file for your particular camera.
When I started using darktable, I wasn't happy with the results I was getting from 'denoise (profiled)', but the theory behind it seemed quite sound, so I generated my own noise profile data...darktable provides advice and tools on how to do this, and I think the results were better.
I have been through this process again for the K-3 II and K-1, but of late have reverted to using the built-in noiseprofiles file.
If you want to try this, be warned it is a bit of a tedious process, using up quite a lot of cpu cycles and disk space with the files generated in the process of getting the noise profile data.
Darktable does provide a means for you to start dt (from the command line) and use your local noiseprofiles file (it is in the docs somewhere), but I took the step of building dt from the git master and including my noise profile data during that process, as I would almost always forget to start dt to use my local data

Cheers,
Terry
05-13-2018, 04:44 AM - 1 Like   #10
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QuoteOriginally posted by BigMackCam Quote
I came across an article on "selective Gaussian blur" for noise reduction, and found that GIMP - the image editor I'm using - has a filter for this, albeit with a very limited number of settings
Another GIMP-on-Linux user here, wondering if you know about the standalone NeatImage package. It's not open source, but to my eyes the free version performs better than any of GIMP's built-in denoising methods including those in G'MIC. Also there is a way, with some putzing, to wire it in as a very basic GIMP plugin. I eventually bought the pro version to lift the jpeg-only restriction (so the plugin can run losslessly as png-out/png-in), and also just to support them for supporting Linux.

I'm not a darktable user and can't speak to any comparisons on that side.

Last edited by Sluggo; 05-13-2018 at 04:55 AM. Reason: words
05-13-2018, 05:03 AM   #11
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QuoteOriginally posted by Sluggo Quote
Another GIMP-on-Linux user here, wondering if you know about standalone NeatImage package. It's not open source, but to my eyes the free version performs better than any of GIMP's built-in denoising methods including those in G'MIC. Also there is a way, with some putzing, to wire it in as a very basic GIMP plugin. I eventually bought the pro version to lift the jpeg-only restriction (so the plugin can run losslessly as png-out/png-in), and also just to support them for supporting Linux.

I'm not a darktable user and can't speak to any comparisons on that side.

Interesting - thanks for the info!


Now that I've got used to using the noise reduction features in Darktable's equalizer module, I'm very happy with it indeed - so I think that's what I'll be using from now on. But it's good to know there are other options for GIMP. Actually, I'm just playing around with G'MIC and the GIMP plug-in. I installed it for the film emulations, but there are some interesting noise reduction capabilities in that too
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