I love the Diffuse or Sharpen module to apply the preset sharpen demosaicing (AA filter). It is now part of my standard work flow. However, If I need more sharpening I defer to other modules in addition to this initial sharpening. I don’t feel this single module is going to replace other well known modules like local contrast. However, I really appreciate the effort AP put into making this module. It is complex and with time I will find more uses for it. The presets are good starting points, but I find fast sharpness a little underwhelming at the default preset values. I suspect this is what the @T_N_Args is referring to.
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Keep in mind that “diffuse & sharpen” uses a completely different method than e.g. “sharpen”.
The latter uses an unsharp mask to enhance contrast over a zone of 2 or more pixels. That enhanced contrast gives the impression of a “sharper” image, but can also cause halo’s (increase the ration and strenght to see that). And you can actually lose some of the finest details when overdoing it
“Diffuse & sharpen” tries to undo the cause of unsharpness, so will be more limited in its effects.
I wonder if it would be a good module to use on export after the resizing to final size (and if that’s even possible).
I think this would be awesome! But I have also had good results with the RL deconvolution Lua plugin that deconvolves after downscaling.
That might be interesting. Though I’ve had some unpleasant experiences with deconvolution when pushed a bit (“ringing” artifacts due to sharp edges)
Thanks for these links. I used them to edit darktablerc and enable OpenCL option in Preferences.
However, 2 things happened after I checked the box for OpenCL.
Firstly, the speed of D or S module was not noticeably changed. It still took 40 seconds to apply the ‘Add Local Contrast’ preset.
Secondly, and this is weird, all my images in Darkroom module went crazy: specifically, much brighter and lower contrast (with no change in histogram). Irrespective of whether the Diffuse or Sharpen module is on or off. A bit of investigation and I found it was the Denoise (profiled) module! I am using its default ‘Wavelets:chroma only’ preset. Which makes no sense at all.
OpenCL off, Denoise (profiled) on:-
OpenCL on, Denoise (profiled) on:-
OpenCL on, Denoise (profiled) off:-
So I am forced to turn OpenCL off, if I want my photos to come out correctly, and retain the use of the Denoise module. I don’t know if this is a bug – but I’m not a developer or beta tester, I don’t do bug reports.
no bug report, no fix - there aren’t developers around being able to reproduce this since they don’t have access to M1 macs.
So you need to bite the bullet and stay without openCL and cross your fingers that there will be a fix if someone else reports that 
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Since this is all FOSS, a fix requires some help from users. There are no beta-tester teams like in SW companies, so it’s the user community here that provides insights and strange behaviors like you observed to the developers.
Filing a bug is really no big thing, you already have all information you would need to provide at hand, by just copying your insights into a bug report. With some luck, this gets fixed.
For further information see
So just feel being invited and welcome to the contributing community 
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OK I’ll do it, but half expecting a reply like, “well duh, why do you think we turned off OpenCL? Maybe because you are not supposed to turn it on?”
As that’s the kind of information one might expect to find in the manual (or at least in release notes), a bug report could still be useful.
Maybe it’s only the display that’s corrupted (the histogram being the same). You could try exporting with OpenCL = on, denoise (profiled) enabled. If that works, disable it in your processing; create a new style with the module enabled; add that profile to your export settings in append mode (darktable 4.0 user manual - export) for images that need denoising.
As for the original post claiming that the effect is ‘almost invisible’: I actually find that unless I’m careful, the images can become oversharpened (e.g., with the dehaze preset, especially if combined with others). I continue to use the ‘old’ local contrast module, too, and if you cannot get diffuse or sharpen to run at a reasonable speed (or you just don’t like what it does), you can still use the other modules like sharpen and contrast equalizer (it was a long-time favourite of mine, before diffuse or sharpen arrived).
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It’s been said more than once in this forum: macOS is not a prioritized OS for the darktable developers. I was unhappy with performance, but I did what I had to: installed Linux after 17 years of macOS. I’m very happy with the decision. darktable runs better in its native environment. My work computer still runs macOS though…
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In version 3.8 I too used the chroma only preset. However, I now get better results in version 4.0 with the default settings of the denoise (profiled) module. It tackles most chroma and luma noise well. I even sometimes push the strength slider up to 1.5 with good results on noisy images. I am really impressed with the improvements in this module in the latest version of darktable. Test it for yourself.
Perhaps I should add that they’re making great strides with https://asahilinux.org/
No GPU acceleration yet though…
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Are you sure?
If you check the commit messages from this year, they are refactorings, UI changes and the like.
The current version of the module, v11, arrived in last August.
New noise profiles have been added, but not many:
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@kofa with the high ISO noise shots from my Canon G16 I get good results with the defaults in the latest version of the denoise (profile). It tackles both chroma and luma noise nicely. In 3.8 it softened the image too much for the luma noise correction so I only did the chroma noise in this module and used other modules to tackle luma noise. Noise is less of an issue on my D-SLR images so my testing for noise reduction is based on my G16 images.
Try the CA module to eliminate most of the chroma noise.
If you are using the official release 4.0 for macos, it run under Rosetta on Apple silicon and opencl is unavailable. If you compile darktable for arm64 it runs natively (at least on my macbook pro 14" with m1pro) and opencl is supported (I suppose it’s emulated by metal) and diffuse and sharpen is pretty fast. But you need to compile dt and it’s not straightforward.
JF
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I recently upgraded to 4.0 and I am impressed with the new Diffuse/sharpen module. Great results, and even without OpenCL on my old Lenovo Ideapad 110 running Linux mint, it is quite fast enough (I use it for sharpening mostly)
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Welcome to the forum… I am the same. I can create enough blurring all on my own when I shoot… 
The presets are great and then you can do some small tweaks to them and get some really nice results…
Would be interested in knowing how, even if only to prove it is beyond my abilities