RE "While it is generally not recommended to combine sharpening with noise reduction, RawTherapee-5.5 has a “contrast threshold” adjuster in the Sharpening tool, thanks to which you can sharpen details while preserving the smoothness of uniform, flat areas."
I start with rawtherapee and then, once close to what I want, spawn Gimp and finish up there.
My ant-line habit is to (usually) include some rawtherapee noise reduction (especially at high ISO) and then, as a finishing touch, to also do at least some sharpening in Gimp, immediately before exporting to *.jpg
But that’s combining noise reduction with sharpening, albeit at two slightly separated software steps. I’ll do some black box testing soon. But I find myself wondering what the low-level, theoretical reasons are for NOT combining noise reduction with sharpening.
I think the least technical explanation is that that you’re going to amplify the noise when you sharpen, thus undoing the noise reduction you’d applied.
If you still want to sharpen, you could wavlet decompose your image in GIMP, then sharpen on the finest detail layers, then recompose the image. Or you can have a play with GMICs Octave Sharpening, which is more or less the same thing, but in one step.
A link to the quote would have been nice: Noise Reduction - RawPedia. The language could probably be improved for clarity…
If you read carefully, it doesn’t discourage you from using both sharpening and noise reduction in the same workflow. Observe the intention of the section:
This section details the order of operations for removing noise.
In context, the quote in question is the last step:
9. While it is generally not recommended to combine sharpening with noise reduction, RawTherapee-5.5 has a “contrast threshold” adjuster in the Sharpening tool, thanks to which you can sharpen details while preserving the smoothness of uniform, flat areas.
The while clause
– isn’t saying that you shouldn’t sharpen and denoise in the same workflow.
– it is saying that it isn’t recommended that you sharpen after you denoise.
– but you could sharpen smartly to recover from smoothing (and prepare for resizing).
I understand that. I think the “order of operations” is not the correct phrase for this section because it is about how one would approach the noise reduction problem, and not what goes on under the hood so to speak. Rather, the steps recommend 1) getting as much good detail as possible, then 2) following a noise reduction path and 3) possibly adding more sharpening for taste afterwards.
In any case, since it was brought up twice, it might be worth it to take another look at how this could be confusing for some people. And maybe I am just off-base with my interpretation…
Unsharpening mask amplifies noise as well. If you “unsharpen” in presence of noise you should not be too aggressive, but you can do it of course. Modern techniques (such those used in RT) actually help to mitigate possible problems.
This is all from what I have read. So, feel free to cringe and correct me.
The thing is there are different strategies to go about sharpening and noise reduction. I will focus on noise. Strictly speaking, there are different types of noise and noise reduction is not the same as smoothing. In an artificial setting, noise reduction is quite simple because we only have one type of noise generated in a predictable manner. In an actual image, it isn’t like that. Many types of noise are present, including the usual and / or random distortions to data that you would encounter in post-processing. And so, signal and noise are intertwined. Thus, the lines between noise reduction and smoothing are blurred (pun intended). I.e., noise reduction proper would soften scene information and techniques with smoothing properties could be incorporated to deal with noise.
And maybe I am just off-base with my interpretation…