Got it. Thanks very much.
Can you further clarify when it might be better to use multiply mode vs multiply reverse and why?
No, that was trial and error. Sometimes both modes behave the same sometimes not. I can’t say why unfortunately. For the same purpose you can also use multiply.
Got it. I’m trying to at least hunt down some documentation for those modes, but the manual doesn’t include them unfortunately.
Hey Boris,
for clarity you can use the blend mode subtract with 1-2%. The best is to use it with the contrast equalizer for fine tuning but a rgb curve works fine too.
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This is the best explanation of what the mode is…[blend mode "multiply reverse" & masks] mask is partially ignored · Issue #7464 · darktable-org/darktable · GitHub
.if you don’t mask it will be the same as multiply but when you mask…then you will see differences due to the order in which the input and output are blended
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If you use a opacity different to 100% the result is different too
I just tested and somehow that with masks is not quite right. Especially when the opacity is smaller. Here is an example where I use local contrast as the last instance and use multiply and multiply reverse modes.
Note that I do not use masks.
multiply:
multiply reverse:
multiply reverse version with low opacity looks much sharper.
similar effect can be achieved if I implement @asn recommendation with new subtract and subtract reverse scene blend modes:
subtract:
subtract reverse:
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Looks like it produces artifacts… What is multiply reverse BTW?
Then the math must not be the same or maybe I don’t understand but without a mask if you multiply input by output or output by input would it not be the same??
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I agree with your observation…I am just trying to understand it based on this statement…
" Because the multiplication is commutative (output * input = input * output ), the effect inside the masked region is the same for the multiply and multiply reverse blending modes. The outside of the masked region will change as it will depend on which image is the base image:" from one of the developers that wrote the code…
Not sure about reverse subtract …maybe try that on a more colorful image I think from what it did to the color checker there will be major color artifacts…maybe not so evident at low opacity though…I have been using subtract with local contrast and a linear tone curve for some time…5%~…it does make for a nice enhancement and provides a dehazing like effect without too much effect on color…maybe deepens them…I guess I will need to experiment some more…
EDIT Maybe its like the input output saturation sliders in color balance so when you drop opacity and the blend order is reversed you are adding multiply to a reduced opacity input and so the effect is less overwhelming darkening everything and thus sharper and bringing out the darkest parts a bit more. .So at 100% opacity they would be similar or very much so but not at lower opacity…does that make sense maybe??
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Wow, I had to read the explanation several times. Mask is reversed (black is mask) and the reverse result has the wrong colour. The language is confusing too but English may be a second or third language for the author.
Old substract mode behaves exactly the way you describes here. New ones are changed. It will bei interesting to see what is behind that. 
I did also note that the effect was not as strong if I did the same thing using the rgb tone curve vs the old tone curve module as well…
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What I’ve readen is:
When you use a reverse blend mode the final image is mixed with the image with the module applied in “normal” mode.
If you use a known blend mode, for example “multiply”, the final image is blended with the input image of the module.
If the opacity is set to 100% “multiply” and “multiply reverse” gives the same results (without masks).
Use a landscape picture (for example) and in Local contrast, apply a multiply blending with opacity of 30%. And change from multiply to multiply reverse. You will see the difference
Agreed I wasn’t taking into account opacity…
I have been wondering if the observed result is produced by this sort of arrangement
“Maybe its like the input / output saturation sliders in color balance, ie you can reduce a bit the input before calculation. So when you drop opacity and the blend order is reversed then the masking now on the input image, so you are blending the output in multiply to a reduced opacity input and so the effect is a less overwhelming darkening of everything and thus sharper and bringing out the darkest parts a bit more. .So at 100% opacity they would be similar or very much so but not at lower opacity…does that make sense maybe??”"