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
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:
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??
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??
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…
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??”"
We have to tattoo it in our arm! hahaha
Experience is the best reminder…I have quite a few presets with low opacity multiply so I just use them….if I had experimented more with the new one I would have likely discovered the nuance of it. I love your edits by the way. I think that you and Boris are great ambassadors for the software. There is a lot of math and technical talk around DT but you and Boris put some creative and artistic flare into it……
The problem of “reverse” blend modes in scene referred is that they respond in a strange way to masks.
Unless I am missing something, there is no way to restrict the effect to a part of the image with a drawn or parametric mask, some effect is applied also outside the mask.
I think only reverese multiply is doing that the other two don’t seem to …ie divide and substract…at least in my limited experience
https://github.com/darktable-org/darktable/issues/7464#issuecomment-766158145
I’m “studying” the code where these blending modes are done “blendif_rgb_jzczhz.c” but I have some difficulties to understand everything.
I’m not a software man as you can see. If I understand everything I will try to find where the problems are. Because I’ve detected they the mask is lost when I do some operations (I have to identify exactly when it happens)
To understand why a mask doesn’t seem to be effective, you need to understand the architecture of how blending modes are working. REfer to the following diagram in the user manual:
https://www.darktable.org/usermanual/en/darkroom/processing-modules-and-pixelpipe/the-anatomy-of-a-module/
This shows a case where, for example, we have a normal “Multiply” operation. See the slider at the top right, it takes a copy of the input image, and a copy of the output of the module+blending operator. The position of the slider for each pixel slider is then controlled by the value of the mask at that pixel position. If the opacity is set to 0, you get 100% the input image, and if the opacity is 1 you get 100% of the output from the module+blend operation.
In the case of multiply reverse, the top of the slider doesn’t connect to the input image, it cnnects to the output of the module, just before the blend operation. This means the final output of the slider is a mix between the module output (eg. local contrast enhancement, for the local contrast module) and the output of the blend operator (eg. multiplying the pixel from the input image with the pixel from the output of the contrast enhancement, in the case of a multiply blending operation). In this case, both sides of the slider input are coming from the output side of the module, which means that the effect of the module (eg. local contrast) will be present in some form regardless of where the mask sets the slider.
Probably we should provide a second copy of that diagram showing the logic of the “reverse” blending modes. The documentation is a bit vague on blending modes in general, passing the buck to the GIMP documentation. I think this was an attempt to keep the documentation from growing too much and being difficult to maintain as new blending modes are added, but it does make this aspect of the darktable behaviour more opaque.
