Is XOR possible in compositing

Roken · December 23, 2022, 5:19pm

I have a scene with three suns, each in its own lightgroup. In compositing, I set up the nodes to use the sun that I want, and to adjust it’s colour, brightness etc.

Is it possible to set up the nodes in such a way that, if the brightness of any of them is set to a non-zero value, the other two must be zero, or even a way to have a check box to select which to use in compositing (this must be a compositing setup, so that I only render once).

I’m guessing not without resorting to python, but if anyone can prove me wrong I’d be grateful.

joseph · December 23, 2022, 8:35pm

You can make any logical gate in Blender very easily with the Math node- some combination of Greater Than, Less Than, Maximum, and Minimum will do it. For example, for an And gate, you plug a Greater Than and a Lesser Than node into a Minimum node. For Or, you change the Minimum to a Maximum. Use that output of that into a Mix node, in your case, or actually you’ll need two Mix nodes- plug suns 1 and 2 into the first one, plug the output of that into the second one, with sun 3 as the second input, and another logical Math combination as the factor

Roken · December 23, 2022, 9:56pm

But surely the nodes are evaluated in a linear manner, and so any solution such as that will always favour the first sun, and then the second. If I change sun 2, and either sun 1 or sun 3 was previously selected, it won’t automatically disable the previous selected sun? In fact, it could end up in a weird loop where the newly selected sun is immediately de-selected again in favour of the one that has priority in the evaluation order.

joseph · December 23, 2022, 10:06pm

I’m not sure, you should try it. There’s definitely a fairly simple combination of Math nodes and Mix nodes that will get close, at least, but I’m not sure if it’ll actually work

lucas.coutin · December 24, 2022, 3:56am

Isn’t quite well explained the exact effect that you’re looking for, but XOR equation in Nuke is A(1-b) + B(1-a), A and B are RGB inputs, a and b alphas.

Okidoki · December 24, 2022, 10:23am

You might want to mix the background with this somehow… value 0, 0.5 and 1 chooses Light1, Light2 or Light3…

Hikmet · December 24, 2022, 11:14am

XOR math is:

IF (x == y)
  return 0
ELSE
  return 1

You can do this with COMPARE Math Node and MIX node easily.

Roken · December 24, 2022, 2:47pm

Thank you, all. I was that focused on an XOR that I missed the blindingly obvious (that you have now given me). A single value input connected to three compare nodes (comparing 1,2 and 3) connected to a multiply node connected to the brightness.

Now I just feed the relevant value, and that’s the only sun used, exactly what I wanted.

Thanks again. This node setup just keeps getting more complicated, with layer upon layer of nested groups. But it gets me this.