Cloud with internal light - v2 vs v3

[Mohawk20]

I have an issue with a rather unique situation.
I'm trying to recreate the passing through the Red Sea, from Exodus.
Making a path through water is not a problem in TG. But there was also a cloud pillar that shone light in the front and caused darkness at the rear.

I made a localized cloud pillar and put 2 planes inside, a little bit apart. One has luminosity of 100, to shine in front. The other plane has luminosity of - 500 to darken the surroundings even when the sun shines. In a v2  cloud, this looks good. But in a v3 cloud it seems to have an inverted effect on the cloud. See below for comparison.

What settings do I need to change to make the v3 cloud look believable?

[Dune]

Firstly, I think you have to be careful of using minus luminosity, as the colors may end up strange and bluish. Same goes for unclamped negative colors. Can't you use the cloud itself to make the shadow? Or do you need the cloud itself to be dark on one side, light on the other? Then the shadow function and a simple shape may help. Or use two half clouds, with different colors, masked by such a simple shape.

[cyphyr]

I'd make two scenes with everything matched apart from the lighting effects of the cloud. You could then animate the transition from light to dark very easily or if it's a still just mask out the areas you don't need.

[Oshyan]

I think Ulco has the best idea here, just use the Shadow input function. Using negative light sources is inherently going to have unpredictable and, well, unrealistic results. After all there is  no real-world equivalent to a negative light.

- Oshyan

[yossam]

A black hole................

[Mohawk20]

I do like to play with things that don't usually occur in nature, and see how a realistic simulator like TG handles it.
Could give some new effects nobody has found yet.

And as I'm trying to recreate a miracle, it's not a 'real world' situation perhaps...

My problem is that I got some nice results in TG 3, which are gone in TG 4. I'll post some examples when I'm at my workstation. 

[Mohawk20]

Below are the results in TG3. Looks as I expected it would look.
It's very different from the TG4 results.
And now I opened the file I saved in TG4 in TG3 and the cloud comes out completely black...

[Mohawk20]

So, Ulco, or Oshyan, how does the shadow function work exactly?
Because I really can't see much effect when I plug the simple shape into the shadow input.
The image below has a simple shape placed below the left side of the cloud.

The effect I'm looking for is a cloud that lights the way in front, bus causes darkness behind.
Exodus 14:19,20 - "And the pillar of cloud that was in front of them moved to the rear and stood behind them. So it came between the camp of the Egyptians and the camp of Israel. On the one side, it was a dark cloud. On the other side, it was lighting up the night."

[Dune]

Very simple, but this is on a V2 cloud.

[Mohawk20]

Can it also be used to make the cloud emit light, like inverting the effect somehow?

[Dune]

Don't think so, perhaps a negative color, but I doubt it.

But you can use a spotlight.