I need a "Channel Combiner" node

Yes, the Steve Wright book is extraordinary. This guy knows his stuff!

And the "copy" node certainly did the trick. That fixed it right up for me.

Seeing the results you've produced, I'm still sort of trying to get my head around exactly what's going on. I know from the Shake Node Reference:[It] "Tells Shake what channels to copy from B to image A."I suppose the "A" and "B" and the 1st & 2nd node knots respectively? Little bit of head scratcher...

Anyway, you solved my dilemma. I was getting frustrated trying to figure it out on my own.

So thank you very much!

Thanks for the confirmation.

So not long after posting my initial query here I clicked the "Contact" link associated with the book's online training material. I wasn't sure who this was going to. Steve Wright? And I had no idea if I'd receive a reply.

Well, I did receive a reply, less than 24 hours later, from Steve himself! Nice.

He gave me the complete unspill script, AND his was much less convoluted than mine...by a long shot. FWIW, here it is:

GREENSCREEN = Reorder(0, "rgba");
B = Reorder(GREENSCREEN, "bbb0");
G = Reorder(GREENSCREEN, "ggg0");
R = Reorder(GREENSCREEN, "rrr0");
average_RB2 = Mix(R, B, 1, 50, "rgba");
spillmap2 = ISub(G, average_RB2, 1, 100);
scale_RGB = Lookup(spillmap2, JSplineV(x,1,0@0,0.173@0.9978),
JSplineV(x,1,0@0,0.8538@0.7783), JSplineV(x,1,0@0,0.432@0.9774),
JSplineV(x,1,0@0,1@1));
G_only = Reorder(scale_RGB, "0g00");
RB = Reorder(scale_RGB, "r0b0");
G min_spillmapG = ISub(GREENSCREEN, G_only, 1, 100);
RB plus_spillmapRB = IAdd(G min_spillmapG, RB, 1, 100);

Thanks Steve Wright!

Patrick,

I think the general theory is (something like) this:

The standard despill strategy is to build a green spill map, then subtract that from the green channel to remove the excess green from your shot. This certainly gets rid of some green from the shot, which is the goal. But since green is typically the brightest channel, this process can also darken the shot. Further, it can also introduce some hue shifts.

The unspill strategy is a similar but different strategy. Instead of subtracting the green you add red & blue. But this only works properly with the scaling node in between the individual channels. Since we're adding some red + blue, we don't want to pull out as much green as before. So we can't use our original spill map at "full strength" on the green channel, like we did with a despill process. The scale_RGB (lookup) node lets us adjust how much each channel contributes to the spillmap before it's final calculations. The addition of the RB channels only works because of the scaling node upstream. We'd scale green down and red/blue up. The exact proportions would depend on the specific shot at hand.

All in all, I think the despill vs. unspill processes are basically "Six Of One, A Half Dozen Of The Other". Different strategies to go at the same issue: removing spill.

But because of the scaling lookup node there's a little more adjustability with the unspill method.

Anyway, the specific scale_RGB values provided in the script-paste above are somewhat arbitrary. In the end, their real utility is twofold: you can use them to limit the green as you add red+blue - in the spillmap. Also they could be used to help "color" any fringing. If you have those tiny (annoying) edges on the key you might try eroding the matte to eliminate them. Not ideal. This scale_RGB node could, alternatively, be used to alter the colors of the fringe so that it mates up with it's background in a cleaner way. Color matching instead of matte eroding.

Hope that all makes sense. I'm still trying to get my head around most of this stuff...