I no longer remember if/why I did the Diffuse_Value = Add(0). It may have even been a behavior of a version of Poser which doesn't exist anymore, because of service releases.

My belief system at the time was this:

Rendering uses all the nodes, and all the math you can build. For rendering purposes, all of the following are the same. (Note: <-- denotes plugging in a node.)

Diffuse_Value = 0
Diffuse_Value = 1 <-- Add(0, 0) # Math:Add node plugged in here, aka 1 * (0 + 0)
Diffuse_Value = 1 <-- Mul(1, 0) # Math:Mul node here, aka 1 * (1 * 0)
Diffuse_Value = 0 <-- Add(x, y) # Note - any value of x, y here would still result in 0, because 0 * (x + y) = 0

Now we can certainly see that all of these would completely disable the built-in Diffuse node in the PoserSurface.

But, what does preview do? It seems to ignore a lot of nodes and connections. I'm not sure, but I have a recollection that it would honor the simple DIffuse_Value = 0 in at least one version of Poser, resulting in a black preview. But it did not "understand" 1 * (0 + 0) as being black, instead it saw that as 1 * <<something I'm too stupid to figure out so I'll just assume it's 1>>, which resulted in a nice bright colored preview image.

Lately I've been just setting Diffuse_Value = 0 because nobody seems to tell me that their NVidia GTXFROMBULOUS90000 board (which I don't have) shows my shaders as black in preview. At one time, they did, and that's when I did the stupid 1 * (0 + 0) trick, because apparently that kind of "complexity" didn't end up in the preview calculation.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)