I can't think of a way to build the shadow catcher using nodes that controls transparency without calibrating. The problem is there is no node that tells you directly how much you are in shadow. All we have is how much light is reflecting, either using a Diffuse node or something similar, such as Clay. I can't even tell how much light is hitting the surface. I can only tell how much is reflecting after hitting, which is based on the normal as well as the light intensity and where the shadow is falling.

So that's why I used the Diffuse node and require that you calibrate, so the shader can know how bright it is when lit and how bright when in shadow.

For example, with an IBL and an infinite, the two values could be .3 and .6, or .1 and .9, or .8 and .9. Which do you look for? Is .5 in shadow? Not in the last one - the last one has .8 in shadow and .9 when lit. So we have to calibrate.

The AO node gives us a direct measurement of occlusion, regardless of how much light is falling. We need a similar node for directional shadows but there is no such thing in Poser.

The information is there, just not accessible or derivable using existing nodes. Only the built-in shadow catcher is able to detect "in shadow %" without regard to light level or angle of incidence.

Also, in P7 it is not possible to build GC into this if we use transparency. We do not have a transparency node either. I wish we did. I would like a Transparency node that gives the color of the object behind the current object, similar to what Refraction does at IOR = 1, but without having to do ray-tracing.

I can build a GC shadow catcher using ray-tracing and refraction instead of transparency. In fact, I already did. But it's slower than the transparency version.

If you really don't care about color or shadow intensity, just use the built-in one.  I know it doesn't do GC, but it is good enough for test renders.I'd argue if you're not interested in shadow color, you're not really interested in shadow intensity that much, right? Because pretty much GC would only change the shadow intensity.

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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)