My point: GC is meant to reduce highlights and brighten up shadows to correct for the various shortcomings of a (Poser) 3D render. Unfortunately, the way it's applied in Poser, also affects transparency and fresnel, and can affect reflection/refraction, while in my opinion it should not.

transparency set to 80% for a block, white ball behind it. Expected: light passes 2 surfaces front and back, that's a 80% x 80% = 64% brightness reduction by the block. Rendered with default gamma, result measured in Photoshop: 82% brightness. Which matches the 64% reduction corrected with GC. It should not, it should remain at 64%.

in math: object color C, gamma g, transparency T.
expected result: C * T^2
render result: ( (C^g) * T * T ) ^ (1/g) = C * T^(2/g)

reflection, plugged into Reflection_Color, Reflection_Value set to 80% for a block, white ball in front of it. Expected: 80% brightness reduction. Photoshop: 90% brightness. Which matches the 80% corrected with GC. It should not, but remain at 80%.
Reflection Value 100%, Reflection_Color reduced instead. Now Photoshop measures 80% brightness as it should - as the color-swatch is involved in GC. Same when reflection is plugged into Alternate_... which offers a color-swatch only.

in math, reflectivity R
expected result: C * R
render result with value: ( (C^g) * R ) ^ (1/g) = C * R^(1/g)
render result with color:  ( (C^g) * (R^g) ) ^ (1/g) = C * R as it should be 

refraction, plugged into Refraction_Color, Refraction_Value set to 80% shows the same behavior as transparency: 80% resulting brightness. When Refraction_Color is set to 80%, or refraction is plugged into Alternate_... with 80% the resulting brightness is 64% as it should.

So when a color_swatch is used to set transparency, reflectivity or refractivity, all is fine. But when a Value is used instead thing go wrong with GC. As is the case everywhere: a diffuse-only material with 50% white in the color and 100% in the value will render different from 100% white in the color and 50% in the value. in Poser, when GC is applied.

Unfortunately, transparency has no color-swatch to use. One can adjust the value for GC, but then its shadows will go wrong. When using swatches, the ratio between reflection and refraction remains okay as well. When values are used instead, the ratio is distorted as well (as transparency and refraction reduce brightness twice, reflection reduces brightness once, and GC is nonlinear).

And now I wonder how Fresnel will behave, because it mixes a reduced reflection and reduced refraction depending on the angle to the camera. Which to me seems value-based and not color-based. When turning the block mentioned above into Fresnel, the brightness ratio between the refracted and reflected balls as measured in Photoshop is extremely sensitive to the gamma value used. Which it should not be. I don't have a clue yet how to correct for it, as its angle-dependant.

And still we need to mix those raytracing nodes with transparency, to get decent shadows behind the block from a direct light. That's where the aspirines kick in :-)

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though