Now do a flip test. The parts that are relatively bright don't look too bad without GC.

But the darker parts - those things down around 20% and below, are very wrong.

The terminator on the cylinder is too gradual. That's not how things look with a point light source. That's because as we get closer to black, the pixels are darker than they should be - they approach black in a hurry, so the terminator is more gradual. The reality is that the luminance does not drop to 10% until you are 84 degrees out of line with the light source. That means that the last 10% of luminance (down to no contribution from the main light) happens in just six degrees of rotation.

Without GC, the 10% level is reached at 69 degrees out of line with the light source. The last 10% of luminance is spread over a rotation of 21 degrees. That's more than twice as wide as it should be, and when I see that, I notice. Many Poser users don't - in fact they have conditioned themselves to think that is normal. It is not.

When I show my wife the non-GC renders that people make, she always asks why are they so muddy? Why do they look like they are shaped funny?

It's because the dropoff in luminance with rotation away from light sources is skewed. It's because this skewing makes the curves and shapes of organic things look different than what they are supposed to be.

In my early days of Poser, I thought there was something wrong with its Diffuse light calculation. I photographed cylinders lit only by flashlight and compared the luminance gradients with renders. They were really off by a lot. I never understood why, until I discovered gamma. Then it all made sense.

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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)