Sigh. Does anybody read (search) anymore? I post this over and over. Makes me nuts.

1) The Poser Primitive One-Sided Square is one sided. The Hi-Res Square is two sided. You cannot see this artifact on a one-sided square, despite what was said above. There is confusion here. If you see this artifact, you are using a two-sided prop. (If you collapse a box to 0 thickness, you just made a two-sided square.)

2) In Poser 5, one-sided or two-sided didn't matter. Starting with P6 and up, you must click NORMALS_FORWARD in the material to get P5 behavior. Without that, which side of a polygon you are on matters.

3) This whole subject, in the context of a "mirror", is irrelevent. Mirrors do not exhibit diffuse reflections. They only do perfect reflections. If you can, in any way, change what you see in a mirror by shining a light on the mirror, or changing how a light shines on a  mirror, you have done it wrong.

Here is how to make a physically correct mirror material.

Diffuse_Value = 0

Specular_Value = 0

Reflect node plugged into Reflection_Color

Reflection_Value = 1

Reflect_Lite_Mult = OFF

Reflect_Kd_Mult = OFF

LAST TWO ARE SUPER IMPORTANT.

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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)