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• reflection=(0.1, 0.1, 0.1), refraction=(1.0, 1.0, 1.0), diffuse=(1.0, 0.0, 0.0) => 10% reflection, 90% refraction and 0% diffuse, so you won't see that red color anywhere
• reflection=(0.1, 0.1, 0.1), refraction=(0.5, 0.5, 0.5), diffuse=(1.0, 0.0, 0.0) => 10% reflection, 45% refraction and 15% diffuse, so you will see a semi-transparent object with some red. * Note that 1/3 of the last 45% of the energy is red scattering, so actually 15% of the original energy is scattered, only in the red channel, and 30% is absorbed in the other channels.
• reflection=(1.0, 1.0, 1.0), refraction=(0.5, 0.5, 0.5), diffuse=(1.0, 0.0, 0.0) => 100% reflection, 0% refraction and 0% diffuse, so you will see a perfect mirror - nothing is seen through it and no red
• reflection=(0.1, 0.1, 0.1), refraction=(0.0, 0.0, 0.0), diffuse=(0.5, 1.0, 0.5) => 10% reflection, 0% refraction, 60% diffuse and 30% absorbed (no energy is absorbed in the green channel)

The absorption of energy in the diffuse layer matters for GI (global illumination). Having a closed room with pure white diffuse walls is like having a room of mirrors. V-Ray will bounce light around until it reaches its maximum GI depth and this may slow rendering down. If the walls were 88% white instead, only 7% of the original energy will remain after 20 bounces. At some point V-Ray will decide that the ray is too weak and insignificant and it will stop tracing it. V-Ray makes a lot of decisions based on light intensity, so this matters. Samples with intense light are considered more important than weak ones.

An important aspect of BRDFVRayMtl is the Fresnel reflection parameter (which is off by default when you create an instance, while it's on by default in V-Ray for 3dsMax). When it's enabled reflection amount depends on the angle of incidence. Glazing angles produce more reflection than looking head-on at the surface. This changes the conclusions we made above about how energy is distributed in the layers of the material. Even with 100% reflection (white color) some of the energy will go through to the reflection and diffuse layer. Fresnel reflections are a must for physically plausible materials. For Fresnel you need to set a correct index of refraction (IOR), even if the material doesn't refract any light. For example metals have very high IORs.

5.1. Material and BRDF plugins

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