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I want to make a surface which has some scattered transmittance. So I made a Fresnel type surface with varing BTDF ratio.

Comparing the ray tracing results of different BTDF ratios, I found all the results are the same. Please let me know how should I express the nature of the rough surface in TracePro?

Synopsis

Scattered plus specular transmittance equals Fresnel transmission

Solution

TracePro calculates the Fresnel Reflection and Transmission, and when scatter terms are added to a Fresnel Type Surface Property, the Fresnel R and T values become the Total R and T. The A, B, and g coefficients define both the distribution and the magnitude of the scatter. In your case of transmissive scatter, the transmissive scatter plus the specular transmission sum to equal the calculated Fresnel transmission based on the indices of refraction on the two sides of the surface and the angle of incidence. Increasing the A coefficient in the surface property should increase the ratio of scattered T / specular T, but Total T should be the same. However, TracePro will not allow the total flux to exceed that dictated by the Fresnel coefficient of transmission.