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So I see there is a discussion about what those colors mean and what they actually do. The logic behind them goes roughly like this:
Each BRDF has three main characteristics with respect to light:
(*) the fraction of light that is reflected by the BRDF layer away from the surface - let us denote this multiplier with R;
(*) the fraction of light that is absorbed by the BRDF layer - let us denote this with A;
(*) the fraction of light that is left unchanged by the BRDF and passes through it to layers below - let us denote this with T.
Obvisouly, each of these multipliers must be less than or equal to 1 (or white, since we are dealing with colors). Also, the sum of these three color multipliers must be exactly 1 (the white color), since they describe everything that can happen to light falling on the BRDF layer:
R<=(1,1,1)
A<=(1,1,1)
T<=(1,1,1)
R+A+T=(1,1,1)
Here is a diagram that I hope makes it a little more clear:
Having all the three multipliers separately adjustable by the user would be too much of a trouble and it would be difficult to enforce the energy preservation equation. To makes things more manageable, in V-Ray, you only specify the transparency multiplier T exactly, and then you specify what portion of the remainder (R+A) is occupied by the reflection multiplier R. This portion is called the filter color (F), and more exactly, F=R/(R+A)=R/(1-T).
So, each BRDF has two adjustable multipliers: the filter color F, and the transparency color T. From here, the other multipliers can be computed as:
R=F*(1-T)
A=(1-T)*(1-F)
In previous versions of V-Ray for Rhino, the filter multiplier F was always assumed to be white - in other words, there was no absorption and light was either reflected or let through. In that specific case, the reflection R is the white complement of the transparency T (R=1-T) and the absorption multiplier A is zero. That means that the reflection color R was enough to define completely the BRDF.
However, since you had no control over the absorption color, there were some possibilities that could not be achieved with just that one parameter. This is why in recent builds, the filter color was introduced. As before, the reflection color is the while complement of the BRDF transparency T, but now you also have control over the filter color.
For example, if the Reflection color is medium grey, that means that half of the light falling on the BRDF will pass through it to BRDF layers below. For the remaining half, the filter color is applied to determine what portion of it is actually reflected. You can think of the filter color as an additional multiplier for the reflection result.
For example, in older builds, you couldn't make a reflection layer that would let half the light through, but reflect only the blue portion of the remaining light. With the filter color, this is now possible.
I hope this clears things up a bit.
Best regards,
Vlado
Each BRDF has three main characteristics with respect to light:
(*) the fraction of light that is reflected by the BRDF layer away from the surface - let us denote this multiplier with R;
(*) the fraction of light that is absorbed by the BRDF layer - let us denote this with A;
(*) the fraction of light that is left unchanged by the BRDF and passes through it to layers below - let us denote this with T.
Obvisouly, each of these multipliers must be less than or equal to 1 (or white, since we are dealing with colors). Also, the sum of these three color multipliers must be exactly 1 (the white color), since they describe everything that can happen to light falling on the BRDF layer:
R<=(1,1,1)
A<=(1,1,1)
T<=(1,1,1)
R+A+T=(1,1,1)
Here is a diagram that I hope makes it a little more clear:
Having all the three multipliers separately adjustable by the user would be too much of a trouble and it would be difficult to enforce the energy preservation equation. To makes things more manageable, in V-Ray, you only specify the transparency multiplier T exactly, and then you specify what portion of the remainder (R+A) is occupied by the reflection multiplier R. This portion is called the filter color (F), and more exactly, F=R/(R+A)=R/(1-T).
So, each BRDF has two adjustable multipliers: the filter color F, and the transparency color T. From here, the other multipliers can be computed as:
R=F*(1-T)
A=(1-T)*(1-F)
In previous versions of V-Ray for Rhino, the filter multiplier F was always assumed to be white - in other words, there was no absorption and light was either reflected or let through. In that specific case, the reflection R is the white complement of the transparency T (R=1-T) and the absorption multiplier A is zero. That means that the reflection color R was enough to define completely the BRDF.
However, since you had no control over the absorption color, there were some possibilities that could not be achieved with just that one parameter. This is why in recent builds, the filter color was introduced. As before, the reflection color is the while complement of the BRDF transparency T, but now you also have control over the filter color.
For example, if the Reflection color is medium grey, that means that half of the light falling on the BRDF will pass through it to BRDF layers below. For the remaining half, the filter color is applied to determine what portion of it is actually reflected. You can think of the filter color as an additional multiplier for the reflection result.
For example, in older builds, you couldn't make a reflection layer that would let half the light through, but reflect only the blue portion of the remaining light. With the filter color, this is now possible.
I hope this clears things up a bit.
Best regards,
Vlado
Thanks vlado!
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