Glossy Hilights/Refl

[bob-cat]

Just trying to get my head around adjusting the glossiness values individually for the hilight and reflective values and was wondering what sort of materials you would use individual values on?

[Morbid Angel]

Usually you wont.
The physically accurate model of the shader should not separate the specular from reflection. However this is added to the shader in order to add more control to the user.
Since the way vray works, it treats reflectance in two different ways:
direct and indirect reflection. Where direct reflection is a specular component, and indirect reflection is a bounced ray.
Simply put, when they are used together, vray does sortof a blend of direct and indirect specular, when specular is traced from light sources. But when they are separated, user has individual control over the spread of the specular vs reflectance.
A good example would be to eather use a layered shader such as vray blend material and blend few different speculars over top of each other, to simulate effects like car paint, or coated surfaces.
The other way would be to simulate effect of blurred material with sharp highlites, where reflection can be blurred and specular can be sharp.

[bob-cat]

Thanks Morbid, was scatching my head trying to think of a material that would have different values, but car paint is a good one.
With the way vray works, are you saying that the direct and bounced rays are calculated seperately and then blended to create the final reflection?
Am getting confused with the specular/indirect reflection terminology.

[Morbid Angel]

from what I understand, and vlado can correct me here if im wrong, but there is different calculations involved when tracing specular component and reflected component. I think specular component is optimized, so it renders faster, however its not entirely physically accurate though it does involve raytracing still, thats why it only ment to work with direct light sources.
For example, if a direct light shines at the glossy surface, there is a way to identify that light source as a uniform single surface which has no unique paramiters, hance it will return pure values without augmentation, and so specular can be calculated fast, based on that formula. But true reflection actually requires to trace rays to each parts of the surface (triangles) to determine differences and unique attributes (rgb values), return that to the reflected surface and blur them accordingly, hance the specular algorythm can no longer work.
Well this is my understanding of it.

[vlado]

In nature, hilights are simply reflections of light sources. In computer graphics, there are two ways to calculate this same effect: the one is to make no distinction between lights and other objects, and to just do reflections (what Morbid refers to as indirect hilight); the other is treat lights separately from all other objects, since they tend to be bright and their reflections are important for the appearance of the object and so it makes sense to calculate their reflections separately (what Morbid refers to as specular hilight).

It is important to realize that both methods produce the same visual results, they are just different approaches to the same effect. However, in certain situations the first method performs better, in other situations - the second. By "better" here I mean the amount of noise in the area light's reflection for a given number of samples.

Luckily, it is possible to combine (blend) both of these methods to produce a result with the least amount of noise for the given number of samples. In this case, the final result in the image is a sum of two components - one computed as a regular reflection, and the other - computed as a hilight. When added together, these produce the final accurate result. Of course, this only works correctly if both the reflection and the hilight are computed with the same glossiness value.

For artistic and render speed reasons however, V-Ray allows you to use a different glossiness for each of these effects. This is not physically accurate, but may be useful in many situations.

For car paint specifically, the correct way to do it is to use a VRayBlendMtl material to overlay several materials with different glossiness values.

Best regards,
Vlado

[Morbid Angel]

I was close thanks for clarifying this vlado !

[bob-cat]

And the mud settles, thanks Vlado and Morbid for the indepth explanation.
Now me understand.