Rough surfaces - a mini tutorial

[Micha_cg]

Hi,

an interesting render theorie says, that rough surfaces need a special calculation model. A good real world examples is the moon: it looks flat like a disk and not like a sphere.

The Vray diffuse layer dosn't act as rough surface, looks a little bit like plastic and so I have tested several materials.

left: matte - middle: rough metal - right: matte bump


And here the materials:






I like the bump control, because I have the feeling it best simulate the real world. Here different map intensities (left to right: 1 - 0.5 - 0.25):


Ciao Micha

[flipside]

Rough surfaces - a mini tutorial

Not sure what you're trying to do here. First thing is that I cannot see the difference in your renderings between noise from low quality QMC sampler and noise from the bump map.

The moon looks flat like a disc because it is lit so much by the sun. A light bulb or any other light source also looks flat if you take a pic of it with long enough exposure.

A rough surface simply needs bump or displacement mapping, and good settings so there is not so much noise from your GI or area shadows.

To effectively see the bump on a non reflective surface you need artificial lights (rect light, spotlight, etc...). If you use only skylight, your bump will only show up a little bit in areas with some shadow variation. In the other places the bump will be lit too evenly from all sides so you don't see the bump. So a good thing to do is using a tiny bit of reflection with some glossiness (with glossiness I mean blurry reflections).

Here's a small test, simple material, diffuse color and a bump map. Lit by HDRI.

[Micha_cg]

Rough surfaces - a mini tutorial

... I hope, you belive me now, that rough surfaces can not rendered per single diffuse layer only. For my example I have used a bump intesity of 0.75. Very interesting for me is the hard shadow of the simulated moon.



The first time I have work with this effect was a few years ago. I have try to render a chair with felt. Try it, you will not get a good look without this method. A rough surface looks a little bit like a textured emitter - the lighting dosn't show so much variation on the surface. In my book "advanced renderman" is a short description of the effect and so I had rewrite a texture shader for the chair project.

For now I would wish, the diffuse layer would support a roughnes parameter. Like you have seen, the fine bump cause a noise and it need much samples to get it clean. I hope, Vlado read here too.
The rough material is very needful for clay, cloth, felt, rough stones ... .

Why is it so? Difficult to explain with my english, but the roughness are very small hills (approx. half spheres on the surface). Each little half sphere colllect the light approx. independent from the place on the big moon surface. I hope my rendering with the wrong bump scale explain it better than I.



PS: Thank you wouter to push me to this experiment. I had much fun.

[flipside]

Rough surfaces - a mini tutorial

So your problem is that you need to use a bump map? Isn't that what it's for, to create bumps, or tiny surface detail? I think if a diffuse layer would have a roughness parameter, it would also need lots of samples to get clean. Just like the glossiness parameter. The glossiness is actually a roughness parameter for reflections (blurry reflections also come from tiny surface detail).

If procedurals could be used as bump maps, it would already be a good start, as mapping isn't an issue then and you could easily change the size and height of the bump.

But of course, if possible, a roughness parameter in diffuse would be cool too, if it is faster than using diffuse + bump!

[Micha_cg]

Rough surfaces - a mini tutorial

So your problem is that you need to use a bump map? Isn't that what it's for, to create bumps, or tiny surface detail? I think if a diffuse layer would have a roughness parameter, it would also need lots of samples to get clean. Just like the glossiness parameter. The glossiness is actually a roughness parameter for reflections (blurry reflections also come from tiny surface detail).
....

I have thought too, that the glossiness is something like a roughness, but try it out, you will not get the "rough surface" effect. Only a simple diffuse look will be possible like you can see here.

I think, we need a build in roughness function here. Like you can see in my first test (firts image), a metal surface with very low glossiness clean up better than a bump mapped rough diffuse surface.

Can you see the rough surface effect in the first image now? The contrast between shadowed surfaces and bright lit surfaces is at the bump mapped surface much lower. I like this felt look.

[Micha_cg]

Rough surfaces - a mini tutorial

After some more experiments I have find a problem with the subdivs of the single reflective layer. I have used the full adaptive sampling and let the default reflection layer subdivs at 8 that has cause a problem. Now I have set the subdivs of the reflective layer at 50 and it works well. Here my last solution:

Left: only diffuse layer - Middle: diffuse and bum - Right: only reflection layer


The rough single reflection layer works phantastic goog - fast and clean. Here the material setup:

[flipside]

Rough surfaces - a mini tutorial

Why do you put the map in the filter slot and not in the reflection slot of the reflection layer?

[Micha_cg]

Rough surfaces - a mini tutorial

I can not explain it, because the truth is, I have not understand this:
http://www.asgvis.com/phpBB2/viewtopic.php?t=491

only I know how I can get the effects I want to get. If I would use the map at the reflection color texture slot, than the reflection color could not be dark or black and would be transparent at dark texture colors.

The reflection color is big secret for me. If I set a color and the next layer is grey diffuse layer, than I get mix of the reflection color and the inverted reflection color. If I set the diffuse layer black and use a reflection texture, than I get the inverted colors of the texture ...
Could be nice, if you could test the usage of the reflection color and filter color. Maybe there is a bug.
Or somebody could create a tutorial with examples. Could be nice.

Back to the rough surfaces topic. I have the feeling the single reflection layer method has a disadvantage - it works with reflected light and not with diffuse light. If "reflection caustic" of GI is off, than no color bleeding should be visible. If no background env is set, but a GI env, than the surface dosn't work with the GI env light only.
It's only a feeling, but I would like to control the rough surface effect at the diffuse layer. A single "roughness" option could control the effect.

[vlado]

Rough surfaces - a mini tutorial

Micha, what you want is something like an Oren-Nayar diffuse shader... I've been thinking about this for some time, so it may be implemented in V-Ray at some point...

Best regards,
Vlado

[Micha_cg]

Rough surfaces - a mini tutorial

Vlado, I'm very curious to use it.

[Micha_cg]

Re: Rough surfaces - a mini tutorial

Damien, did you know from 3dsmax, is Oren-Nayar available for Vray? Could we get it for VfR?

[dalomar_cg]

Re: Rough surfaces - a mini tutorial

I'm not sure exactly...with 1.5 they introduced a roughness value, but beyond that I'm not certain...what exactly are you looking to do with this?

[Micha_cg]

Re: Rough surfaces - a mini tutorial

Rough surfaces dosn't detailed react on the lighting, the difference between shadowed and unshadowed side is less, the shape isn't so clear visible. The reason is, every particle at the surface collect light of approx. the full hemisphere - the flat moon effect.
This type of surface is needed for all very rough surface like cloth, dry earth ... . For example a good felt look isn't possible per standard diffuse function only.
OK, my reflection trick works, but need longer render times. It's like using the reflections slot for a diffuse surface. A special rough-surface function could be a time saver.

[dalomar_cg]

Re: Rough surfaces - a mini tutorial

As I said, its there http://www.spot3d.com/vray/help/150R....htm#roughness
I don't know if its in the SDK though, and don't really know what the plans for it are.

[Micha_cg]

Re: Rough surfaces - a mini tutorial

Thanks Damien, that is it. Nice help page.

It seems to be, that some images at this help page are at the false place, for example the "The Refraction depth parameter" images - refraction depth 8 looks like refraction depth 3 or somthing like this.