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View Full Version : Mental Ray, DSG Shiny Value, and Artifacts, oh my!


Theta-Dot
05-26-2006, 01:40 AM
Been spending a decent amount of time looking through the various (and somewhat old/lengthy) Mental Ray threads. Came accross one part which spoke of colored dots / artifacts but that's not quite my problem I believe.

First, have a look-see:

http://www-personal.engin.umich.edu/%7Eaustincb/generated/CgTalk/MRSpots.jpg

After throwing my hands in the air over another scene file which I could not get to look decent, I decided to hit reset and see what I could do with a classic plane/teapot combination scene. It has one MR-Spotlight pointing down.

The materials applied to both the teapot and plane are mental ray materials, with DGS sub-materials applied to both the surface and photon slots (instances of each other).

I suppose my first question should be: Is DSG to be used only in very specific instances? In my real scene, I was trying to light a simple window with shutters and applied a DSG material to it to make it appear like white painted wood. I ended up with the same problem as you see in the picture above (red circled).

Seems as if I take the "Shiny" value of the plane to zero, those go away. The higher the shiny value, the closer and denser the tiny dots become as they give a more mirror-like reflection of the teapot.

From my 1+ hour spent on fiddling with various render settings, it would appear that no combination of values -- ranging from the very low to the very high -- in the render settings dialog for GI and FG will have any impact on the dots, as they are in the exact same location and intensity every time.

Any clue on how to make these go away? Or should I not be using DSG at all?

Thanks for any assistance.

Theta-Dot
05-26-2006, 01:55 AM
I spelled it "DSG" instead of "DGS" in the title :D
oopsie!

3DMadness
05-29-2006, 02:30 PM
The problem is with your Glossy Highlights, this is what is causing the white dots. I've replicated your scene here and if I put black in the glossy highlights these white dots disappear. So you should use the glossy highlights only when the you have a high shiny value, in this case you can add some specular to give it some reflection.

But I think DGS should be used in specific situation because it's a physic phenomen and can slow your render, so in animations I usually don't use DGS.

Theta-Dot
05-29-2006, 04:53 PM
I see. I've been out of town for a few days, but before I left I just started using DGS materials directly (Instead of assigning DSG to slots of a Mental Ray first-tier shader). It fixed things but I guess I don't fundamentally understand the difference between the two.

I guess I also don't get why using a physics related material would ever NOT be desired (at least in single frame renders). Aren't they supposed to be the most physically accurate?

There seems to be a redundancy, or overlapping of sorts in a lot of these shaders. You can achieve a similar look in several different ones. It almost makes choosing one over another completely arbitrary.

3DMadness
05-29-2006, 06:43 PM
I guess someone like jeff patton could answer better than me, but what I know is that the DGS Material have all the shaders inside like surface, shadow, photons, etc... when you use the DGS as a surface shader in a MentalRay material, you have more control over the other shaders.

And we always want to use physical accurated materials in our render, but sometimes we can fake it with a differente material, like a raytrace material, getting much faster render times. Of course that if you have a fast computer or render farm you don't bother with render time, but in small companies and with short deadlines we have to avoid long render times.

And I agree with you, we have many differents ways to get to the same result. ;)

rodrigogua
05-31-2006, 03:28 AM
dgs materials work in a very particular way. they replicate in a physically accurate way real-world materials. it is called DGS responding to a Diffuse color or map, a Glossy color or map, and a Specular color or map.

Diffuse. This is the main "color" of the material. A wooden table, for example, would have a wooden texture map applied here.

Glossy. This parameter determines how blurred the reflections on your object will be. When you click the color, the Value parameter can range from 0-1. 0 means the object will b 0 percent glossy. that is, it won't have glossy reflections, (it won't have 'blurry' reflections). a value of 1 will mean 100% glossy reflections, that is, very blurred. Glossy reflections take much longer to render than straightforward reflections and will need much more samples for them to look any good. those white points in your render are caused by the fact that you have glossy reflections in your material and not enough samples to make them look smooth, at least not as smooth as you'd like them to.

Specular. this parameter determines the specularity of the material, that is, the straightforward, raytraced reflections in your object. as in the glossy parameter, 0 means no reflections at all, and 1 means 100% reflective. speculars don't take as long to render as glossiness, but it will still take longer to render than a 0% specular, 0% glossy material. It will take as long to render as an ordinary raytraced material.

For instance, let's say you want a chrome material. then, you would set D to black, G to black (V:0) and S to 1, so it would be 100% reflective.

An important thing is that the values of G and S, added, SHOULD NOT exceed 1, or you might get unexpected and unrealistic results.

good luck

Theta-Dot
05-31-2006, 05:00 PM
Oops. I thought glossy, specular, AND diffuse had to add up to 1...
Knowing that it's only glossy and specular is helpful. Thanks.

3DMadness
05-31-2006, 08:21 PM
Oops. I thought glossy, specular, AND diffuse had to add up to 1...
Knowing that it's only glossy and specular is helpful. Thanks.
I thought that too.

Thanks Rodrigo for the explanation. ;)

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