View Full Version : Mental ray issue.

06 June 2009, 08:32 AM

I'm experiencing a very annoying problem in mental ray as I try to make a human eye. The eye itself is ok but I have a cornea layer wrapping it. I want to make it wet and reflective and no reflections are showing up. I tried Arch and design with the following settings : Fully refractive, fully reflective, with a lot of glossiness with an IOR of 1.4.

Also I have a question, is it possible to make a refraction map in mental ray? I had to remodel the eye cause I couldn't tell the shader what area would refract. The only option I got was the cutout one that disabled the reflections in the refracted area :/

When I put an HDRI map it does indeed reflect but in a very ugly way. What I'm trying to do is very easy in Vray so I suppose I must have made a mistake somewhere :P

Could it be that I have to show the light source in order to see the highlights?

06 June 2009, 03:03 PM
Show us a picture, it'll help enormously.

06 June 2009, 09:37 PM
Hi, thanks for the reply :).

So this is what I got when I wrote this message :

And this is the latest rendering :

So I understood quite some stuff but I still have a problem. No specular Highlights are showing up it looks like the lights dont cast any.
It's automatic in Vray but maybe I have to do something in MR.

I'm currently using a raytraced material cause the mia shader has some sort of fog effect upon refraction so the other side of the object is blured.

Also there is something called Vray displacement mod that extrudes all the white values of the displacement map. It can extrude them up to any value. In MR it looks like my displacement is smoothed :/ Would they be a way to fix this?

06 June 2009, 10:14 PM
Firstly standard lights all emit specular as a default, you can see that it is in your rendering through specs on the hat, it's just that you can't see it on your skin (I am assuming it is a misss_fast_skin_maya shader). You need to increase a few settings in the 'Specularity' rollout of the shader - I won't go into which ones, it is better for you to read the help docs and have a play. :)

I'm currently using a raytraced material cause the mia shader has some sort of fog effect upon refraction so the other side of the object is blured.
I'm assuming this is for your eyes, you were right to use the mia_material, but you must have set the Glossy reflection control to something other than 1.000. 'Glossy' reflection/refraction controls how much the 'out' ray is deviated from a perfect reflection/refraction, a setting of 1 means that the surface is perfectly smooth, while 0.0 is so blurry that no reflections can be seen - this is an optimisation setting to simulate tiny imperfections in the surface.

I don't know where the displacement is on your model, but if it is 'smoothed' then the chances are that you need to increase the settings in the 'Tessellation Attributes' rollout of the shape node of your mesh - again, consult the help documents for more details. If the displacement appears to have the 'tips' of the most displaced parts sliced off, it means you need to increase the 'Max Displace' in the Options tab of the Render Settings window. I have to admit, displacement in MR is a pain in the ass compared to other render engines, but it's also very powerful and very adaptable - again, consult the help docs.

Nice head BTW :), she could do with some eyelashes though... I'd play with putting very fine veins in a texture for the back SSS weighting, it would really enhance the ears.

06 June 2009, 12:09 AM
Hi Weihalter,

Most of the things you can do in Vray can be reproduced in MR. the options are just, often, not as easy to find!

To have white specular highlights in the character's eye, you must choose the right kind of lights. What are the one you are using? Are they area lights? Other lights will fake a highlight resulting in a non physical light simulation and an ugly round highlight. The lights I use most often are standard/ mr area spot and photmetric/ target light. The first one is not a physicaly correct light but will give you very decent results. The second one is physicaly correct if set with the principles of optic physic in mind. I suggest you tu use the standrad mr area spot. Under the modify panel, under the area light parameters, you'll find the settings to adjust the size of the light. In Vray you automaticly see the size of the area light in the viewport but with the mr area spot, you will only see it while playing with the height and width spinners. (with the photometric one you see it all the time in the viewport).

When the light as a good size, be sure that it's position will cast visible highlights from the camera's perspective on the eyes.

Hope it helped!


06 June 2009, 11:57 AM
Thanks for the feedback guys.

@cbamber85 :
The mia shader rollout is about the same in Vray, so the material is fully reflective with a glossiness of 1.0 and the same for the reflectivity to have the most possible glossiness. So according to the doc the material should be behaving like a mirror.

So I do the same for the refraction, perfectly refractive with gloss values of 1.0 to get a perfectly clear transparency according to the documentation. In the preview window the shader looks exactly like I want it to be.

Also I found a very cool setting called Round corners which enables additional highlights where the eyes meet the skin :).

I also discovered the Highlight intensity option. Upon increasing it I see them but in a very blurry way. So I assume the lights I'm using are causing the problem since with the HDRI lights the reflections are perfectly clear.

@cani :

The lights I'm using are MR area omni and spot arround the head but I'm also using a photometric light just in front of it. All of them use raytraced shadows.

I'm using real world scale so I assume my problem comes from the size of the lights. I didn't pay attention to this detail and they are pretty big. For example the one in front of the head is a rectangle shaped photometric light measuring 122x61cm.

I'm using FG could this also have a role to play in the issues I'm dealing with?

About the displacement here is what I'm talking about :

With Vray the plason is perfectly extruded according to the grayscale values contained in the bitmap and this on a single polygon with alpha masking. In mental ray the result is visible on the first pic I posted.

I'm going to try what you guys suggested and will post the result.

Thanks a lot!

06 June 2009, 10:22 PM
You should start by testing the lighting with FG and Gi turned off. Always try to isolate the problem. Try hiding your mesh, placing a sphere with a relfective mia material and see what it looks like. Do you see highlights?

How far are your lights? They might be too far or too close.
your light size is a bit big but you should be able to achieve smooth soft ligthing with that. A screenshot of your light setup might help.

the round corners is a great tool when you have complex mechanic meshes for faking a bevel on the corners. For better results, it is better to actually model the concave thin layer between the eye and the eyelids.

For your displacement problem, how do you map it? I don't have Max right here but from memory you have to go in the mental ray connections, under the material. Then add a 3d displacement map, then load your bitmap or something like that. There is also more options under the renderer tab in the render settings.

Keep on trying!

06 June 2009, 08:22 PM
Hi again guys, I solved the problem. It came from the photometric light. It was rectangle shaped so I changed it to point shaped and my material is a clear as glass :).

I don't understand why this occurs, I mean it's not very logical. I never heard anything about the glossiness of specular highlights beeing altered by the shape of the light :/

Anyway thanks for helping me out, your help was greatly apreciated!

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