View Full Version : MR_Lighting Techniques_Feedback Please

Daniel Whitton
03 March 2004, 11:59 PM
I am placing this post with hoes of receiving feedback from the comunity on developing a strong understanding of some basic/advanced techiques and concepts on lighting with mental ray for maya.

I have been trying to establish a lighting model for a BMW Z 4 that I have modeled 3 months ago. the last 3 months have been put to researching light and rendering. I have been using Rendering with Mental Ray and Digital Lighting and Rendering as my guides.

I'm posting a rendering of my car. It is by no means qualtiy and I hope to improve it with your help.
The scene is set up with 3 point lights emitting 100,000 photons each and sharing the same physical light shader with and intensity of 300. To make it easy to debug the light color and environment color are both white. Also I am using 500 Final gather rays.

What I can't figure out is the render is completly washed out when I add the GI to my render. One would think this to be light intensity or photon energy. I am baffled.

The wall is a DGS shader that is Diffusing white light. While the ground is a DGS Diffusing Black while have 100% Glossy refelctions. The render however is to different tones of grey.

If I omit GI and only use Fg the colors render as expected.

From what I understand so far is that when I use a physical light shader it suppresses the maya light shader and only calculates the physical shader which works from the inverse square law.The exponent value of the photon energy is suppresed when physical is checked on. Photonic energy is the intesity of the indirect illumination passed off by the photons as the bounce through the scene.
With that said I have tried atleast 20 different renders with various settings of photonic energy values and light energy values. I either receive dark or nuclear renders.
I set this up with a black floor and white wall and grey reflective car so I can hopefully grasp the concept a little better then render out more impressive scenes.

I beg anyone to please help with concepts,ideas,theories,anything to get this going in the right direction.

My main goal is to correct the GI in this scene and talk about the DGS shader for my car with specular and glossy reflections for silver paint. I'm obviosuly not a lighting artist, but I want to get stronger. This is just a simply scene inpspired by a render form the Rendering with Mental Ray hanbook that shows a red BMW with a white wall and Black floor. I was trying to match it,but falied.
Thanks in advance.:blush:

03 March 2004, 12:10 PM
Hi, I'm currently working on a similar scene and your photon and final gathering setting should be o.k.. but the most biggest problem for such scenes is that the combination of a dgs shader together with high contrast/filtering and sampling values is really timewaisting, because they working against each other. probably you need at least 3 min and 4 max-samples and the highest filtering to get a clean combination of smooth microfacets and sharp edges --and this would take, for example, about 12 hours in pal quality on a 3ghz i tried the custom blinn shader, which simulatates the microfacets by a non-physical way and isn't affected by the samplling for every single micofaccets like dgs. the second thing is that you need a layered shader for the fresnel effects of carpaint and it seems that the layered shader from maya doesnt work for mental ray custom shaders... I hope they'll fix that in Maya 6.0:D

03 March 2004, 01:23 PM
Nice model !
If you're just starting in lighting, why bother using all this stuff?

I'd just use the 3points lighting, then add a big sphere as a dome and enable the FG. It's easy and gives good results.

For the glossy reflections, switch to the ReflectionUtility (form the it's much faster.

But you should focus on your 3points lighting first, it's where you could adjust your setup, leave the FG just for the last improvements.
Hope this helps a little bit.

Daniel Whitton
03 March 2004, 08:03 PM

Thanks, yeha I have gotten feadback from other posts that the integrated DGS in maya is just a hack, and I'm starting to agree with that. To get the correct smooth render I need to over sample at 4 to get it right. That leaves me with a 7 - 8hour render. I thought it was just that long because of my wimpy machine. But seeing that you have a 3ghz.hmmm.
What shader are you in refence to. Do you mean shaders by Pixero, where can I find the custom Blinn.


I have followed a lot of your posts looking at your work. You and several other people that have posted great renders with mental ray are the reason that I am trying to capture mental ray.

I have be using mental ray since beta and found this discussion which orignally got me hyped about mental ray.
I'm not desiring 100% physical correctness, but some correctness just the same. I'm actually deveopling my demo rel for hard surface and organic modelling. I would love to render amazing images. I know the time and dpain involved withrendering. It's nothing easy. I just trying to get it right.
I'm trying what you suggest with three point lighting. One basic question is when setting GI. Do you still setup with 3 lights, or only need 1 light emiting photons. I know it sounds stupid, It just seems easier to light outdoor illumination with GI than an indoor scene.

03 March 2004, 08:24 PM
Yes, when you're happy with your 3 points lighting, you could think in adding global illumination. But for outdoor rendering, i think you have to use Final Gathering. If you want download the scene maya file posted by Jeremybirn in Lighting&Rendering section here in cgtalk and use it as a start point for your FG rendering.
You could also think in faking the GI by using some Ambient Occlusion, as an easy solution for that, try the dirtmap, really nice shader.

Good luck.

Daniel Whitton
03 March 2004, 09:13 PM
Thanks Lazzar

I just went to Lighting and Rendering, did a quick search for Jeremybirn , and found about 6 threads. I coulnd't find a link to the scene your refering to 2. I went onto his website and couldn't find it there either:hmm:

03 March 2004, 09:58 PM
Here is the thread:

Daniel Whitton
03 March 2004, 11:48 PM

03 March 2004, 09:44 PM
I would stay away from GI when doing cars for now.
Good lighting is crucial but maybe even more than that is
environment where you put your car in. Additional reflective
surfaces stratecigaly positioned can make or brake your scene.
In this example I used final gather with couple of mr area lights with raytraced shadows.

You can see how simple environment was build to get nice reflections. On the top is classical highly white plane for windshield and bonnet reflection. Car material is only blinn with
fresnel reflection, maybe it would look better with layered shader.

Daniel Whitton
03 March 2004, 06:50 PM
thanks I.D
it make sense, I just downloaded a few shaders for maya that support fresenl reflections and blurred reflections, and render faster than thecustom mr_shaders. Could you possibly post a wire showing the position of your area lights and the light intensity. ALso did you use Final gather in your scene or just raytrayced shadows.

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