Mental Ray GI doesn't seem to be doing anything

[JohNny45]

Hi Folks,

I'm working through the Intro to Mental Ray in 3DS Max, but also working on the July Member Challenge. So using my own prop.

I followed the directions, selected Mental Ray as the default renderer, turned on GI, but there is no difference between it and the direct illumination. I'm sure I've done something wrong, but I can't figure out what. The difference in scale between my file and the DT one is pretty extreme. I've attached an image. Any help would be appreciated. Thanks.

-Johnny

[roxo]

Hey.. You need to shoot photons from the light. Go to the light's modifier stack and search for mental ray photons. Try about 5000 or higher...I don't know exactly but I had the same question back in the day. Use Final gather instead, it can be more accurate if the interpolation is set right and it's a lot faster...plus you don't have to click on additional buttons like photon casting stuff.

[JohNny45]

Hey Thanks Roxo. That seemed to work a little...definitely saw some GI in the shadows, but just a touch. I vaguely remember hearing that lighting in Mental Ray relies on real distances...is this accurate? So I changed the distance settings in Max to be 1 unit = 1 inch...and scaled all the objects in my scene down and I got the GI to start working.

Ideally what should your settings be when using Mental Ray?

-JohNny

[roxo]

I always try to work with real units. If I want to model something like a chair I try not to model it 100ft tall. I usually start with a bounding box just to get it at the right scale.

I often have the display units and the system units both as centimeters. At work when working with large architectural stuff I use system units as meters and display as centimeters. This way I can view numbers and parameters as cm and things can be at the same size. Also you get rid of crazy viewport artifacts when working with cm with a large scale scene. This applies to rendering settings too but mostly with global Illumination. I prefer to use final gather with a medium preset or as brute force (setting the interpolation to 0 makes final gather realistic - you don't even need ambient occlusion to get detail). I always use photographic exposure control and tweak the highlight burn and shadows value.

Also, it's very important to work with a linear workflow. Go to the preferences menu and at LUT gamma tab change the gamma to 2.2. (also check the material and color picker checkboxes) This way the renders won't be as dark and the image will have a different range of values. This is definitly a setting I cannot render without. Our monitors are set to handle gamma display at 2.2 so it wouldn't make sense render an image that clamps all the shadow information. Do some test renderings... some will look washed out but remember that you get a lot more color information. When rendering a scene with a bitmap you can choose at the import dialog if you want to correct it at 2.2 or at 1.0. Usually the default is always good because most images are at 1.0. (if you 2.2correct an image with a 2.2 gamma it will have a 4.4 gamma and will be almost white). Also if you are using normal maps, MAKE SURE TO import them at 1.0 overide. This is the only type of bitmap that gives you terrible seams when rendering. If you import it at 1.0 override the problem is avoided.
There is a lot of help about linear workflow and gamma correcting on the internet. I strongly encourage you to learn about it.

[roxo]

to be more on the practical side, draw a big plane on the ground and a simple teapot. go to create - systems and draw a daylight system. Change the sun to "mr sun" and sky to "mr sky". Say yes to exposure control (because without it the image would be very bright). Render and BAM.!!! Instant day light configuration. You don't need Global Illumination nut you definitely need final gather to get good results.
If you are rendering and interior scene, do exactly the same as above but put a camera inside a room with a window. You'll have to crank up the exposure control so that the image is not so dark and place a SKYPORTAL at the window (with the exact same dimensions as the window) with the use exterior lighting option checked on (or something with a similar name). That way you get raytracing where you really want it and not shooting everywhere at the scene.

MAKE SURE TO LEARN EVERYTHING I SAID AS IT'S VERY IMPORTANT. hohohohohohoh

[JohNny45]

Wow Roxo...thanks a ton for that information. I'll get studying asap . Funny I was just posting on these forums about a normal map seams issue when rendering and I think it might be related to what you're talking about. How do you import a normal map at 1.0 override? Where is that setting? Thanks again.

-JohNny

[roxo]

Hey, sorry for the delay.. When you import a bitmap it opens a window. In that window you have override gamma. For normal maps, use 1.0 as override...when using 2.2 gamma configuration!