Strange Hot Spots / Reflections / Speckles when Rendering w/ Mental Ray

[jeremyvandyke]

Hey everyone,

 

This is my first post, so I apologize for all the crucial information I'm sure I'm missing in the post. I've just step up a model for an interior fit-up I'm doing at work. I've set up a mr Sun and Sky set-up for the geographical position of my project, and have added a mr Sky Portal on the windows entering the space (space is going to be a lunch lounge for an office building). The floor plan of the space is too deep to be lit exclusively from light from the exterior, so I have added 3 photometric lights (for now), on the interior.

 

The obvious issue that came from the first render was the hot spots / white specks over the majority of the flat surfaces. The light levels are generally ok and what I'm looking for, but the walls / floors/ ceilings are all giving off these terrible highlights. All of the materials giving off these bad reflections are Arch. + Design Materials, with bitmaps taken from the company collection for the diffuse and bump maps. I have been trying to solve this problem for a few days, but all I've managed to narrow it down to is the fact that it is only occuring in the Arch + Design materials - all the ones I have created from scratch using "standard" materials as the default are fine.

 

I have tried the following:

 

-played with the reflectivity and glossiness of each material within the Arch + Design materials properties

-Increased the number of gathering points and interpolation substantially

-Decreased the intensity of the interior light sources

- Increased the amount of glossy samples in the overall render

 

 

A run down on my render Settings :

 

Renderer = Mental Ray

 

Final Gather = Enabled, gather precision = high, FG bounces = 3

 

Indirect Illumiation = Enabled, optimized for Final Gather Gather. Number of photons per sample = 4000. I have no idea if that is low or hi

 

Glossy reflections = High

 

Lights are all emitting about 200 000 photons

 

Again, Only the Arch + D materials are doing this to me. I would like to use them as their built in occlusion etc. make them faster to work with, but I can't get away from these stupid hot spots... Thanks

[Matt McDonald]

you don't have any "raytrace" maps or materials in there do you? If you do, get rid of them as they are not completely compatible with MR.

 

Another thought, when you use a sun & sky system with GI turned on the sun emits 99.9% of the scene photons and then any lights in the scene divide up the remaining .1%. So each light emits like 10 photons and creates a spotty interior. You might try setting the photon count on the daylight system to very low values and see if that fixes anything.

 

Last thought, you can run into troubles when your light object affects specular and the light material can be seen in reflections.

[Francisco Penaloza]

Well this may be a late reply but it may help you for the next time

I am almost sure that those bright spots are specular samples from your glossy materials.

The default for Mental Ray is 8 that is way too low for any decent rendering, I may suggest 64 or 128 or more, depending of your light situations, (More lights, more hot spots more, glossy blurry materials = more samples)

Now other reason can be that your light are going through a glass material first IE light equipment a box with a glass with a light inside, one solution is increase the light samples and no make the glass blurry or just make the glass invisible for that light.

 

as a new Mental Ray user your optimal learning workflow should be

- set up your models

- apply a flat mid gray color to all surfaces ( you can use material overwrite for this)

- then place the lights need it, per CAD drawings or so.

- Turn on FG only and see how it goes

- Play with the values of exposure in your camera.

- if the scene is to tricky for light, then you can try FG & GI, if you turn on both and you are not sure how they works, it will be more frustration that help.

 

For what I see in your image Final Gather will be enough to lit your scene.

 

Things to notice, although you are using Mental Ray Sun/Sky your background seems very dark. Do you have mental ray in the environment

 

When you use Final Gather and GI (photon maps) the bounces of Final Gather are neglected

 

4000 photons is small number, 20000 is default and that should be increase for cleaner solutions.

Do not confuse Maximum Num. Photons per Sample and Average Photons per lights.

Try to avoid using Standard materials, they will not react in a proper way with Mental Ray lights, once you find your self comfortable with Arch&design materials you'll see they are faster and flexible, keeping all the physical properties of Mental Ray.