Mental Ray - Render Troubleshooting


Hi Jeff,
I understand the Physically correct aspect of photography, but when i my clients are trying to see and Ocean view they don’t care for physical correct :slight_smile:

Thanks Jeff,I normally set up another scene and render the background scanline.

This is exactly what we need, thanks for paying attention Jeff :slight_smile: With guys like you understanding what we and our clients need Autodesk and Mental Images are sure to listen to you.



What about using the render elements? Passes like the background element isn’t affected by the exposure control. Of course that leads straight to compositing work…and that probably defeats the point here since you can easily add the background without having to render a specific pass for it…but for animated backgrounds it could help.


Probably a good idea, I will have to look into it.
The last time I used elements, the whole scene had to be render instead of just
the pass I was after. Does this still happen ?


Not sure how to answer that…They fixed the render elements a few versions back so it now creates all the passes at the end of a render (it doesn’t re-render for each pass, they are all generated at once).

But no, you can’t render out just an element. You have to render the scene, then the elements are instantly generated at the end of the scene render.


ehy Jeff,

just a little correction. 'images' are generated at the end of the rendering, while 'elements' are collected during rendering and wrote down as images at the end. the distinction is important because on an adv workflow you could even interact with 'elements' and let a shader sources one and outputs to another one for example. 
then a little lens shader could also allow to switch what is interactively displayed in the vfb. in 3dsmax is not really necessary but on workflows where the main render out is not a 'sum up' of all the other elements you could just switch 'element' to be previewed without having to wait till the end of the rendering. 

eventually what ‘they fixed’, is that now ‘elements’ are based on mentalray user framebuffers. while before a whole new render was necessary to render each element… and that’s because color data were not stored during rendering.



Cool, thanks for that clarification Max. :beer:


Hi guys!
playing around with the mr photographic exposure control, I found a way to get the backgrounds “almost untouched”.

Select the mr Phographic Exposure Control, and ajust the Exposure Value (EV) or Photographic Exposure to get your image correctly lighted.

Set the physical scale to “UNITLESS”. The value here, will be the result from the next formulae:
(note: EV is the value you get from the Exposure Value (EV), even if you used Phothographic Exposure, and the Exposure Value (EV) is greyed out…)
Unitless value = PI*(2^EV)
EV = 0
Unitless value = 3.14 * (2^0) = 3.14 * 1 = 3.14

EV = 1
Unitless value = 3.14 * (2^1) = 3.14 * 2 = 6.28

EV = 10
Unitless value = 3.14 * (2^10) = 3.14 * 1024 = 3215

What will mess up a little bit your picture, will be the Image Control…
But I found some values, that works when you have gamma off (have to look for some others when you work with gamma 2.2, 1.8, etc)
Highlights: 0.83
Midtones: 0.43
Shadows: 0.0
Color saturation: 1.0
Whitepoint: 100.0 (YES!, 100!!)
Vigneting: 0.0

To probe this, I post here a snapshot from the RAM player with two pictures, the left with mr Photographic Exposure Control, and the right one without control. You will notice the background (a gradient) is the same on both pictures. I used the ramp, to be able to adjust the subtles differences on the midtones.

So, it would be REALLY nice, if the “Process Background and environment maps” control, would do this automatically… Or at least have a button that calculate all of this!!!

Well… I hope this helps you. And I’ll be waiting for comments and suggestions for better techniques.
(sorry for my bad english…)


and for gamma 2.2, you can use these parameters:
highlights: 0.83
Midtones: 2.12
Shadows: 0.0
Color saturation: 1.0
Whitepoint: 100.0
Vigneting: 0.0


Sorry! Wrong midtones value for gamma…
With gamme 2.2 or 1.8, the midtone should be .946
I get this multiplying .43 * 2.2 = .946
When I used gamma 1.8, I thought the midtone value should be: .43 * 1.8 = .774, but it’s not…

Does anybody knows if is there any way to calculate the midtone value when using gamma?
Wich is the maths behind all this?

Thanks in advance for your responses.


Hello, I am using mental ray on my Mac Pro Intel 2.33Ghz Quad Core with 5GB RAM and Ati Radeon 512MB X1900XT graphics card in a Tiger operating system. Two things are bugging me:

  1. In mentalrayglobals in the AE, I click on it and get an error message:
    //Error: Cannot convert data from type int[] to float or something like that
    what does this mean?

  2. I don’t know why Maya does not use the memory available to it. In mental ray globals memory limit, I tried setting it to 0 for infinite, or a high number (it will only allow 32500) but it jumps back to about 1410 after I render it. Right now I set it to 32500 right before I render and I am getting 380% CPU 16 threads 1.5GB Physical Memory and 2.3GB Virtual Memory. Previously it was running at 400% CPU 21 threads 700MB Physical Memory and 1.3GB Virtual Memory. I’d like the program to stop limiting itself so it can use the resources available on my computer.

I’m rendering an architectural scene with raytraced shadows, GI and FG. I have a sculpted NURBS surface U26 V26 I scaled up 60 times or so and put a painteffects fur grass preset on it with a density of 90000. Is there an easier way to make some pretty grass?

Here’s a test render. The light is way too intense and I’ve fixed the ground a little since this pic. I tried fixing the lighting by making the multiplier for physical sun/sky .75 and the intensity of the sundirection .91, but it’s still a little on the bright side.


Hi Guys,

we from german mental ray forum have a big Problem with the new mental ray 3.6 in 3dsmax2009. I comes sometime a Black Bucket and after this rendering, is the Bucket a Black Line cross over the picture to the top.

look here GMRF-Tread:

hot chip


Hotchip, I’ve had that too, I think it was to do with Satelite rendering.
When I turned it off, it no longer happened.


we have for each rendering a solution, but all the time different and all renderings are on a single machine, not satillite rendering. :cry:

some solution

  • switch from 32Bit Buffer to 16 Bit Buffer
  • turn off Shadow from tranparency Objects
  • delete a portal light and use the mia_portal_light shader in a rectangle area light.



What about changing resolution ?
Does it make a difference ?


not resolution, a mean solution


Hotchip, I had a similar problem using an output camera lens shader.
But the lines were RED… I solved the problem removing the HDR Image Motion Blur (mi).

I used to put a Shader List as an Output Camera Shader, and use HDR Image Motion Blur (mi) with a Glare shader. But everytime I close and open the file, the Motion Blur gets dupplicated on the shader list… (this occurs almost always, with every file).

So… until there is a fix for this problems, I prefer to not use an output camera shader.

Did you use one on your render with the black lines ?


I have experienced similar problems in a scene with lots of trees, using leaves with cutout transparency. In the end the solution was really simple. I was using FG without GI. When I turned on GI my rendertimes were more than 5x faster(!) and all the problems were gone. It looks like FG gets “lost” when there is no GI and there’s too much going on in the scene (like your grass and perhaps the geisha’s hair?). (This was in Max 9 though.)

Do you have both FG and GI enabled?



the errors comes is both enable, and the errors comes it not both enable. The rendering was without a output shader, with is not the bug.


Hi dudes, i was playing around with the ivygenerator and now i have problems with cutout map in the arch material. it all looks good in the material editor but when i render i get this black line around the edge of my leafs. please look at the attached image. or do you guys have a good leaf material to share… :slight_smile:



I have expirenced the same problem.
I also had a problem where if the glare shader was in the camera output slot I would get horrible anti-aliasing issues. As soon as I remove it all is fine.