View Full Version : Mental ray, sampling/undersampling

01 January 2005, 08:00 PM
Hello everybody,

I have a question for you. How do you undersample in Mental Ray 3.3 included with maya 6.0?

The thing is I find it to be a terribly slow renderer using fg, I want less samples and to smear them out over a larger amount of pixels. I use this all the time in Brazil. But I haven't found a way to do this in Mental Ray yet.
So if you have a way, please let me know it would be much appreciated.


01 January 2005, 10:13 PM
To undersample the antialiasing/regular samples you just do something like set the Min samples to -2 and the Max samples to 0. That way the minimum number of samples that will be taken is one every 16 pixels and up to a maximum of 1 per pixel determined by the contrast settings.

To undersample Final Gather, basically the higher the radius sizes and the lower the number of FG samples/rays, the more undersampled it'll be.

I think, hehe.


01 January 2005, 03:29 PM
Thanks I think I'll give it a go right away,

I saw that the quality didn't go down a lot when I lowered the fg rays to 150 on a non textured object. So it's definitely a good speed hitch. If I use a higher radius, will I be spreading out samples over a larger screen area or will I just be undersampling?


01 January 2005, 08:50 PM
Thats a quote out of 'Charlotte Manning's Options Notes' ( from

Final gathering uses quasi-random sampling over a hemisphere centered at a sample point. To get an accurate solution, usually hundreds of samples are needed (a sample is obtained by shooting a ray). If a renderer performed finalgather for every eye ray (an eye ray is the initial ray originating from the camera to sample a pixel in the image), the render would take forever.

mental ray will choose - based on your finalgather parameters - only select points in the scene to perform this expensive operation. These FG points are determined during the finalgather precomputation phase just before the main render. The idea is to interpolate between these points to cover the entire image. The placement of the points depends on the finalgather accuracy settings:

The FG accuracy setting has two parameters:

Number of Rays (= # of samples) and
Max Radius (= the maximum radius in which a final gather result can be interpolated or extrapolated).

More rays gives better sampling for a given finalgather point. The larger the radius, the less dense the FG points are, the less accurate the solution, and the faster the render time.
I also recommend going through the Samples Tips ( over there for a detailed understanding of adaptive sampling in mr.

02 February 2005, 12:52 AM

you are a walking talking library. You amaze me! Thank you very much. I'm looking over it right now.:thumbsup:

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