View Full Version : Mental RAY GI - MAX/XSI problems with FG vs materials ???

03 March 2002, 02:45 AM
I have been using Mental Ray with Max3-4 for some time now..

I have experienced a lot with GI rendering with MR photon maps method.. but there seems to be a problem in the way that max/mr communicates that refrend me/us to be able to use the GI rendering to it's true potential ..

after reading MR's rendering book lik 2-3 times..i emailed directly Mental Images ...even if they don't give direct support they neway replied to me ( maybe cause i was pointing out something that is important )

so here is what they told me :

> how are you ?
Fine, thanks.
> I'v recently noted that lot of people are using FG without GI for rendering purpose...
> after having read the rendering book, this seem awfully non phisically correct it ?
No, it's not correct because later light bounces are discarded, but
it often comes very close. It's usually good enough for entertainment
projects, where absolute correctness is not required. Final gathering
without photons is certainly supported by mental ray.
> and then i try and use only FG
> i get washed out white result and i have to turn down light intesity
You are probably not using a physically correct illumination model.
Phong is not correct, for example; it reflects more energy than came
in. It's an amplifier. Since final gathering samples and integrates
many light paths, light multiplies arbitrarily. The same problem exists
with light shaders that do not properly implement inverse-square falloff.
This causes final gathering to experience light that gets _stronger_
with the square of the distance.
Try the dgs_material and physical_light shaders from our physics library
instead. You can even find source code on
> isn't FG supposed to only sharpen and help distribute photons by ray means when both FG and GI are turned on ?
It's an extra integration step over a photon distribution one generation
closer to the light, which means you can get away with far fewer photons.
But this won't help any with the shader issue.
> Then in the rference book, you are talking about reflection coefficient...what is that ?
The amount of radiated energy divided by the incoming energy. Again, the
Phong model with "specular" and "sharpness" parameters is bogus.

i tried to work with what he told me..but got nowehere more :(

I have talked with Discreet people on that and they were not of any help :(

Maybe i should go look into XSI forums to see if i can get something there .. i'm sure that someone has found the answer..

would it be possible to use external MR material shaders ?
could that resolve the problem?

if anyone could be of any help ?
or could think of how to resolve problem?new testing steps ?

thanks :)

03 March 2002, 10:29 AM
thanks for the tips
I have this book too, is very good.But i use MR with max and render with max, not with the standalone render.The discreet connection with MR is a big shit at the moment i thing.
I wait for good integration with the render but who know?

but my opinion is:
too many "photoreal" and "fast" renders are on the market at the moment, but the real really photorealistic rendering is Mental Ray.


03 March 2002, 07:32 PM
that is what i think too :)

but my problem is how to be able to use mental ray to it's full potential while working with MAX for scenes...

how do you use MR ? as a standalone ?
are you able to use shaders ?

i think i'll start studying the mi2 file structure soon :)

03 March 2002, 04:41 PM
my max crash every time when i go to translate the scene to .mi file.Thats i mean for the connection at the moment.Or maybe the problem is the max, i don't know.Only i managed to render the default .mi file that coming with MR / the cornell box and the 2 spheres /
i'll start studyng shaders too, i hope

sorry for the english bro


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