View Full Version : MR - Final Gather & Shadow Layer

04 April 2003, 08:21 AM
hello folks,

I'm having issues with Mental Ray.

I have no lights in my scene and I'm using final gather to light it. I want to capture the shadows its generates into a layer so I can composite it. BUT I can't seem to get them, I've tried using a "usebackground" shader to no avail. I'm using the shader correctly but can't seem to get the shadows into the alpha.

Any help much appreciated.

04 April 2003, 08:42 AM
I'm not sure if this is what you want to know, but;

If you try to use final gathering without lights in your scene, Maya will by default put in (I think three point lights) lights into your scene. Probably making your shadows disappear or become unnoticable (if you made a light source with hdri/maps or incandescence).

Don't know how to turn off the default lights per se, but you can create a (say a) point light and set its intensity to "0".

Then the lighting you made for final gathering will kick in and render the way you want it.

Correct me if I understood you wrong.

04 April 2003, 08:52 PM
I think I might be able to better express whats happening if I post the images and scene setup...

My setup:
- no lights
- default light still enabled
- an object with a shader consisting of a near white diffuse and ambient colours sitting above my character to act as the light source.
- final gather preview selected in the quality settings. default setting for final gather.

Here is the scene rendered:

Now, this second image is the image I want to composite onto:
I tried rendering it out in one scene with the character and mental ray rendering...but it renders the grid texture out like "crap", I had to crank the sample settings to ungodly levels to get anything half I thought I would attempt to comp the shadows from the first onto the second and have a nice clean image, sound easy doesn't it?

All I need is to get the shadows from the ground plane onto the grid image. Like a stated earlier, I tried using a usebackground shader but when I do the shadows just dissappear. I know my settings for the usebackground shader are ok because I tested it in a basic file with mayas renderer and it worked fine getting the shadows into the alpha channel but mental ray won't let it happen.


04 April 2003, 08:04 AM
Since the shadows in the final gather setup isn't actual 'cast shadows', but rather lack of light bouncing into the area, I suppose this is what is causing your problem...

To be honest I don't quite know how to solve it though..

04 April 2003, 08:39 AM
yeah that sounds about right to me. thanks for the insight!

I wonder if there IS a solution.

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