VraylightMtl with Matte

[TheLooopCGI]

Could somebody please explain to me why an object with VraylightMtl brightens up a matte object and regular lights don't? I would think it has something to do with GI element of VraylightMtl but if I use a GI override in the Environment options instead then this doesn't bright up the matte. How come?

Thanks!
David

[ChaosGroup]

You need to enable Direct Illumination option of the VRayLightMtl, this way the "light object" will affect the direct illumination instead of the GI.

[TheLooopCGI]

Right, but it doesn't answer the question I asked.

VrayLightMtl with either Direct Illumination OFF or ON still influences matte objects while it should not.

[Zdravko]

Would it be possible to attach an example scene or send it to: support@chaosgroup.com

[TheLooopCGI]

It is really easy to recreate:

1. Assign a non-black color to the environment.
2. Make GI override in VRay settings black (so the environment color is actually not lighting up the scene).
3. Create a plane, make it matte.
4. Create a box and place it on the matte plane.
5. Create a small ball close to the box and assign a VRayLightMtl to it.
6. Render.

You will see two things:
1. The ball is influencing the matte object making it bright.
2. The ball is not casting the shadows of the box on the matte.

Is this a desired and correct behavior?

We basicaly came to this because we're making a lot of location shots for cars - we photograph a background, an HDR 360 degree panorama and then put a 3D car on a matte plane for casting shadows and render. As long as we use a GI override slot for emitting light from the HDR panorama all is fine. But once we use a geometrical sphere around the car instead of GI override it doesn't work anymore. In essence the GI override is creating a large sphere around the scene and emits GI light into the scene the same way as a large geometrical sphere with a VrayLightMtl does, correct?

Thanks for clearing this up.

[Zdravko]

The light is influencing the matte object because its only affects the GI. If you want to use it as a light try to enable Direct Illumination option.
Also in your scenario I thing it will be best to use VRayDomeLight with the HDRI that you have (http://www.spot3d.com/vray/help/200R...lights.htm#ex5) instead of geometric planes. This is very popular approach for rendering and since the dome light emits direct light you get better shadows and faster render times.
Set a dome light in the scene, assign the HDRI to it as well as the background and make sure the Override environment is set to black. For the dome light make sure you are using VRayHDRI texture in its Use texture slot.

[TheLooopCGI]

I understand the dome light, I'm just curious about the VrayLightMtl and the way it works. Or more importantly why it doesn't work well with mattes.

Turning the direct illumination ON doesn't change the way it affects matte unfortunately.

[Zdravko]

You where right about the shadows on matte surfaces. We made a note about that.
The Direct Illumination option in another hand is working fine. In the image one of the spheres uses Direct illumination and the other doesn't.
Can you attach simple scene?