Strange shadow color on VrayMatWrapper

[Fabrice]

Hello,
have prob with color spotlight (with Vray shadow) casting strange colored shadow on the ground.
The ground has a white VrayMat with a VrayMatWrapper above it. "Mate surface" option is selected.
The shadow casted on it are allways the opposite color of the light :
Yellow light casts purple shadow, red light cast green shadows...

Any solution about it or am I doing something wrong ?

Vray 1.5 rc3 + Max 9 sp1 32bit

http://www.protozoaire.com/data/blue_shadows.max

[RErender]

It is kind of weird, but to get the right result just render in front of a black background.

[Fabrice]

No change ... any color on the background gives strange colored shadows, even very dark grey.
And with black you don't see the shadows on the ground ...

I tried to put geometry on the background and I looked at the color mapping settings with no results.

[vlado]

In fact this is the correct result.

If you think about it, the matte shadow is a color that you must remove from the background, in order to get the impression that it is illuminated by the same light.

For example, if you have a red light illuminating the scene, shadows cast by that light are characterized by absence of red, compared to the rest of the scene, resulting in a cyan matte shadow color. When you multiply this with the proper background (which is assumed to be under similar lighting conditions, e.g. also illuminated by a red light), this removes the red color from it, producing the correct shadow result.

If, for whatever reason, you want the shadows to actually match the color of the light, you can use a VRayShadows render element.

Best regards,
Vlado

[Fabrice]

Ok Vlado I understand the "logical reason" but the "optical result" is a bit strange ... no ?
I don't remember to have encountered this problem in previous versions.

Now I don't do know how to achieve a white seemless background ... with shadows on it.
Could you explain the way ? (VRayShadows render element ?)