I use Maya to make icons, and I've been trying on and off for quite some time to make the "Use Background" shader work as I like.

The end result should be a icon which is a transparent PNG file where the shadow is projected in a supposed ground plane, but is transparent, enter "Use Background", right?

It works, but not as I'd like. I get a shadow, but it's way too light, and not only that, it's also not black, but rather colored grey, which is wrong. See the images I've attached. On the one with white background, you can see how the background on the "ground" is a lot lighter than the shadow on the other objects in the scene. And in the image on a red background, you can see how the shadow isn't realistically "black" either. Not looking for pure black here, but the object appears to cast a white shadow!

I'm using Raytracing and the settings for the Use Background shader is:
Specular color: black
Reflectivity: 0
Reflection limit: 0
Shadow Mask: 1
Matte Opacity mode: Opacity Gain
Matte Opacity: 1

Any ideas?