Re: How do XSI Alpha Channels Work?

["guillaume laforge" <guillaume.laforge.3d(at)gmail.com>]

AO with alpha set to 1 on both colors should work. Do you use a custom lens shader ?

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Guillaume Laforge
freelance TD | cg Artist
my blog ! http://vol2blog.blogspot.com/

On 10/2/06, Thomas Helzle <xsi(at)screendream.de> wrote:

Hi,

although I am quite comfortable with rendering in XSI, one thing is a complete mystery to me:

How does XSI/Mental Ray gather it's Alpha Channel values?

I render a scene with a reflective glass sphere on a ground plane to RGBA. One point light and a HDRI Environment.

Now I would expect the alpha channel to contain the transparency of the material or even the reflected transparency of the background, but what I usually get is a weird mixture of the reflected and the refracted stuff around the sphere - basically the surface texture/brightness that is reflected and refracted. In other cases it looks like the texture is written into the Alpha. Why would I want that in an alpha channel?

Then, on the floor there is only an ambient occlusion shader, and there the alpha is the amount of occlusion...? Huh?

I tried everything I could think of, setting the alpha values of the AO shader both to 0 or 1 - no change.
Setting the Alpha of the environment shaders to 0 or 1. nope.

I even wrote a simple shader that overwrites the alpha of anything plugged into it with a user defined value. No luck.

To me it looks completely erratic.

What I would like to have is the behavior of Lightwave, where I was able to set on a per surface basis what is written into the alpha channel.
I am willing to write a special shader for it if needed, but I fail to understand what I have to overwrite to get this effect.

And yes, I could create a matte pass, but I would prefer a usable Alpha Channel in the first place and I would especially love to understand how this part of MR works.

Thanks for a bit of explanation and guidance! :-)

Thomas Helzle
www.screendream.de
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