|> In
|> other cases it looks like the texture is written into the
|> Alpha. Why would I want that in an alpha channel?
RTFM :-)
Have you tried the "Color clipping" in your render options?
Usally it is set to RGB. If Alpha<max(R,G,B) then Alpha=max(R,G,B)
This is good for "default" compositing. Your rendered images are pre-multipled.
Images where RGB and Alpha is linked together. If you want un-premultiply them,
the alpha should not be lower than a RGB value.
If you set up special passes where you use the image channels for different stuff,
then set the color clipping to RAW mode.
Holger Schönberger
technical director
The day has 24 hours, if that does not suffice, I will take the night
|> -----Original Message-----
|> From: owner-xsi(at)Softimage.COM
|> [mailto:owner-xsi(at)Softimage.COM] On Behalf Of Thomas Helzle
|> Sent: Monday, October 02, 2006 11:17 PM
|> To: XSI - Mailinglist
|> Subject: How do XSI Alpha Channels Work?
|>
|> 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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