Thank you Holger!
Setting the color clipping to RAW seems to do what I need - my recently written "Replace Alpha" node proves useful then.
RTFM is fine with me in principle, but I don't think I would ever have found that solution, since I didn't know what to look for.
Even now I will have to read this several times to really get it...
I have worked with straight Alphas and premultiplied Alphas before, but the way it works in XSI seems to be very different from Lightwave etc.
I owe you a beer now :-)
Cheers and thanks again,
Thomas Helzle
On Tue, 03 Oct 2006 23:38:49 +0200, Schoenberger <XSI(at)digidragon.de> wrote:
>
>
> |> 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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> |>
>
>
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