Perhaps I'm joining this thread to late.. but..
To bounce off this quote:
"In general though I do find many situations where I want to
control the alpha channel in the same manner as an RGB channel
in both the fxTree and the render Tree."
How do we do this? Is there any color node that allows for independent
R,G,B,A control in the render tree?
I am attempting to create a massive key Matte pass that export RGB CMY,
both with and without alphas. That way I end up with 12 independent
mattes from one pass.
Nathan Turner
EA-Chicago
312-661-7626
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf
Of dbarosin(at)optonline.net
Sent: Tuesday, August 29, 2006 2:18 PM
To: XSI(at)Softimage.COM
Subject: Re: RE: alphas - was: fxTree - mask shapes
Ok well that makes it clear regarding the paint ops.
It threw me off a little when I was getting the desired behavior without
using edge blur. I wish it would just sustain the same behavior when
blurring.
In general though I do find many situations where I want to control the
alpha channel in the same manner as an RGB channel in both the fxTree
and the render Tree.
I was quite deflated when I saw a 'different' Mental Ray compatible
software package slinging values into each RGB & A channel quite easily.
Maybe there is still a trick to doing this that I've missed. There are
many times I just want to pack 4 mattes into one render.
But once again I appreciate your help.
----- Original Message -----
From: Luc-Eric Rousseau <lucer(at)Softimage.COM>
Date: Tuesday, August 29, 2006 12:37 pm
Subject: RE: alphas - was: fxTree - mask shapes
> It's because alpha is used for compositing and generally means
> pixel coverage, or the weight of the color in that pixel, it's not
> an additionnal color channel. It's closely linked to the RGB value.
>
>
> It's equivalent to the opacity channel, which isn't directly
> editable in adobe product.
>
> For the fxtree case, the mask shapes operator is meant to be used
> to produce gray scale matte. When connecting an RGBA image to its
> input, it get into this state where it isn't really a mask shapes
> operator anymore, but a normal color paint node. So it pastes the
> grayscale shape onto the background image, using its mask. The mask
> of the shape is white everywhere there is full pixel coverage, and
> a gradient where partial information, for the antialiasing at the
> edges.
>
> In other words the output of paint is alpha pre-multiplied. If the
> alpha of the shape were zero, it would mean that nothing would be
> painted, i.e. composited on the backgroud. So to paint a black
> shape in the color channel, one must paint a black shape with
> alpha.
>
> That's why you can't use the input of the mask shape operator to
> put black in the alpha channel of a background, you have to combine
> it with the original image with a compositing operator that doesn't
> use alpha for compositing, which is the only behaviour available
> with the paint operators inputs
>
>
> > From: dbarosin(at)optonline.net
> >
> > Thanks Luc. I'll give it a shot.
> >
> > Seems like there should be an option regarding the behavior
> > of the alpha channel.
> >
> > On another note I was having a similar issue with the render
> > tree and alpha channels. If you take a RGBA combine node and
> > plug it into a material you'll see that you have control of
> > the alpha while its rendering (need to have a render region
> > displaying the alpha channel to see this). But at the last
> > moment the alpha pops to white.
> >
> > I appreciate the help.
>
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