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spheratu
03-11-2003, 08:41 PM
ok,here's the problem:
I render images from maya with alpha channel,tiff format and layers in order to compose them afterwards in combustion or shake.
I load the images in combustion,premultiply them with black and.....theres the black rim!!! always the black rim on every layer,specially visible if there was motion blur in the maya render.
What do I do? I use matte controls/shrink to get rid of it,but most of the times this isn't a good solution.
The funny part is that when I load the same footage in shake and premultiply it....everything is right,NO BLACK RIM!!!
Can somebody tell me what's going on here? Am I doing something wrong in combustion? is it a bug or something?
Thanks

GrantKay
03-12-2003, 10:09 AM
Hi,

Select the layer that you imported. Expand the layer and select your footage operator. In the source options you will see the switch which says "pre-multiply". Try turning this option on and off.

Regards
Grant

Discreet UK

JoshKirk
09-27-2005, 10:56 PM
Im having the same problem and when I click premultiply on and off it doesnt change anything at all... Any other solutions?
PS its the button that says "Premultiply with" then it has a color picker right?

Kai01W
09-28-2005, 12:51 PM
ok,here's the problem:
I render images from maya with alpha channel,tiff format and layers in order to compose them afterwards in combustion or shake.
I load the images in combustion,premultiply them with black and.....theres the black rim!!! always the black rim on every layer,specially visible if there was motion blur in the maya render.
What do I do? I use matte controls/shrink to get rid of it,but most of the times this isn't a good solution.
The funny part is that when I load the same footage in shake and premultiply it....everything is right,NO BLACK RIM!!!
Can somebody tell me what's going on here? Am I doing something wrong in combustion? is it a bug or something?
Thanks
Works just fine for me. You did "unmultiply" them by checking the premultiplied checkbox in the footage controls? (Sorry, if this is a dumb question...)

-k

gemcap
09-28-2005, 10:51 PM
Not sure in Maya, but in Max this is fixed by un-checking post-multiply alpha in the file output settings (I use TGA myself, so I know its there)

This results in incredibly bad looking rgb info, but totally correct results once you apply the alpha. the reason for this is that the color info is rendered 100% opaque even if the alpha will only reveal that area at 1% transparency.

My understanding is that this would be the only way to permanently banish those edging issues.

G

dprgb
09-29-2005, 07:08 PM
Have you tried using the g-buffer builder and adding the alpha tiff in there?

DrachenVFX
10-19-2005, 07:40 PM
I am going to preface this with the fact that everything I know about compositing has been learned by trial and error between 2 and 4 AM the morning of the deadline. Anyway . . .

What we have resorted to is render out each elemnt in order. First the sky, then the environment, then the trees, then the building, etc. When you render the environment, load the sky as a background. When you render the trees, load the background+sky, and so on. So when you go back into Combustion and use alphas (to make the trees go behind the building) that rim around the edges of your object will be the same color as the background, and will mesh perfectly with your scene.

gemcap
10-20-2005, 03:57 PM
Just for the record...


This is exactly what you shouldn't need to do if you can disable pre-multiply alpha...the whole point is to prevent the software from mixing in the current background anywhere the alpha is non-solid. Basically what happens is the RGB values of your foreground objects get rendered at 100% opacity, negating the background influence. The alpha still retains the softened edge/gradient mix, and the mix occurs in your compositing software, ensuring that the blend into the bakground happens only at that stage.

The only time this won't work for you is when your foreground needs to distort the transparency (ie refracting).

Hope that helps.

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