obtaining shadow alpha maps

# 1 16-09-2004 , 07:59 PM

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Join Date: Dec 2003

Location: USA

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Ok I need to render a cactus with an invisible shadow(it will not be visible since the shadow is made of black pixels). However, I only want the cactus and the cactus's shadow to show up in the alpha mapping. Now I am casting the cactus's shadow onto a plane, however setting the plane's matte opacity to 0 results in the shadow's alpha being destroyed with it. I tried using render layers but I need to have only one alpha layer. I am making graphics for a 2d prerender game, and the lead wants shadowing. He uses 3dsmax however. Also, the image plane and shadow can be visible, I can just put that image's alpha onto another without the plane, and delete its alpha channel. I also tried rendering a top veiw of the shadow with render layers, and making that to map the color of the image plane. It worked, it just looked cruddy. I think mental ray image flags might be helpful but unfortunately I am not that advanced yet.

thanks.

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# 2 17-09-2004 , 08:58 AM

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you can do it with render layers. Put the cactus on one layer and the plane on another. Turn on beauty for the cactus layer and shadow only for the other one. Now when you render you should get the cactus in one picture and the plane with the cactus' shadow in the other. You can then use shake or photoshop to take the shadow out ofthe second pic and comp it with the first.

Alan

Technical Director - Framestore

Currently working on: Your Highness

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# 3 17-09-2004 , 09:43 PM

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I need the cactus's alpha, and the shadow's alpha in one alpha channel. Is there a way to merge channels in photoshop maybe?

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# 4 20-09-2004 , 08:28 AM

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yes you will have to do that. You wont get an alpha from the ground plane though. You should get a greyscale image of where the shadow is though. what you can then do is use contrast settings to take it out and form a channel and then as you said layer it in phtoshop and combine them.

what you seem to want to do (get a shadow as an alpha) is impossible without having a plane there to cast the shadow onto...

user added image

Alan

Technical Director - Framestore

Currently working on: Your Highness

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obtaining shadow alpha maps

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