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threehundredbpmshredder
04-03-2007, 02:49 PM
Hello there. I want to render the shadow of an object above another object with both objects invisible (alpha=0%). I used a point light (light color=black, shadow color=green) and unchecked the primary visibilty of the object that will cast a shadow. Everything works fine except for the floor (where the shadow sits). It's still black. I tried using the shader "Use Background" but it didn't leave any shadow at all. I want both the object and the floor to be invisible. Does anybody know how to do this in maya? Thanks in advance.
jeremybirn
04-03-2007, 04:27 PM
You haven't mentioned why you're not seeing shadows, such as if you turned on shadowing in the light, whether you're using raytracing. But don't use special shaders, try Lambert (white) for the ground and a surface shader (black) for the ball. You can leave primary visibility on for everything.

-jeremy
threehundredbpmshredder
04-03-2007, 05:00 PM
Well, there's nothing wrong with lights and the other configuration. I am able to cast shadows as long as the object where the shadow will sit is visible.

To make it simple, what I want to do is to render a shadow pass in maya with everything except the shadows transparent. My problem is that I couldn't get the floor to be transparent. I wonder how some people use 3D rendered shadows over live action footage. If it's frame by frame tweaking, isn't it a bit tedious and time consuming?

Can anybody give me a step by step procedure on how they overlay shadows in live action footage? Thanks in advance your help will be greatly appreciated.
jeremybirn
04-03-2007, 05:16 PM
You don't need to use an alpha channel to composite a shadow pass onto a background. Pretty much any compositing program you can buy will let you use a single image or even a single channel (like green) and use that as the mask for the adjustment to the brightness in your background plate.

-jeremy
threehundredbpmshredder
04-04-2007, 05:00 AM
Oh so now I get it. When creating shadows for compositing all you need is a chroma keyable background and a the shadow itself. Then compositing software will do the trick like blur the edges of the shadow and alpha the shadow itself. Is that how they do it? If that's the way to do it, in your opinion what would be a good compositing software application? I heard of after effects, digital fusion, combustion, etc. but I heard some of them are better than the others. By the way, thanks jeremybirn for bearing with me.
jeremybirn
04-04-2007, 05:38 AM
You shouldn't need a chroma key or any other keying, what you render should be the mask. But yes, you can use what you render as a mask to darken or do anything else to your background plate.

-jeremy
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