Rendering just a shadow

#1

Hey everyone.

Im doing some 3d and live action compositing. I have a 3d cube i want moving on a table with a shadow, but i cant figure out how to set up all the layers so that the shadow renders and not the object the shadow is present on.

So i have the cube, a light, and a plane, i want the cube to render, the light to render, and the cube’s shadow on the plane to render, but not the plane itself. Ive tried setting the background image, but the shadow and everything disappears, so now im not quite sure what to do.

Thanks for the input.

#2

Which software?
in 3ds Max you could use a matte/Shadow material for the plane or outputting a shadow render element.

#3

It would be the easiest way I think. Another one with standard lights is setting the light’s color to black, the shadow color to white, and the shadow density to -1.

#4

Never thought of that. Nice woekaround.

#5

Im using Maya.

#6

If you are using MR there’s a Mip_matte_shadow shader that you can use in conjunction with the usebackground shader on your model which would render just the shadow.

Since your object is simple you might also be able to go into it’s shape node and just turn off primary visibility but leave cast shadows turned on although that doesn’t always work.

You could also just render your cube and shadow together and then do a seperate matte pass of just your cube and in compositing just seperate the two, probably the easiest.

Hope that helps!
Zach

#7

Thank you so much for telling us what software you use, now we know where to direct our efforts to help you. In maya, try setting he light color to black and setting negative shadow color.

#8

Hey guys, i figured out how to do it. I read online that if I give the plane the “usebackground” mat, set the shadow to black, “set reflectivity” and “reflection limit” both to 0, leave “shadow mask” alone at 1 and then disable primary visibility for the cube in the render stats, it basically renders a black screen. However, the alpha channel stores the shadow data. So then i just render the cube without “shadow casting” and the plane without “receive shadows”, and i can composite the two in After Effects well enough. Now to just touch it up.

Also, im using maya 2010 which doesnt seem to have all those fun MR shaders. I have 2011 on my desktop at home and it does have them. I use 2010 down in my school’s computer lab because i can call it work study and get payed to mess around in Maya :slight_smile:

#9

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