View Full Version : Render layers and Shadows?

11 November 2009, 11:03 PM
Lets say I have a building I want to render that has grass (fur) around the base of it.

For simplicity sake, lets say the building is just a basic poly cube
Around the base of the cube is a poly plane with grass (fur) applied to it

Since the grass takes a long time to render and I'm making lots of changes to the building, I want to render the grass out in its own render layer all by itself, so I don't need to render it out everytime I make a change to the building's color or something. This way, when I'm all done with the building I can render it out without the grass and then I'll just comp the previously rendered grass into the image. Simple enough right?

Well certain parts of the grass is going to receive shadows from the building/cube. How do I render out JUST the grass by itself but still get the accurate shadows? Do I have to render out the building at the same time? Cause it seems like that defeats the purpose of trying to use render layers in the first place...

I'm a bit of noob with render layers so maybe there is just something I'm missing.

If anyone could point me in the right direction I'd appreciate it.

11 November 2009, 02:34 AM
Okay after some investigation I've figured out that I can use a "Use Background" shader on my building in order to make it 'transparent' but still apply shadows to the grass.

That is a decent solution, however, how does this really work in a production environment where a building has hundreds of parts with hundreds of shaders applied to different parts of it? Is something setup to swap the shaders between the Use Background shader and the proper shader for every single object when switching between different render passes? Or are the shadow passes from a duplicate maya scene in which all the shaders have been replaced with Use Background shader?

11 November 2009, 03:35 AM
You could use regular layers. Place the grass plane on its own layer then hide it until your satisfied with the building.

11 November 2009, 10:23 PM
I think you are missing the point of render layers...

11 November 2009, 10:57 PM

U can also apply a use_backgroud shader to the plane and have the shadows in the alpha channel of the plane and keep the original shader to the bulding

not sure if u mean just render layers/render passes or just render layers

if u are using render layers only
u can have in one layer the building and in the other the plane
in the building render layer, go to the building geometry and in the render stat turn off
primary visibility, but cast shadows on, that will cast shadows onto the plane

in the plane render layer just render as usual

try this with ur basic polys

CGTalk Moderation
11 November 2009, 10:57 PM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.