Expert: Andre Hickman - 11/16/2010

Question
I'm new to using 3ds max, and usually I can pick up
software, but this is particularly frustrating. I was hoping
you could tell me what's happening with the roof I'm trying
to render - for the moment its a 2d plane, and is set to
black and glossy. When I render with a camera facing down so
you can see the top, everything is peachy keen. Then when I
render from the outside of the building, but looking 'up' so
you can see the underside of the roof, the roof disappears
in the render (it shows up in the viewport still). Someone
suggests that I need to check or uncheck a box to do with
backface culling. First of all, I have found several places
to do this, and none seem to help. (Ive even tried creating
a new plane after checking/unchecking). Also, shouldn't
backface culling make it transparent on the underside
anyway, since the light source is above it? Isnt that the
point?
Thanks so much for your help.

Answer
Hey Andrea...(nice name by the way)

Here is what I think is happening.  When you use a plane, it really only has one surface, which is the top.  There is no such thing as a 2-sided polygon, even if it appears like you are seeing the backside in the viewport.  This is because the vertices that create the plane reference a "normal" direction from the surface.  There can't be 2 normals on one surface.

In your material editor however, you can check the 2-sided box in the Shader Basic Parameters section of the material in question, which will simulate adding the material to the back (non-existing) face of the plane.

An alternative approach, which would probably be a more accurate way of doing things is to use a very thin box for your roof, instead of a plane.  That way, you have a "real" surface facing outward, and a real surface facing inward.  You can also apply different materials to each side, usind a multi/sub-object map, and editing the id numbers of the boxes polygon faces (by applying an "edit mesh" modifier)

A third way to do it would be to add a "shell" modifier to the plane, by which you could designate a thickness to the plane.

I believe that that backface culling is really just an aid for modelling purposes, but for rendering, always keep in mind that a polygon only has one normal direction/face. If you create a box or sphere and go inside of it and render, you'll realize that the only surface that renders is the outside.  If you wanted to render the inside of a sphere like a dome, you would have to add a "normal" modifier to it, and flip the normals.

I hope this helps.  Keep working at it, and don't be afraid to ask questions, because as a noobie, some of this stuff seems strange...but pretty soon you'll laugh at this misunderstanding.

Cheers,
Andre