🚀 go-pugleaf

In article <101668@bu.edu> ambi@it-next2.bu.edu (Michael Amirault) writes:
> Anyone noticed that the brick texture looks inside out?  I was 
>wondering if maybe I'm looking at the wrong side of the polygon??
>The brick does look nice on a plane facing the camera, but on one
>that's diagonal, it doesn't seem to draw individual bricks - just
>rows.

[Yet another episode in the series  "Daryll's Tips for RenderMan" ;-)]

The problem is that you can't make a smooth parameter space for
polygons. Normally your objects have some nice smooth parameter space to
do texture mapping. For example, in all the quadratics, u runs along the
longitude from 0 to 1 and v runs along latitude.

What do you do with a polygon? The problem is that you don't have a
complete surface, it could have holes, it has multiple vertices, it
could be non-planar, etc. RenderMan defines a standard that says that
parameter space on a polygon is the X and Y coordinates of the polygon
in object space. There is no perfect answer of how to handle polygons.
The biggest flaw of this answer is that any polygon that is parallel to
the XY plane has a constant parameter value (and weird things happen!)

You can define the u,v values at each vertex of the polygon. RenderMan
would love that, because it could then interpolate between the values
and be happy. Unforetunetly, you need a modeller that allows it.

The bottom line of all this is that you have to be very careful if you
plan on applying texture maps to polygons. If you can avoid polygons,
you probably should. Realize that a biliear patch doesn't have these
nasty properties and works just as well for polygons with up to 3 or 4
vertices.