[Radiance-general] Radiance-general Digest, Vol 142, Issue 5

[Thomas Bleicher]

Per

Basically your command traces rays from the view point (inside the box) to
the surface of the box. Where it intersects with the polygon it creates a
new ray to render everything on the other side of the box with rtrace.

When you use boxes as cut-away shapes for this technique you have to think
from "inside the box". I assume that your clipping boxes are very large,
both enclose the view point and the top and front of the building
respectively. In this case you would see from the inside viewpoint the
following surfaces (top to bottom in an imaginary image):

a) The inside of the box cutting away the front of the building (green
below)
b) The eave of the building (blue below)
c) The bottom of the box cutting away the top of the building (red below)

A bit of ascii art to confuse things:

   +--------------------------------+
   |                                | top cutting box
+-------------------------+         |
|  | * viewpoint          |         |
|  |                      |         |
|  |                      |         |
|  |                      |         |
|  |                      |         |
|  |                      |         |
|  |                      | b       |
|  |                      b   b     |
|  |                    b |     b   |
|  +------------------b-----------b-+
|                     b   |       b
|                     b   |       b
|                     b   |       b
|                         |
+-------------------------+
  front cutting box


Your command will use the first ray intersection with the boxes to generate
the cutting edges: the green and red lines. Everything _behind_ these
cutting planes will be rendered. Therefore the only thing you cut out of
the image is the eave of the building.

To create the effect you want with your current command create a single box
that encloses the building core (the part you want to see). Think of it
more of a "gift wrap" than a "clipping box": It encloses the parts you
want, not the part you don't want.

Hth,
Thomas
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