[Radiance-general] Rendering large space with small detail (small picture now attached!)

[Rob Guglielmetti]

Great follow-ups Jack. A couple of points within...

On Thu, May 28, 2009 7:50 am, Jack de Valpine wrote:

> If this is the case and all the geometry is black, then you can
> do
>
> xform -I original_geometry.rad > original_geometry.flipped.rad
>
> This will flip the orientation of all the normals in
> original_geometry.rad.

Cool, all these years I never noticed that option to xform. The cool thing
about Radiance is you never will know everything about it.

What we really need is some kind of program to re-orient normals based on
some point. I thought I had an old AutoLISP routine that did that, but
man, that is long since gone from my backups (both the DVD kind and the
grey matter kind)!

> In Autocad (for example) 3dfaces that
> are built in a clockwise manner will have the normal pointing out of
> the clockwise face (I hope I am remembering this correctly).

It's hard to tell from your description if you are or not. Think of it
this way: let's say you are drawing a polygon that represents the face
(screen)  of the computer monitor you are now staring at. If you draw the
points in a clockwise fashion, the surface normal will be pointing away
from you (out the back of the monitor). If you define it with points in a
COUNTER-clockwise fashion, the polygon's surface normal will be pointing
at you. IOW, in the first example, with a glow applied, the monitor face
would render black. In the latter example, the monitor face would glow.

This can be reinforced by a permutation of the so-called "right hand
rule". Take your right hand and point at your face with your thumb, and
then curve your fingers around the axis defined by your thumb; if you
imagine your fingers pointing in the direction of rotation (and direction
of point creation), your thumb describes the surface normal (vector).

===================
 Rob Guglielmetti
www.rumblestrip.org