[AD] Patch for the documentation of polygon_z_normal()

[Andrei Ellman <ae-a-alleg@xxxxxxxxxx>]

Attatched is a patch for the documentation of polygon_z_normal() in 
allegro._tx. What this does is to tell the user that if using the 
function for culling back-facing polygons, they must convert the 
co-ordinates into screen-space. I've also mentioned an alternative 
method for culling back-facing polygons, but I'm not sure if the docs 
for polygon_z_normal() are the right place to mention this, as this 
alternative method does not use polygon_z_normal().


AE.

--- allegro._tx.old	2005-05-19 16:07:30.908598400 +0200
+++ allegro._tx	2005-06-05 21:30:37.318049600 +0200
@@ -10794,7 +10794,13 @@
    to the viewer, therefore they never need to be drawn. This can cull on 
    average half the polygons from a scene. If the normal is negative the 
    polygon can safely be culled. If it is zero, the polygon is perpendicular 
-   to the screen.
+   to the screen. However, this method of culling back-faces should only be 
+   used once the X and Y coordinates have been projected into screen-space 
+   using persp_project() (or if an isometric-projection is being used). In 
+   order to cull back-faces before projection into screen-space, find the 
+   dot-product of a vector from the camera to a point on the polygon with 
+   the polygon's normal, and if the sign of the result is positive, the 
+   polygon can be culled.
 
 @@void @apply_matrix(const MATRIX *m, fixed x, y, z, *xout, *yout, *zout);
 @@void @apply_matrix_f(const MATRIX_f *m, float x, y, z, *xout, *yout, *zout);