Re: [AD] Patch for the documentation of polygon_z_normal()

[Elias Pschernig <elias@xxxxxxxxxx>]

On Sun, 2005-06-05 at 21:44 +0200, Andrei Ellman wrote:
> 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().
> 

I think it's a good place. Else you could move it to the beginning of
the chapter..

And while you're add it, maybe add a note that the function will fail if
the three vectors lie on one line.

+   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 

I'd replace "should" by "can" and "isometric" by "orthographic".

+   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.

Maybe also add a short example code how to get the normal of a triangle.
I mean, if someone doesn't know the above information already, then that
person certainly also doesn't know how to get the normal in the first
place.

-- 
Elias Pschernig <elias@xxxxxxxxxx>