From: T DOT Harte AT btinternet DOT com (Thomas Harte)
Newsgroups: comp.os.msdos.djgpp
Subject: Clipping lines on the third dimension in Allegro
Date: Mon, 30 Jun 1997 17:12:55 GMT
Organization: BT Internet 
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To: djgpp AT delorie DOT com
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	I wrote earlier about wanting a 3d lib which clipped polygons at the
line z=0. And I haven't received a reply yet, so I've been looking into doing it
myself and then using Allegro's polygon functions. I have yet to fix the u and v
co-ordinates (shouldn't take more than an hour, probably just a stupid mistype
somewhere, as x,y,z & c already work), but I thought I'd let you know because
I've seen a few people asking for code which does this. The reason I'm writing
now and not then is because this code :-

void line3d(BITMAP *bitmap, float x1, float y1, float z1, float x2, float y2,
float z2, int colour)
{
   V3D_f points[2];
   float ratio;

   points[0].x=x1;
   points[0].y=y1;
   points[0].z=z1;

   points[1].x=x2;
   points[1].y=y2;
   points[1].z=z2;

   if(!(points[0].z < 1 && points[1].z < 1))
     {
      if(points[0].z < 1)
        {
         ratio=(1-points[0].z)/(points[0].z-points[1].z);
         points[0].x = ((points[0].x-points[1].x) * ratio) + points[0].x;
         points[0].y = ((points[0].y-points[1].y) * ratio) + points[0].y;
         points[0].z = 1;
        }

      if(points[1].z < 1)
        {
         ratio=(1-points[1].z)/(points[1].z-points[0].z);
         points[1].x = ((points[1].x-points[0].x) * ratio) + points[1].x;
         points[1].y = ((points[1].y-points[0].y) * ratio) + points[1].y;
         points[1].z = 1;
        }

      persp_project_f(points[0].x,  points[0].y,  points[0].z,
                        &points[0].x, &points[0].y);

      persp_project_f(points[1].x,  points[1].y,  points[1].z,
                        &points[1].x, &points[1].y);

      line(bitmap, points[0].x, points[0].y, points[1].x, points[1].y, colour);
     }
}

	. . . which draws a line between two positions in 3d space, clipping at
z=1 does work, and I thought someone might want to use it. It's probably not
optimised for speed or anything, but it works. Could it be included as a new
function in the next Allegro?

	An important thing to remember is not to perspective project the points
before you pass them. Apart from the added z co-ordinate, this works exactly the
same as the regular line function in Allegro.

	Well, anyone who wants to - enjoy using this code! And come back later
(hopefully) for a polygon clipping one.

		-Thomas
http://www.softysoft.home.ml.org