COIL.EXE v2.0  7/25/92 W. D. Kirby

Description:

   COIL.EXE is a simple utility to create an endless coil type object for the 
Persistance of Vision (PoV-Ray v1.0) raytracer.  It is set up to create coiled 
strings of spheres that twist about a circle a number of times and 
terminate at the point of origin. It generates a data file to be called 
from the users main PoV input script file.

Files included:

   COIL.EXE       DOS executable program 
   COIL.C         C source code for coil.exe
   COIL.INC       Example generated data file for coil object
   COIL.POV       Example PoV v1.0 main script file that calls COIL.INC
   COIL.GIF       GIF image of example coils (640x480x256)
   COIL.PRJ       BC++ v3.1 project file
   COIL.MAK       BC++ v3.1 make file
   Coil.DOC       This file

Usage:

   The COIL program is easy to use since it is interactive. Just type COIL 
at the DOS prompt, and enter the parameters required, and most of the 
questions are straight forward.  The parameters requested are briefly defined 
as follows with defaults show in brackets:

Output filname [coil.inc]:   enter the output file to be created- note this 
   file does not contain all of the PoV inputs required. 

Union name [coil]:   name of object union to be used to identify the created 
   object

Number of spheres [100]:  number of spheres to be used in total

Number of spheres in cross section [2]:  number of spheres to be contained 
   within the cross section of an imaginary torus about the center position. 
   See illustration below. Spheres are equally separated in minor angle.

Number of twists [2]:  number of twists the coil is to make within a single 
   revolution about the center position (i.e., major radius).

Major radius [1.0]:  major radius of coil (i.e., center of torus).

Minor radius [0.25]:  minor radius of imaginary torus of sphere center 
   positions.

Sphere radius [0.25]:  radius of the individual spheres that make up coil.

Overall scale factor [1.0]:  scale factor for all x,y,z positions, but not 
   the radius of the individual spheres.


                                           + <= 1st sphere center
           object                        /
           center                      /  minor radius and angle
              + -------------------->+ -------------------
                major radius       /
                                 /
                               + <= 2nd sphere center

         Illustration of two spheres in cross section

Source code:

COIL was created using Borland C++ version 3.1, and the compiled program 
(COIL.EXE) provided will work with an 8088 without a coprocessor or better. 
The COIL output has been tested with the Persistance of Vision (PoV) raytracer 
version 1.0. Testing has included objects containing over 4000 spheres. The 
example PoV input file (coil.pov & coil.inc ) for a 320x200 image with 
100 spheres and a plane requires 2 min 19 sec on a 486/33Mhz using 
the 386 protected mode version of PoV-Ray v1.0.

Fragment of code for generation of sphere center positions

/* Compute twisted coil object */

    for(i=0;i<steps;++i){      /* loop on number of spheres in object */
      angle1 = 2 * PI * Ntube * (double)i/ (double)steps; /* major angle */
      x1 = cos( angle1 );
      y1 = sin( angle1 );
      angle2 = (double)( Ntwist + 1.0/Ntube) * angle1; /* minor angle */
      x2 = cos( angle2 );
      z2 = sin( angle2);
      xpos = k * ((Rad1 * x1) + (rad2 * x2 * x1));   \
      ypos = k * ((Rad1 * y1) + (rad2 * x2 * y1));    > sphere center position
      zpos = k * rad2 * z2;                          /
      write_piece(xpos,ypos,zpos,radius);   /* sphere position and radius */
    }

   Besides computing sphere positions the computes bounds for the generated 
coil using the min and max coil shape x,y,z extent plus a 1% fudge factor to 
insure it encompasses the whole coil. The code is true brute force with 
little optimization.  This is justified since the code executes so quickly.

Enjoy

Bill Kirby



Bill Kirby        CIS [70711,2407], COMART forum
3527 Cornell Road
Fairfax, VA 22030

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