/*
   This is a ray tracing program which works with spheres.
*/

#include <math.h>
#include <conio.h>      // getch
#define V_800x600x256
#define V_FILE
#include <graphx.h>

/*
       +---x
       |\
       | \
       y  z
*/

struct VECTOR {

   float x,y,z;

} E = { V_CENX, V_CENY, -550 };   // "E" for eye

#define SPHERES 4

struct SPHERE {

   float x,y,z,   // center
         r;       // radius

/*
for 320x200x256

} S[SPHERES] = { 159,  90, 100, 50,   // "S" for sphere
                  50,  30, 150, 45,
                 290, 170,  75, 60,
                  40, 180, 160, 50 };
*/
/* for 800x600x256 */

} S[SPHERES] = { 398, 270, 275, 138,   // "S" for sphere
                 125,  90, 413, 124,
                 725, 510, 206, 165,
                 100, 540, 440, 138 };
/* 640x480x16
} S[SPHERES] = { 340, 190, 220, 110,   // "S" for sphere
                 100,  72, 330,  99,
                 580, 408, 165, 132,
                  80, 432, 352, 110 };
*/

void SphereIntersection(VECTOR p1,VECTOR m,int s,float &d,VECTOR &p2,int &s2) {

   float A,B,C,t,d2,x,y,z;

   p1.x -= S[s].x;
   p1.y -= S[s].y;
   p1.z -= S[s].z;
   A = 2*(m.x*m.x + m.y*m.y + m.z*m.z);
   B = 2*(m.x*p1.x + m.y*p1.y + m.z*p1.z);
   C = B*B - 2*A*(p1.x*p1.x + p1.y*p1.y + p1.z*p1.z - S[s].r*S[s].r);
   if (C < 0) return;   // no intersection
   t = (-B - sqrt(C))/A;
   x = m.x*t;
   y = m.y*t;
   z = m.z*t;
   d2 = sqrt(x*x + y*y + z*z);
   if (t < 0 || (d2 >= d && d >= 0)) return;   // intersection farther away than currently found one
   p1.x += S[s].x;
   p1.y += S[s].y;
   p1.z += S[s].z;
   x += p1.x;
   y += p1.y;
   z += p1.z;
   p2.x = x;
   p2.y = y;
   p2.z = z;
   d = d2;
   s2 = s;
}

void ReflectRay(VECTOR &p1,VECTOR &m,int s,VECTOR p2) {

   float k;
   VECTOR TP1,CT,B;

   // T = p2     C = S[s].x,y,z

   CT.x = p2.x - S[s].x;
   CT.y = p2.y - S[s].y;
   CT.z = p2.z - S[s].z;

   TP1.x = p1.x - p2.x;
   TP1.y = p1.y - p2.y;
   TP1.z = p1.z - p2.z;

   k = (TP1.x*CT.x + TP1.y*CT.y + TP1.z*CT.z)/(CT.x*CT.x + CT.y*CT.y + CT.z*CT.z);

   B.x = k*CT.x + p2.x;
   B.y = k*CT.y + p2.y;
   B.z = k*CT.z + p2.z;

   m.x = 2*B.x - p1.x - p2.x;
   m.y = 2*B.y - p1.y - p2.y;
   m.z = 2*B.z - p1.z - p2.z;

   p1.x = p2.x;
   p1.y = p2.y;
   p1.z = p2.z;
}

int WhatColour(float x,float y) {

   VECTOR p1,   // point on line
          p2,
          m;   // direction vector
   float d;   // distance
   int s,s2,olds;   // sphere index

   p1.x = E.x;
   p1.y = E.y;
   p1.z = E.z;
   m.x = x - E.x;
   m.y = y - E.y;
   m.z = 0 - E.z;
   olds = -1;
   jump:
   d = -1;
   for (s = 0; s < SPHERES; s++) if (s != olds)
      SphereIntersection(p1,m,s,d,p2,s2);
   if (d > 0) {
      ReflectRay(p1,m,s2,p2);
      olds = s2;
      goto jump;
   }
   /*
      At this point, "m" and "p" contain a line pointing towards outer
      space.
   */
   if (m.y == 0) return 101;
   if (m.y > 0) {
      // k = 580
      p2.x = p1.x - (p1.y-580)*m.x/m.y;
      p2.z = p1.z - (p1.y-580)*m.z/m.y;
      s = ((long int)(p2.x/200)) % 2;
      if (p2.x < 0) s = !(-s);
      s2 = ((long int)(p2.z/200)) % 2;
      if (p2.z < 0) s2 = !(-s2);
      if (s == s2) return 164; else return 101;
   }
   else return (-63.0*m.y/sqrt(m.x*m.x+m.y*m.y+m.z*m.z) + 1);
   /*
   if (m.y > 0)   // pointing down
      return (63.0*m.y/sqrt(m.x*m.x+m.y*m.y+m.z*m.z) + 101);
   else return (-63.0*m.y/sqrt(m.x*m.x+m.y*m.y+m.z*m.z) + 1);
   */
   /*
   if (m.y == 0) return 0;
   if (m.y > 0) {
      // k = 580
      p2.x = p1.x - (p1.y-580)*m.x/m.y;
      p2.z = p1.z - (p1.y-580)*m.z/m.y;
      s = ((long int)(p2.x/200)) % 2;
      if (p2.x < 0) s = !(-s);
      s2 = ((long int)(p2.z/200)) % 2;
      if (p2.z < 0) s2 = !(-s2);
      if (s == s2) return 15; else return 0;
   }
   else {
      // k = -100
      p2.x = p1.x - (p1.y+100)*m.x/m.y;
      p2.z = p1.z - (p1.y+100)*m.z/m.y;
      s = ((long int)(p2.x/200)) % 2;
      if (p2.x < 0) s = !(-s);
      s2 = ((long int)(p2.z/200)) % 2;
      if (p2.z < 0) s2 = !(-s2);
      if (s == s2) return LIGHTBLUE; else return LIGHTCYAN;
   }
   */
}

void Rect(int x,int y,int c,int x2,int y2) {

   int a,b;

   for (a = x; a < x+x2; a++)
      for (b = y; b < y+y2; b++)
         PutPixel(a,b,c);
}

main() {

   int x,y;

   GoGraph();

   for (x = 1; x <= 64; x++)
      SetPalette(x,0,64-x,63);   // sky
   for (x = 101; x <= 164; x++)
      SetPalette(x,0,x-101,0);

   /*
   for (x = 0; x <= V_MAXX; x+=20)
      for (y = 0; y <= V_MAXY; y+=20)
         // PutPixel(x,y,WhatColour(x,y));
         Rect(x,y,WhatColour(x,y),19,19);
   */

   for (x = 0; x <= V_MAXX; x++)
      for (y = 0; y <= V_MAXY; y++)
         PutPixel(x,y,WhatColour(x,y));

   getch();
   getch();
   getch();

   save_screen("C:\\BCPP2\\GRAPHX.DAT\\RAYS.DAT");
}