Cara Membuat Proyeksi Bayangan Gambar Teko dengan OpenGL

Syntax :
#include <math.h>
#include <stdio.h>
#include <GL/glut.h>

double rx = 0.0;
double ry = 0.0;

float l[] = { 0.0,  80.0, 0.0 }; // koordinat sumber cahaya
float n[] = { 0.0,  -1.0, 0.0 };
float e[] = { 0.0, -60.0, 0.0 };

void help();

// obyek yang akan digambar
void draw()
{
glutSolidTeapot(30.0);
}

//membuat proyeksi bayangan
void glShadowProjection(float * l, float * e, float * n)
{
  float d, c;
  float mat[16];

  d = n[0]*l[0] + n[1]*l[1] + n[2]*l[2];
  c = e[0]*n[0] + e[1]*n[1] + e[2]*n[2] - d;

  mat[0]  = l[0]*n[0]+c;            // membuat matrik. OpenGL menggunakan kolom matrik
  mat[4]  = n[1]*l[0];
  mat[8]  = n[2]*l[0];
  mat[12] = -l[0]*c-l[0]*d;
 
  mat[1]  = n[0]*l[1];       
  mat[5]  = l[1]*n[1]+c;
  mat[9]  = n[2]*l[1];
  mat[13] = -l[1]*c-l[1]*d;
 
  mat[2]  = n[0]*l[2];       
  mat[6]  = n[1]*l[2];
  mat[10] = l[2]*n[2]+c;
  mat[14] = -l[2]*c-l[2]*d;
 
  mat[3]  = n[0];       
  mat[7]  = n[1];
  mat[11] = n[2];
  mat[15] = -d;
 
  glMultMatrixf(mat);              // kalikan matrik
}

void render()
{
  glClearColor(0.0,0.6,0.5,0.0);
  glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

  glLightfv(GL_LIGHT0, GL_POSITION, l);
  glDisable(GL_CULL_FACE);
  glDisable(GL_LIGHTING);
  glColor3f(1.0,1.0,1.0);
 
  glBegin(GL_POINTS);
     glVertex3f(l[0],l[1],l[2]);
  glEnd();
 
  glColor3f(0.8,0.8,0.8);
  glBegin(GL_QUADS);
     glNormal3f(0.0,1.0,0.0);
     glVertex3f(-1300.0,e[1]-0.1, 1300.0);
     glVertex3f( 1300.0,e[1]-0.1, 1300.0);
     glVertex3f( 1300.0,e[1]-0.1,-1300.0);
     glVertex3f(-1300.0,e[1]-0.1,-1300.0);
  glEnd();

  // gambar bayangan
  glPushMatrix();
  glRotatef(ry,0,1,0);
  glRotatef(rx,1,0,0);
  glEnable(GL_LIGHTING);
  glColor3f(0.0,0.0,0.8);
  draw();
  glPopMatrix();
 
  //sekarang  gambar bayangan yang muncul
  glPushMatrix();
  glShadowProjection(l,e,n); 
  glRotatef(ry,0,1,0);
  glRotatef(rx,1,0,0);
  glDisable(GL_LIGHTING);
  glColor3f(0.4,0.4,0.4);
 
  draw();
  glPopMatrix();
  glutSwapBuffers();
}


void keypress(unsigned char c, int a, int b)
{
  if ( c==27 ) exit(0);
  else if ( c=='s' ) l[1]-=5.0;
  else if ( c=='w' ) l[1]+=5.0;
  else if ( c=='a' ) l[0]-=5.0;
  else if ( c=='d' ) l[0]+=5.0;
  else if ( c=='q' ) l[2]-=5.0;
  else if ( c=='e' ) l[2]+=5.0;
  else if ( c=='h' ) help();
}

void help()
{
  printf("proyeksi contoh bayangan sebuah obyek teapot\n");
}

void idle()
{
  rx+=0.4;
  ry+=0.7;
  render();
}

void resize(int w, int h)
{
  glViewport(0, 0, w, h);
}

int main(int argc, char * argv[])
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
  glutCreateWindow("proyeksi bayangan");
  glutReshapeFunc(resize);
  glutReshapeWindow(400,400);
  glutKeyboardFunc(keypress);
  glutDisplayFunc(render);
  glutIdleFunc(idle);

  glEnable(GL_NORMALIZE);
  glEnable(GL_LIGHTING);
  glEnable(GL_COLOR_MATERIAL);
  glEnable(GL_DEPTH_TEST);
  glEnable(GL_LIGHT0);
  glEnable(GL_TEXTURE_2D);
  glMatrixMode(GL_PROJECTION);

  glLoadIdentity();
  gluPerspective(60.0f, 1.0, 1.0, 400.0);

  // Reset koordinat  sebelum dimodifikasi/diubah
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glTranslatef(0.0, 0.0, -150.0);

  glutMainLoop();
  return 0;
}

Run :