/* shadow.c */

/*
 * Use the stencil buffer to generate shadows.  This demo inspired by
 * an article in IRIS Universe Magazine, No. 18.  This program is in
 * the public domain.
 *
 * Brian Paul
 */

#include <windows.h>

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glaux.h>

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

float light_pos[] = { 0.0F,10.0F,0.0F,1.0F } ;
float view_rotx ;
float view_roty ;
float ground_xrot ;
float ground_yrot ;
float ground_zrot ;
float ground_y ;
int num_tri = 4 ;
float triangle[] = {
    0.5F,4.0F,0.5F,-0.5F,4.0F,0.5F,0.0F,4.0F,2.0F,0.5F,4.0F,0.5F,0.5F,4.0F,-0.5F,2.0F,4.0F,0.0F,
    0.5F,4.0F,-0.5F,-0.5F,4.0F,-0.5F,0.0F,4.0F,-2.0F,-0.5F,4.0F,-0.5F,-0.5F,4.0F,0.5F,-2.0F,4.0F,0.0F };

void draw_shadow_volumes() {
   int tri;
   int i,j;
   float v0[3] ;
   float v1[3] ;
   float p0[3] ;
   float p1[3] ;
   float p2[3] ;
   float p3[3] ;
   for (tri=0;tri<num_tri;tri++) {
     for (i=0;i<3;i++) {
       j = (i+1) % 3;
     v0[0] = triangle[tri*9+i*3+0] - light_pos[0];
     v0[1] = triangle[tri*9+i*3+1] - light_pos[1];
     v0[2] = triangle[tri*9+i*3+2] - light_pos[2];
     v1[0] = triangle[tri*9+j*3+0] - light_pos[0];
     v1[1] = triangle[tri*9+j*3+1] - light_pos[1];
     v1[2] = triangle[tri*9+j*3+2] - light_pos[2];
     p0[0] = triangle[tri*9+i*3+0];
     p0[1] = triangle[tri*9+i*3+1];
     p0[2] = triangle[tri*9+i*3+2];
     p1[0] = triangle[tri*9+j*3+0];
     p1[1] = triangle[tri*9+j*3+1];
     p1[2] = triangle[tri*9+j*3+2];
     p2[0] = p1[0] + v1[0] * 10.0F;
     p2[1] = p1[1] + v1[1] * 10.0F;
     p2[2] = p1[2] + v1[2] * 10.0F;
     p3[0] = p0[0] + v0[0] * 10.0F;
     p3[1] = p0[1] + v0[1] * 10.0F;
     p3[2] = p0[2] + v0[2] * 10.0F;
     glBegin(GL_POLYGON);
     glVertex3fv(p0);
     glVertex3fv(p1);
     glVertex3fv(p2);
     glVertex3fv(p3);
     glEnd(); } }
  }

void drawScene() {
    int ground = -1;
    int i,j;
    float x,y,z;
    glPushMatrix();
    glRotatef(ground_xrot,1.0F,0.0F,0.0F);
    glRotatef(ground_yrot,0.0F,1.0F,0.0F);
    glRotatef(ground_zrot,0.0F,0.0F,1.0F);
    glTranslatef(0.0F,ground_y,0.0F);
    if ( ground == -1 ) {
      ground = glGenLists(1);
      glNewList(ground,GL_COMPILE_AND_EXECUTE);
      for ( i=0 ; i<5 ; i++ ) {
        for ( j=0 ; j<5 ; j++ ) {
         if ( (i+j)%2 != 0 ) {
          glColor3f(0.0F,1.0F,0.0F); }
          else {
          glColor3f(0.0F,0.0F,1.0F); }
        glNormal3f(0.0F,1.0F,0.0F);
        x =(float) (i*2.0 - 5.0);
        y = 0.0F;
        z =(float) (j*2.0 - 5.0);
        glBegin(GL_POLYGON);
        glVertex3f(x,y,z+2.0F);
        glVertex3f(x+2.0F,y,z+2.0F);
        glVertex3f(x+2.0F,y,z);
        glVertex3f(x,y,z);
        glEnd(); } }
      glEndList(); }
      else {
      glCallList(ground); }
    glPopMatrix();
    glColor3f(1.0F,0.0F,0.0F);
    glNormal3f(0.0F,1.0F,0.0F);
    glBegin(GL_TRIANGLES);
    for (i=0;i<num_tri;i++) {
      glVertex3f(triangle[(i*3)*3],triangle[(i*3)*3+1],triangle[(i*3)*3+2]);
      glVertex3f(triangle[(i*3+1)*3],triangle[(i*3+1)*3+1],triangle[(i*3+1)*3+2]);
      glVertex3f(triangle[(i*3+2)*3],triangle[(i*3+2)*3+1],triangle[(i*3+2)*3+2]); }
    glEnd();
  } 

void CALLBACK display() {
    glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
    glPushMatrix();
    glRotatef(view_rotx,1.0F,0.0F,0.0F);
    glRotatef(view_roty,0.0F,0.0F,1.0F);
    glLightfv(GL_LIGHT0,GL_POSITION,light_pos);
    glEnable(GL_LIGHTING);
    glDisable(GL_LIGHT0);
    glColorMaterial(GL_FRONT_AND_BACK,GL_AMBIENT);
    glEnable(GL_COLOR_MATERIAL);
    drawScene();
    glDisable(GL_LIGHTING);
    glStencilFunc(GL_ALWAYS,0,0xffffffff);
    glStencilOp(GL_KEEP,GL_KEEP,GL_INVERT);
    glEnable(GL_STENCIL_TEST);
    glColorMask(GL_FALSE,GL_FALSE,GL_FALSE,GL_FALSE);
    glDepthMask(GL_FALSE);
    draw_shadow_volumes();
    glColorMask(GL_TRUE,GL_TRUE,GL_TRUE,GL_TRUE);
    glDepthMask(GL_TRUE);
    glEnable(GL_LIGHT0);
    glEnable(GL_LIGHTING);
    glColorMaterial(GL_FRONT_AND_BACK,GL_AMBIENT_AND_DIFFUSE);
    glEnable(GL_COLOR_MATERIAL);
    glStencilFunc(GL_EQUAL,0,0xffffffff);
    glStencilOp(GL_KEEP,GL_KEEP,GL_KEEP);
    drawScene();
    glDisable(GL_STENCIL_TEST);
    glDisable(GL_LIGHT0);
    glPopMatrix();
    glFlush();
    auxSwapBuffers();
  }

void CALLBACK idle() {
    ground_yrot -= 2.0F;
    ground_xrot =(float) (sin(ground_yrot*0.1) * 10.0);
    ground_zrot =(float) (cos(ground_yrot*0.1) * 10.0);
    ground_y =(float) (sin(ground_yrot*0.15) * 2.0);
    display();
  }

void initialise() {
    view_rotx = 50.0F;
    view_roty = 20.0F;
    float ambient[] = {0.3F,0.3F,0.3F,1.0F};
    ground_xrot = ground_yrot = ground_zrot = 0.0F;
    ground_y = 0.0F;
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
    glShadeModel(GL_FLAT);
    glLightModelfv(GL_LIGHT_MODEL_AMBIENT,ambient);
    glLightfv(GL_LIGHT0,GL_AMBIENT,ambient);
    }

void CALLBACK reshape(int width,int height) {
    glViewport(0,0,width,height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-1.0,1.0,-1.0,1.0,5.0,50.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0.0F,0.0F,-35.0F);
  }

void main(void) {
  auxInitDisplayMode(AUX_DOUBLE|AUX_RGBA|AUX_DEPTH);
  auxInitPosition (0,0,300,300);
  auxInitWindow("Shadows") ;
  initialise();
  auxReshapeFunc(reshape);
  auxIdleFunc(idle);
  auxMainLoop(display);
}