/*****************************
Example Program to illustrate
  1 - creating simple shadows
  2 - querying the system state

A simple triangle and its shadow are drawn.
A light is at location 5,20,0
******************************/

#include <GL/glut.h>
#include <stdlib.h>

/*******************************************************************/
void print_matrix()
{
   GLfloat mv[16];  //matrix vector
   int r,c;

   glGetFloatv(GL_MODELVIEW_MATRIX,mv);
   for (c=0;c<4;c++)
     {
      for (r=0;r<4;r++)
         printf("%5.2f ",mv[c+r*4]);
      printf("\n");
     }
   printf("\n\n");
}

/*******************************************************************/
void display(void)
{
   GLfloat shadow_matrix[16];
   int i;

   /**** build the shadow matrix *****/
   for (i=0;i<16;i++)
      shadow_matrix[i] = 0.0;
   shadow_matrix[0] = shadow_matrix[5] = shadow_matrix[10] = 1.0;
   shadow_matrix[7] = -1.0/20;

   /***** clear the screen ******/
   glClearColor (0.0, 0.0, 0.0, 0.0);  // clear to black
   glClear(GL_COLOR_BUFFER_BIT);

   /**** draw a triangle in white ****/
   glColor3f(1.0,1.0,1.0);
   glBegin(GL_POLYGON);
     glVertex3i(10,14,-10);
     glVertex3i(-10,14,-10);
     glVertex3i(0,9,-5);
   glEnd();

   /***** reset the Modelview Matrix to create shadow ****/
   glTranslatef(5,20,0);         // step 3: move light back to original position
   print_matrix();
   /*** when debuging it was useful to tilt the shadow
   glRotatef(45,1,0,0);
   ****/
   glMultMatrixf(shadow_matrix); // step 2: add the shadow effect matrix
   print_matrix();
   glTranslatef(-5,-20,0);       // step 1: move the light to the origin
   print_matrix();

   /***** redraw the triangle in a shadow color *****/
   glColor3f(0.3,0.3,0.3);       
   glBegin(GL_POLYGON);
     glVertex3i(10,14,-10);
     glVertex3i(-10,14,-10);
     glVertex3i(0,9,-5);
   glEnd();

   glFlush();
}

/********************************************************/
void reshape(int w, int h)
{
   glViewport (0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glFrustum(-15.0, 15.0, -1.0, 15.0, 5.0, 80.0);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
}
/********************************************************/
 
int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode (GLUT_RGB);
   glutInitWindowSize (450, 450); 
   glutInitWindowPosition (100, 100);
   glutCreateWindow (argv[0]);
   glutDisplayFunc(display); 
   glutReshapeFunc(reshape); 
   glutMainLoop();
   return 0;  
}