/********************************************************************
Program fog.cpp
Adapted from the example program in Chapter 6 of the 
OpenGL Programming Guide for OpenGL 2.1

by H Masterman  April 2009

Changes made:  perspective projection
				added some comments
				added the ability to change the density interactively

********************************************************************/


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

static GLint fogMode;
float density = 0.3;

static void init(void)
{
   GLfloat position[] = { 0.5, 0.5, 3.0, 0.0 };  //for locating the light

   glEnable(GL_DEPTH_TEST);

   glLightfv(GL_LIGHT0, GL_POSITION, position);  //position the light
   glEnable(GL_LIGHTING);
   glEnable(GL_LIGHT0);
   {
      GLfloat mat[3] = {0.1745, 0.01175, 0.01175};      //set the material properties for the spheres
      glMaterialfv (GL_FRONT, GL_AMBIENT, mat);
      mat[0] = 0.61424; mat[1] = 0.04136; mat[2] = 0.04136;     
      glMaterialfv (GL_FRONT, GL_DIFFUSE, mat);
      mat[0] = 0.727811; mat[1] = 0.626959; mat[2] = 0.626959;
      glMaterialfv (GL_FRONT, GL_SPECULAR, mat);
      glMaterialf (GL_FRONT, GL_SHININESS, 0.6*128.0);
   }

   glEnable(GL_FOG);  //fog must always be enabled
   {
      GLfloat fogColor[4] = {0.5, 0.5, 0.5, 1.0};  

      fogMode = GL_EXP;  //initial fog mode
      glFogi (GL_FOG_MODE, fogMode);
      glFogfv (GL_FOG_COLOR, fogColor);  //set the fog color to grey
      glFogf (GL_FOG_DENSITY, density);   //set the fog density
      glHint (GL_FOG_HINT, GL_NICEST);  //nicest possible rendering
      glFogf (GL_FOG_START, 2.0);  //start distance for linear fog
      glFogf (GL_FOG_END, 6.0);  //end distance for linear fog
   }
   glClearColor(0.5, 0.5, 0.5, 1.0);  // set the background color = fog color
}

static void renderSphere (GLfloat x, GLfloat y, GLfloat z)  //render a sphere at x,y,z
{
   glPushMatrix();
   glTranslatef (x, y, z);
   glutSolidSphere(0.4, 32, 32);
   glPopMatrix();
}

/* display() draws 5 spheres at different z positions.
 */
void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   renderSphere (-2., -0.5, -2.0);
   renderSphere (-1., -0.5, -3.0);
   renderSphere (0., -0.5, -4.0);
   renderSphere (1., -0.5, -5.0);
   renderSphere (2., -0.5, -6.0);
   glFlush();
}

void reshape(int w, int h)
{
   glViewport(0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   /*
   if (w <= h)
      glOrtho (-2.5, 2.5, -2.5*(GLfloat)h/(GLfloat)w,
         2.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);
   else
      glOrtho (-2.5*(GLfloat)w/(GLfloat)h,
         2.5*(GLfloat)w/(GLfloat)h, -2.5, 2.5, -10.0, 10.0);
		 */
   gluPerspective(75, 1.0,1.0,20.0);

   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity ();
}

void keyboard(unsigned char key, int x, int y)  //user can toggle the fog mode
{
   switch (key) {
      case 'f':
      case 'F':
         if (fogMode == GL_EXP) {
            fogMode = GL_EXP2;
            printf ("Fog mode is GL_EXP2\n");
         }
         else if (fogMode == GL_EXP2) {
            fogMode = GL_LINEAR;
            printf ("Fog mode is GL_LINEAR\n");
         }
         else if (fogMode == GL_LINEAR) {
            fogMode = GL_EXP;
            printf ("Fog mode is GL_EXP\n");
         }
         glFogi (GL_FOG_MODE, fogMode);
         glutPostRedisplay();
         break;

	  case 'D':  //increase the density
		  density += 0.1;
		  glFogf (GL_FOG_DENSITY, density);   //set the fog density
		  glutPostRedisplay();
		  printf("Density = %2.1f\n",density);
		  break;
	  case 'd':  //decrease the density
		  density -= 0.1;
		  printf("Density = %2.1f\n",density);
		  glFogf (GL_FOG_DENSITY, density);   //set the fog density
		  glutPostRedisplay();
		  break;
      case 27:
         exit(0);
         break;
      default:
         break;
   }
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
   glutInitWindowSize(700, 700);
   glutCreateWindow(argv[0]);
   init();
   glutReshapeFunc (reshape);
   glutKeyboardFunc (keyboard);
   glutDisplayFunc (display);
   glutMainLoop();
   return 0;
}