// here I will show you the different types of fog
// they are GL_EXP, GL_EXP2 and GL_LINEAR

// You would use a different one depending on how 
// you would like the area to look. It is more a trial 
// and error thing, to choose which you want.

// In this tutorial I am using the keys, a,s and d to change
// the fog type on demand with the line:
// glFogi (GL_FOG_MODE, GL_EXP);
// which simply changes the GL_FOG_MODE or in English, the fog mode.

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

GLfloat angle = 0.0;

GLfloat density = 0.3;
GLfloat fogColor[4] = {0.5, 0.5, 0.5, 1.0};

void cube (void) {
	glRotatef(angle, 1.0, 0.0, 0.0);
	glRotatef(angle, 0.0, 1.0, 0.0);
	glRotatef(angle, 0.0, 0.0, 1.0);
	glColor3f(1.0, 0.0, 0.0);
	glutSolidCube(2);
}

void init (void) {
	glEnable (GL_DEPTH_TEST);
	glEnable (GL_FOG);
    glFogi (GL_FOG_MODE, GL_LINEAR);
    glFogfv (GL_FOG_COLOR, fogColor);
    glFogf (GL_FOG_DENSITY, density);
    glHint (GL_FOG_HINT, GL_NICEST);
}

void display (void) {
	glClearColor (0.0,0.0,0.0,1.0);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();  
	gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
	cube();
	glutSwapBuffers();
	angle ++;
}

void reshape (int w, int h) {
	glViewport (0, 0, (GLsizei)w, (GLsizei)h);
	glMatrixMode (GL_PROJECTION);
	glLoadIdentity ();
	gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0);
	glMatrixMode (GL_MODELVIEW);
}

void keyboard (unsigned char key, int x, int y) {
	if (key=='a') {
	glFogi (GL_FOG_MODE, GL_EXP);
	}
	if (key=='s') {
	glFogi (GL_FOG_MODE, GL_EXP2);
	}
	if (key=='d') {
	glFogi (GL_FOG_MODE, GL_LINEAR);
	}
}

int main (int argc, char **argv) {
    glutInit (&argc, argv);
	glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
	glutInitWindowSize (500, 500);
	glutInitWindowPosition (100, 100);
    glutCreateWindow ("A basic OpenGL Window");
	init ();
    glutDisplayFunc (display);
	glutIdleFunc (display);
	glutReshapeFunc (reshape);
	glutKeyboardFunc (keyboard);
    glutMainLoop ();
    return 0;
}