/* walk.c                                          */
/* walk "eye" x, X, y, Y, z, Z */

/* We use the Lookat function in the display callback to point
   the eye, whose position can be altered by the
   x,X,y,Y,z, and Z keys.
   The perspective view is set in the reshape callback */
   
#include <stdlib.h>
#include <GL/glut.h>

static GLfloat nearv=2.0;
static GLfloat farv=50.0;
/* initial eye location */
static GLdouble eye[]= {0.0, 0.0, 10.0}; 

void display(void)
{
  char text[]="move eye using x, X, y, Y, z, Z";
  char *p;
  
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  /* Update eye position in modelview matrix */
  glLoadIdentity();
  /* from eye to center of scene */
  gluLookAt(eye[0],eye[1],eye[2],
            0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
  glColor3f(1.0, 0.0, 0.0);
  glTranslatef(-1.0, -1.0, 0.0);
  glutWireSphere(1.0, 12, 12);
  glColor3f(0.0, 1.0, 1.0);
  glTranslatef(2.0, 0.0, 0.0);
  glutWireSphere(1.0, 12, 12);
  glColor3f(0.0, 1.0, 0.0);
  glTranslatef(0.0, 2.0, 0.0);
  glutWireSphere(1.0, 12, 12);
  glColor3f(0.0, 0.0, 1.0);
  glTranslatef(-2.0, 0.0, 0.0);
  glutWireSphere(1.0, 12, 12);
  glColor3f(1.0, 1.0, 1.0);
  glTranslatef(-7.0, -7.0, 0.0);
  glScalef(0.005, 0.005, 1.0);
  for(p=text; *p; p++)
    glutStrokeCharacter(GLUT_STROKE_ROMAN, *p);

  glFlush();
  glutSwapBuffers();
}

void keys(unsigned char key, int x, int y)
{
  /* Use x, X, y, Y, z, and Z keys to move eye */
  if(key == 'x') eye[0]-= 1.0;
  if(key == 'X') eye[0]+= 1.0;
  if(key == 'y') eye[1]-= 1.0;
  if(key == 'Y') eye[1]+= 1.0;
  if(key == 'z') {eye[2]-= 1.0; if(eye[2]<nearv) eye[2]=nearv;}
  if(key == 'Z') {eye[2]+= 1.0; if(eye[2]>farv)  eye[2]=farv;}
  display();
}

void myReshape(int w, int h)
{
  glViewport(0, 0, w, h);
  /* Use a perspective view */
  glMatrixMode(GL_PROJECTION); 
  glLoadIdentity();
  if(w<=h)
    glFrustum(-2.0, 2.0, -2.0 * (GLfloat) h/ (GLfloat) w,
              2.0* (GLfloat) h / (GLfloat) w, nearv, farv);
  else
    glFrustum(-2.0, 2.0, -2.0 * (GLfloat) w/ (GLfloat) h, 
              2.0* (GLfloat) w / (GLfloat) h, nearv, farv);
  /* Or we can use gluPerspective */
  /* gluPerspective(45.0, w/h, -10.0, 10.0); */
  glMatrixMode(GL_MODELVIEW);
}

int main(int argc, char *argv[])
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
  glutInitWindowSize(500, 500);
  glutCreateWindow(argv[0]);
  glutReshapeFunc(myReshape);
  glutDisplayFunc(display);
  glutKeyboardFunc(keys);
  glEnable(GL_DEPTH_TEST);
  glutMainLoop();
  return 0;
}