// A new variable, for gravity (i.e. acceleration).   
// We use a relatively small number (0.1) because this accelerations accumulates over time, increasing the speed.   
// Try changing this number to 2.0 and see what happens.
PFont  fontA;
float  w=100;
float  angleX=0;
float  angleY=0;
float  angleZ=0;
float  theta=0.01;
int    mouseX_old;
int    mouseY_old;
int    i, j;

void setup() {
  size(600,600,P3D);
  smooth();
  fontA = loadFont("ComicSansMS-Bold-32.vlw");
  textAlign(CENTER);
  textFont(fontA, 8);
}

void draw() {
  background(200);
  fill(0);
  rectMode(CENTER);

  pushMatrix();
  translate(width/2, height/2, 0);
  rotateX(angleY);
  rotateY(angleX);
  rotateZ(angleZ);
  scale(1);
  
  stroke(200,100,100);
  for(i=(int)-w; i<w; i++) {
    for(j=(int)-w; j<w; j++) {
      point(i,j,ft(i,j));
//      line(i,j,0,i,j,ft(i,j));
    }  
  }    
  drawBox();
  popMatrix();
}

void mouseDragged() 
{
  angleX=2.*PI*(mouseX-mouseX_old)/width;
  angleY=-2.*PI*(mouseY-mouseY_old)/height;
}

void mousePressed() {
  mouseX_old=mouseX;
  mouseY_old=mouseY;
}

float ft(float x, float y)
{
  return (x*x-x*y)/w-2.0;
}

void drawBox()
{
  stroke(128);

  text("O", 0,0,0); 
  text("x", w,0,0); 
  text("y", 0,w,0); 
  text("z", 0,0,w); 

  line(-w,0,0,w,0,0);
  line(0,-w,0,0,w,0);
  line(0,0,0,0,0,w);

  line(-w,-w,0,w,-w,0);
  line(w,-w,0,w,w,0);
  line(w,w,0,-w,w,0);
  line(-w,w,0,-w,-w,0);
  
  line(-w,-w,w,w,-w,w);
  line(w,-w,w,w,w,w);
  line(w,w,w,-w,w,w);
  line(-w,w,w,-w,-w,w);

  line(-w,-w,0,-w,-w,w);
  line(w,-w,0,w,-w,w);
  line(w,w,0,w,w,w);
  line(-w,w,0,-w,w,w);
}