// 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, x, y, z, dx, dy;
float[][][] data;  
float  angleX=0;
float  angleY=0;
float  angleZ=0;
float  scaleZ=50;
float  theta=0.01;
int    mouseX_old;
int    mouseY_old;
int    i, j, xsize, ysize;

void setup() {
  size(600,600,P3D);
  smooth();
  fontA = loadFont("ComicSansMS-Bold-32.vlw");
  textAlign(CENTER);
  textFont(fontA, 8);
  w=2.;
  dx=dy=0.1;
  xsize=(int)(2*w/dx+1);
  ysize=(int)(2*w/dy+1);
  data = new float[xsize][ysize][3];
  for(i=0; i<xsize; i++) {
    for(j=0; j<ysize; j++) {
      x=i*dx-w;
      y=j*dy-w;
      z=ft(x, y);
      data[i][j][0]=x;
      data[i][j][1]=y;
      data[i][j][2]=z;
    }  
  }      
}

void draw() {
  background(200);
  pushMatrix();
    translate(width/2, height/2, 0);
    rotateX(angleY);
    rotateY(angleX);
    rotateZ(angleZ);
    scale(scaleZ);    
    drawBox();
    drawFt();
  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;
}

void keyPressed()
{
  if( key == 'Z' || key == 'z') {
    scaleZ*=1.25;
  } 
  else if( key == 'X' || key == 'x') {
    scaleZ/=1.25;
  }
}

float ft(float x, float y)
{
  return (x*x-x*y)/w-.5;
}

void drawFt()
{
  stroke(192,128,128);
  for(i=0; i<xsize; i++) {
    for(j=0; j<ysize; j++) {
      point(data[i][j][0],data[i][j][1],data[i][j][2]);
    }  
  }      
  for(i=0; i<xsize-1; i++) {
    for(j=0; j<ysize-1; j++) {
      line(data[i][j][0],data[i][j][1],data[i][j][2],data[i][j+1][0],data[i][j+1][1],data[i][j+1][2]);
      line(data[i][j][0],data[i][j][1],data[i][j][2],data[i+1][j][0],data[i+1][j][1],data[i+1][j][2]);
    }  
    line(data[i][j][0],data[i][j][1],data[i][j][2],data[i+1][j][0],data[i+1][j][1],data[i+1][j][2]);
  }      
  for(j=0; j<ysize-1; j++) {
    line(data[i][j][0],data[i][j][1],data[i][j][2],data[i][j+1][0],data[i][j+1][1],data[i][j+1][2]);
  }  
  stroke(192);
  for(i=0; i<xsize-1; i++) {
    for(j=0; j<ysize-1; j++) {
      line(data[i][j][0],data[i][j][1],0,data[i][j+1][0],data[i][j+1][1],0);
      line(data[i][j][0],data[i][j][1],0,data[i+1][j][0],data[i+1][j][1],0);
    }  
  }      
}

void drawBox()
{
  stroke(128);
  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);
}