/**
  3D Surface from rotating a square function <br/>
  by <a href="http://www.local-guru.net/blog">guru</a>
*/

void setup() {
  size(300,300,P3D );
  frameRate(25);
}

float a =0;
void draw() {
  a+= 0.1;
  background(0);
  lights();
  translate( width/2, height/2 );
  rotateY(a);
  rotateX(a/2);
  fill(255,0,0);
  noStroke();
  beginShape(QUAD_STRIP);
  for( int j =-60; j < 60; j++ ) {
    for( int i =0; i < 360; i++ ) {
      PVector v1 = func(i,j);
      PVector v2 = func(i, j+1 );
      vertex( v1.x,v1.y, v1.z);
      vertex( v2.x,v2.y, v2.z);
    }
  }
  endShape();
}

PVector func( int i, int j ) {
      float y = 5 + sq(float(j)/10);
      float x = cos( radians(i)) * y;
      float z = sin( radians(i)) * y;
  return new PVector( x, j, z );
}