Rotation problem

gistfile1.txt

// Processing script to resolve a frame rotation problem

float zoom;
PVector a, b, pvec;

void setup()
{
  size(800, 600, P3D);
  a = new PVector(0.0, 0.0, 0.0);
  b = new PVector(200.0, 400.0, -600.0);
}

void draw()
{
  background(255);
  directionalLight(150, 150, 150, 0.3, -0.6, 0.3);
  ambientLight(100, 100, 100);
  camera(1200 * sin(mouseX/800.0), 1200 * cos(mouseX/800.0), -mouseY*4, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0);

  stroke(0);
  line(a.x, a.y, a.z, b.x, b.y, b.z );

  // Plot coloured axes
  stroke(255, 0, 0);
  line(-1000, 0, 0, 1000, 0, 0);
  stroke(0, 255, 0);
  line(0, -1000, 0, 0, 1000, 0);
  stroke(0, 0, 255);
  line(0, 0, -1000, 0, 0, 1000);
  noStroke();

  pushMatrix();
  pvec = PVector.sub(b, a);
  translate(pvec.x/2, pvec.y/2, pvec.z/2);
  
  // ROTATE FRAME OF REFERENCE TO PLOT A BOX AS A LINE
  rotateZ(atan(-pvec.x/pvec.y));
  rotateX(atan(-pvec.y/pvec.z));
  //rotateY(atan(pvec.x/pvec.y));

  box(10.0, 10.0, pvec.mag());
  popMatrix();
}