Spin3d spin = new Spin3d();

void setup() {
  size(200,200);
  framerate(20);  
}
void mouseDragged() {
  spin.mouseMoved();
}

void loop() {
  spin.update();
  
  background(240);
  translate(width/2, height/2);
  
    spin.apply();
    scale(50);
    stroke(0);
    drawShape();
}
final float a = 1.0f;
final float b = a * 0.8f;
final float c = a * 0.5f;

void drawShape() {
  drawPair();
  rotateY(HALF_PI);
  rotateZ(HALF_PI);
  drawPair();
  rotateY(HALF_PI);
  rotateZ(HALF_PI);
  drawPair();
}

void drawPair() {
  push();
    translate(0,0,(a-c)/2+c);
    drawPiece();
  pop();
  push();
    translate(0,0,-(a-c)/2-c);
    rotateX(PI);
    drawPiece();
  pop();
}
void drawPiece() {
  // front
  beginShape(POLYGON);
  // right top
    vertex(a/2,        b+a/2,  0);
    vertex(a/2,        a/2,      0);
    vertex((a-c)/2,    a/2,      0);
  // right bottom
    vertex((a-c)/2,    -a/2,     0);
    vertex(a/2,        -a/2,     0);
    vertex(a/2,        -b-a/2, 0);
    
  // left bottom
    vertex(-a/2,        -b-a/2, 0);
    vertex(-a/2,        -a/2,     0);
    vertex(-(a-c)/2,    -a/2,     0);
    
  // left top
    vertex(-(a-c)/2,    a/2,     0);
    vertex(-a/2,        a/2,     0);
    vertex(-a/2,        b+a/2, 0);
  endShape();
  
  float y = 1;
  for (int i=0; i<2; i++) {
  // top cap
    beginShape(POLYGON);
      vertex(a/2,        y*(b+a/2),  0);
      vertex(-a/2,       y*(b+a/2),  0);
      
      vertex(-a/2,       y*(b+a/2),  -c);
      vertex(a/2,        y*(b+a/2),  -c);
    endShape();
  // back cap
    beginShape(POLYGON);
      vertex(a/2,        y*(b+a/2),  -c);
      vertex(-a/2,       y*(b+a/2),  -c);
      
      vertex(-a/2,       y*(a/2+c/2),  -c);
      vertex(a/2,        y*(a/2+c/2),  -c);
    endShape();
  
  
  // side panels  
    float x = 1;
    for (int j=0; j<2; j++) {
      beginShape(POLYGON);
        vertex(x*-a/2,       y*(b+a/2),  0);
        vertex(x*-a/2,       y*(b+a/2),  -c);
        vertex(x*-a/2,       y*(c/2+a/2),-c);
        vertex(x*-a/2,       y*(c/2+a/2),-c/2);
        vertex(x*-a/2,       y*(a/2),    -c/2);
        vertex(x*-a/2,       y*(a/2),    0);
        
      endShape();
      x = -1;
    }
    
    y = -1;
  }
}














/*******************************************************************************
 * Spin3d - Handles nice trackball style camera movement in Processing
 *   - based on Matrix3D code from cello3d by Marcello
 ********************************************************************************/

public class Spin3d {
  float x= 0, y = 0;
  float tdx = 0, tdy = 0;
  float dx = tdx, dy = tdy;


  public void mouseMoved() {
    // GOTCHA: x and y are switched!
    tdx=0.01*(mouseY-pmouseY);
    tdy=0.01*(mouseX-pmouseX);
  }
  public void apply() {
    g.transform(m_0_0, m_0_1, m_0_2, m_0_3,
                m_1_0, m_1_1, m_1_2, m_1_3,
                m_2_0, m_2_1, m_2_2, m_2_3,
                m_3_0, m_3_1, m_3_2, m_3_3);
  }
  void update() {
    x += dx;
    y += dy;
    dx += (tdx-dx)/20;
    dy += (tdy-dy)/20;
    tdx += (0-tdx)/20;
    tdy += (0-tdy)/20;
    rotateX(dx);
    rotateY(dy);
  }

	public Spin3d() {
	}
	public float	
		m_0_0 = 1, m_0_1 = 0, m_0_2 = 0, m_0_3 = 0,
		m_1_0 = 0, m_1_1 = 1, m_1_2 = 0, m_1_3 = 0,
		m_2_0 = 0, m_2_1 = 0, m_2_2 = 1, m_2_3 = 0,
		m_3_0 = 0, m_3_1 = 0, m_3_2 = 0, m_3_3 = 1;

	public void rotateX(float t) {
		if (t==0) return;
		float sin = sin(t);
		float cos = cos(t);
		transform(
			1,    0,   0,   0,
			0,    cos, sin, 0,
			0,    -sin,cos, 0,
			0,    0,   0,   1
		);
	}
	public void rotateY(float t) {
		if (t==0) return;
		float sin = sin(t);
		float cos = cos(t);
		transform(
			cos, 0,   sin, 0,
			0,   1,   0,   0,
			-sin,0,   cos, 0,
			0,   0,   0,   1
		);
	}
	
	public void transform(float nm_0_0, float nm_0_1, float nm_0_2, float nm_0_3,
						float nm_1_0, float nm_1_1, float nm_1_2, float nm_1_3,
						float nm_2_0, float nm_2_1, float nm_2_2, float nm_2_3,
						float nm_3_0, float nm_3_1, float nm_3_2, float nm_3_3) {
							
		Spin3d m =  new Spin3d();

		m.m_0_0 = m_0_0; m.m_0_1 = m_0_1; m.m_0_2 = m_0_2; m.m_0_3 = m_0_3;
		m.m_1_0 = m_1_0; m.m_1_1 = m_1_1; m.m_1_2 = m_1_2; m.m_1_3 = m_1_3;
		m.m_2_0 = m_2_0; m.m_2_1 = m_2_1; m.m_2_2 = m_2_2; m.m_2_3 = m_2_3;
		m.m_3_0 = m_3_0; m.m_3_1 = m_3_1; m.m_3_2 = m_3_2; m.m_3_3 = m_3_3;

		m_0_0 = nm_0_0*m.m_0_0 + nm_0_1*m.m_1_0 + nm_0_2*m.m_2_0;
		m_0_1 = nm_0_0*m.m_0_1 + nm_0_1*m.m_1_1 + nm_0_2*m.m_2_1;
		m_0_2 = nm_0_0*m.m_0_2 + nm_0_1*m.m_1_2 + nm_0_2*m.m_2_2;
		m_0_3 = nm_0_0*m.m_0_3 + nm_0_1*m.m_1_3 + nm_0_2*m.m_2_3 + nm_0_3;
		m_1_0 = nm_1_0*m.m_0_0 + nm_1_1*m.m_1_0 + nm_1_2*m.m_2_0;
		m_1_1 = nm_1_0*m.m_0_1 + nm_1_1*m.m_1_1 + nm_1_2*m.m_2_1;
		m_1_2 = nm_1_0*m.m_0_2 + nm_1_1*m.m_1_2 + nm_1_2*m.m_2_2;
		m_1_3 = nm_1_0*m.m_0_3 + nm_1_1*m.m_1_3 + nm_1_2*m.m_2_3 + nm_1_3;
		m_2_0 = nm_2_0*m.m_0_0 + nm_2_1*m.m_1_0 + nm_2_2*m.m_2_0;
		m_2_1 = nm_2_0*m.m_0_1 + nm_2_1*m.m_1_1 + nm_2_2*m.m_2_1;
		m_2_2 = nm_2_0*m.m_0_2 + nm_2_1*m.m_1_2 + nm_2_2*m.m_2_2;
		m_2_3 = nm_2_0*m.m_0_3 + nm_2_1*m.m_1_3 + nm_2_2*m.m_2_3 + nm_2_3;
	}
}