/**
   * Called back immediately after the OpenGL context is initialized. Can be used to perform
   * one-time initialization. Run only once.
   */
  @Override
  public void init(GLAutoDrawable drawable) {
    GL2 gl = drawable.getGL().getGL2(); // get the OpenGL graphics context
    glu = new GLU(); // get GL Utilities
    gl.glClearColor(0.0f, 0.0f, 0.0f, 0.0f); // set background (clear) color
    gl.glClearDepth(1.0f); // set clear depth value to farthest
    // Do not enable depth test
    //      gl.glEnable(GL_DEPTH_TEST); // enables depth testing
    //      gl.glDepthFunc(GL_LEQUAL);  // the type of depth test to do
    gl.glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); // best perspective correction
    gl.glShadeModel(GL_SMOOTH); // blends colors nicely, and smoothes out lighting

    // Load the texture image
    try {
      // Use URL so that can read from JAR and disk file.
      BufferedImage image = ImageIO.read(this.getClass().getResource(textureFileName));

      // Create a OpenGL Texture object
      texture = AWTTextureIO.newTexture(GLProfile.getDefault(), image, false);
      // Use linear filter if image is larger than the original texture
      gl.glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
      // Use linear filter if image is smaller than the original texture
      gl.glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

    } catch (GLException | IOException e) {
      e.printStackTrace();
    }

    // Texture image flips vertically. Shall use TextureCoords class to retrieve
    // the top, bottom, left and right coordinates, instead of using 0.0f and 1.0f.
    TextureCoords textureCoords = texture.getImageTexCoords();
    textureCoordTop = textureCoords.top();
    textureCoordBottom = textureCoords.bottom();
    textureCoordLeft = textureCoords.left();
    textureCoordRight = textureCoords.right();

    // Enable the texture
    texture.enable(gl);
    texture.bind(gl);
    // gl.glEnable(GL_TEXTURE_2D);

    // Enable blending
    gl.glEnable(GL_BLEND);
    gl.glBlendFunc(GL_SRC_ALPHA, GL_ONE);

    // Allocate the stars
    for (int i = 0; i < stars.length; i++) {
      stars[i] = new Star();
      // Linearly distributed according to the star number
      stars[i].distance = ((float) i / numStars) * 5.0f;
    }
  }
  /**
   * Call-back handler for window re-size event. Also called when the drawable is first set to
   * visible.
   */
  @Override
  public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) {
    GL2 gl = drawable.getGL().getGL2(); // get the OpenGL 2 graphics context

    if (height == 0) height = 1; // prevent divide by zero
    float aspect = (float) width / height;

    // Set the view port (display area) to cover the entire window
    gl.glViewport(0, 0, width, height);

    // Setup perspective projection, with aspect ratio matches viewport
    gl.glMatrixMode(GL_PROJECTION); // choose projection matrix
    gl.glLoadIdentity(); // reset projection matrix
    glu.gluPerspective(45.0, aspect, 0.1, 100.0); // fovy, aspect, zNear, zFar

    // Enable the model-view transform
    gl.glMatrixMode(GL_MODELVIEW);
    gl.glLoadIdentity(); // reset
  }
  /** Called back by the animator to perform rendering. */
  @Override
  public void display(GLAutoDrawable drawable) {
    GL2 gl = drawable.getGL().getGL2(); // get the OpenGL 2 graphics context
    gl.glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // clear color and depth buffers

    for (int i = 0; i < stars.length; i++) {
      // Reset the view (x, y, z axes back to normal)
      gl.glLoadIdentity();
      gl.glTranslatef(0.0f, 0.0f, z);

      // The stars are texture quad square drawn on x-y plane and
      // distributed on on x-z plane around the y-axis

      // Initial 90 degree tile in the x-axis, y-axis pointing out of screen
      gl.glRotatef(tilt, 1.0f, 0.0f, 0.0f);
      // Rotate about y-axis (pointing out of screen), initial angle is 0
      gl.glRotatef(stars[i].angle, 0.0f, 1.0f, 0.0f);
      // Translate about the x-axis (pointing right) to its current distance
      gl.glTranslatef(stars[i].distance, 0.0f, 0.0f);

      // The stars have initial angle of 0, and initial distance linearly
      // distributed between 0 and 5.0f

      // Rotate the axes back, so that z-axis is again facing us, to ensure that
      // the quad (with texture) on x-y plane is facing us
      gl.glRotatef(-stars[i].angle, 0.0f, 1.0f, 0.0f);
      gl.glRotatef(-tilt, 1.0f, 0.0f, 0.0f);

      // Take note that without the two rotations and undo, there is only one
      // translation along the x-axis
      // Matrix operation is non-commutative. That is, AB != BA.
      // Hence, ABCB'A' != C

      // Draw the star, which spins on the z axis (pointing out of the screen)
      gl.glRotatef(starSpinAngle, 0.0f, 0.0f, 1.0f);
      // Set the star's color using bytes (why bytes? not float or int?)
      gl.glColor4ub(stars[i].r, stars[i].g, stars[i].b, (byte) 255);
      gl.glBegin(GL_QUADS);
      // draw a square on x-y plane
      gl.glTexCoord2f(textureCoordLeft, textureCoordBottom);
      gl.glVertex3f(-1.0f, -1.0f, 0.0f);
      gl.glTexCoord2f(textureCoordRight, textureCoordBottom);
      gl.glVertex3f(1.0f, -1.0f, 0.0f);
      gl.glTexCoord2f(textureCoordRight, textureCoordTop);
      gl.glVertex3f(1.0f, 1.0f, 0.0f);
      gl.glTexCoord2f(textureCoordLeft, textureCoordTop);
      gl.glVertex3f(-1.0f, 1.0f, 0.0f);
      gl.glEnd();

      // If twinkling, overlay with another drawing of an arbitrary color
      if (twinkleOn) {
        // Assign a color using bytes
        gl.glColor4ub(
            stars[(numStars - i) - 1].r,
            stars[(numStars - i) - 1].g,
            stars[(numStars - i) - 1].b,
            (byte) 255);
        gl.glBegin(GL_QUADS);
        // draw a square on x-y plane
        gl.glTexCoord2f(textureCoordLeft, textureCoordBottom);
        gl.glVertex3f(-1.0f, -1.0f, 0.0f);
        gl.glTexCoord2f(textureCoordRight, textureCoordBottom);
        gl.glVertex3f(1.0f, -1.0f, 0.0f);
        gl.glTexCoord2f(textureCoordRight, textureCoordTop);
        gl.glVertex3f(1.0f, 1.0f, 0.0f);
        gl.glTexCoord2f(textureCoordLeft, textureCoordTop);
        gl.glVertex3f(-1.0f, 1.0f, 0.0f);
        gl.glEnd();
      }

      // Update for the next refresh
      // The star spins about the z-axis (pointing out of the screen), and spiral
      // inwards and collapse towards the center, by increasing the angle on x-y
      // plane and reducing the distance.

      starSpinAngle += 0.01f; // used to spin the stars about the z-axis
      // spiral pattern
      stars[i].angle += (float) i / numStars; // changes the angle of a star
      // collapsing the star to the center
      stars[i].distance -= 0.01f; // changes the distance of a star
      // re-bone at the edge
      if (stars[i].distance < 0.0f) { // Is the star collapsed to the center?
        stars[i].distance += 5.0f; // move to the outer ring
        stars[i].setRandomRGB(); // choose a random color for the star
      }
    }
  }