/** 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
}
}
}
