gl.glVertex2d(centerX - halfWidth, centerY - halfWidth);
gl.glVertex2d(centerX - halfHeight, centerY - halfWidth);
// right part.
gl.glVertex2d(centerX + halfWidth, centerY + halfWidth);
gl.glVertex2d(centerX + halfHeight, centerY + halfWidth);
gl.glVertex2d(centerX + halfHeight, centerY - halfWidth);
gl.glVertex2d(centerX + halfWidth, centerY - halfWidth);
gl.glVertex3d(0, 0, 0);
gl.glEnd();
}