public void begin () {
if (drawing) throw new IllegalStateException("end must be called before begin.");
if (Gdx.graphics.isGL20Available() == false) {
GL10 gl = Gdx.gl10;
gl.glDepthMask(false);
gl.glEnable(GL10.GL_TEXTURE_2D);
gl.glMatrixMode(GL10.GL_PROJECTION);
gl.glLoadMatrixf(projectionMatrix.val, 0);
gl.glMatrixMode(GL10.GL_MODELVIEW);