AffineTransformで数字パネルの擬似3D回転を表現するフリップ時計を作成する

class FlipPair extends JPanel {
  private static final Color CARD_UPPER_BG = new Color(45, 45, 45);
  private static final Color CARD_LOWER_BG = new Color(25, 25, 25);
  private static final Color TEXT_COLOR = new Color(230, 230, 230);
  private static final int SHADOW_MAX_ALPHA = 130;
  private static final int WIDTH = 80;
  private static final int HEIGHT = 100;
  private static final int SLIT_HEIGHT = 4;
  private static final int FONT_SIZE = 64;
  private static final String FONT_NAME = "Impact"; // "Impact" or "Arial"

  private int currentVal;
  private int nextVal;
  private double angle;
  private boolean isAnimating;
  private final Timer animTimer = new Timer(15, null);

  public FlipPair(int startValue) {
    super();
    this.currentVal = startValue;
    this.nextVal = startValue;
    animTimer.addActionListener(e -> {
      angle -= 15;
      if (angle <= 0) {
        angle = 0;
        isAnimating = false;
        currentVal = nextVal;
        animTimer.stop();
      }
      repaint();
    });
  }

  public void setValue(int newValue) {
    if (newValue != nextVal && !isAnimating) {
      nextVal = newValue;
      angle = 180;
      isAnimating = true;
      animTimer.start();
    }
  }

  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    d.setSize(WIDTH, HEIGHT);
    return d;
  }

  @Override public boolean isOpaque() {
    return false;
  }

  @Override protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setRenderingHint(
        RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    g2.setRenderingHint(
        RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
    g2.setFont(new Font(FONT_NAME, Font.PLAIN, FONT_SIZE));
    AffineTransform at = AffineTransform.getScaleInstance(1d, 1.5);
    g2.setFont(g2.getFont().deriveFont(at));

    int cx = getWidth() / 2;
    int cy = getHeight() / 2;
    String curStr = String.format("%02d", currentVal);
    String nxtStr = String.format("%02d", nextVal);
    drawHalf(g2, nxtStr, cx, cy, true, CARD_UPPER_BG);
    drawHalf(g2, curStr, cx, cy, false, CARD_LOWER_BG);
    if (isAnimating) {
      double rad = Math.toRadians(angle);
      double scaleY = Math.abs(Math.cos(rad));
      g2.translate(cx, cy);
      g2.scale(1d, scaleY);
      boolean isTop = angle > 90;
      Color bg = isTop ? CARD_LOWER_BG : CARD_UPPER_BG;
      drawHalf(g2, curStr, 0, 0, isTop, bg);
      int alpha = (int) ((1d - scaleY) * SHADOW_MAX_ALPHA);
      drawShadow(g2, 0, 0, isTop, alpha);
    }
    g2.dispose();
  }

  private void drawHalf(Graphics g, String txt, int cx, int cy, boolean isTop, Color bg) {
    int x = cx - WIDTH / 2;
    int height = HEIGHT / 2 - SLIT_HEIGHT / 2;
    if (isTop) {
      g.setClip(x, cy - HEIGHT / 2, WIDTH, height);
    } else {
      g.setClip(x, cy + SLIT_HEIGHT / 2, WIDTH, height);
    }
    g.setColor(bg);
    g.fillRoundRect(x, cy - HEIGHT / 2, WIDTH, HEIGHT, 18, 18);
    g.setColor(TEXT_COLOR);
    FontMetrics fm = g.getFontMetrics();
    g.drawString(txt, cx - fm.stringWidth(txt) / 2, cy + fm.getAscent() / 2 - 12);
  }

  private void drawShadow(Graphics g, int cx, int cy, boolean isTop, int alpha) {
    g.setColor(new Color(0, 0, 0, Math.min(255, alpha)));
    int h = HEIGHT / 2 - SLIT_HEIGHT / 2;
    if (isTop) {
      g.fillRect(cx - WIDTH / 2, cy - HEIGHT / 2, WIDTH, h);
    } else {
      g.fillRect(cx - WIDTH / 2, cy + SLIT_HEIGHT / 2, WIDTH, h);
    }
  }

  public static JLabel makeColonLabel() {
    JLabel colon = new JLabel(":");
    colon.setFont(colon.getFont().deriveFont(Font.BOLD, FONT_SIZE));
    colon.setForeground(TEXT_COLOR);
    return colon;
  }
}