JLabelの文字列をシマー効果で装飾する

class ShimmerLayerUI extends LayerUI<JLabel> {
  private static final int FPS = 16;
  private static final int BAND_WIDTH = 100;
  private static final float SPEED = 4f;
  private static final int ALPHA = 180;
  private static final Color TRANSPARENT = new Color(0x0, true);
  private final Timer timer = new Timer(FPS, null);
  private final float[] fractions = {0f, .5f, 1f};
  private float animX;
  private transient BufferedImage buffer;

  @Override public void installUI(JComponent c) {
    super.installUI(c);
    timer.addActionListener(ev -> {
      animX += SPEED;
      if (animX > c.getWidth() + BAND_WIDTH) {
        animX = -BAND_WIDTH;
      }
      c.repaint();
    });
    timer.start();
  }

  @Override public void uninstallUI(JComponent c) {
    timer.stop();
    super.uninstallUI(c);
  }

  @Override public void paint(Graphics g, JComponent c) {
    // super.paint(g, c);
    @SuppressWarnings("unchecked")
    JLayer<? extends JLabel> layer = (JLayer<? extends JLabel>) c;
    JLabel label = layer.getView();
    if (label == null || label.getText() == null || label.getText().isEmpty()) {
      super.paint(g, c);
    } else {
      int w = Math.max(1, c.getWidth());
      int h = Math.max(1, c.getHeight());
      ensureBuffer(w, h);
      paintLayerToBuffer(c, w, h);
      BufferedImage shimBuf = createShimmerBuffer(label, c, w, h);
      paintShimmerBuffer(shimBuf);
      g.drawImage(buffer, 0, 0, c);
    }
  }

  private void ensureBuffer(int w, int h) {
    if (buffer == null || buffer.getWidth() != w || buffer.getHeight() != h) {
      buffer = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
    }
  }

  private void paintLayerToBuffer(JComponent c, int w, int h) {
    Graphics2D buf = buffer.createGraphics();
    buf.setComposite(AlphaComposite.Clear);
    buf.fillRect(0, 0, w, h);
    buf.setComposite(AlphaComposite.SrcOver);
    c.paint(buf);
    buf.dispose();
  }

  private BufferedImage createShimmerBuffer(JLabel label, JComponent c, int w, int h) {
    BufferedImage shimBuf = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
    Graphics2D sg = shimBuf.createGraphics();

    AbstractShimmerLabel.applyRenderingHints(sg);
    Rectangle[] rects = ShimmerLayout.layoutLabel(label, sg);
    Rectangle textRect = rects[1];
    FontMetrics fm = sg.getFontMetrics(label.getFont());
    Point labelOrigin = SwingUtilities.convertPoint(label, 0, 0, c);
    float x = labelOrigin.x + textRect.x;
    float y = labelOrigin.y + textRect.y + fm.getAscent();

    // 1. Draw the text outline to the shimmer buffer (to serve as a mask).
    sg.setPaint(label.getForeground());
    FontRenderContext frc = sg.getFontRenderContext();
    new TextLayout(label.getText(), label.getFont(), frc).draw(sg, x, y);

    // 2. Compose the gradient using SrcAtop (masked by the text's alpha channel).
    sg.setComposite(AlphaComposite.SrcAtop);
    Color[] colors = {
        TRANSPARENT,
        ShimmerColors.shimmerBright(label.getForeground(), ALPHA),
        TRANSPARENT,
    };
    float endX = animX + BAND_WIDTH;
    sg.setPaint(new LinearGradientPaint(animX, 0f, endX, 0f, fractions, colors));
    sg.fillRect(0, 0, w, h);
    sg.dispose();

    // Clear the text area painted
    Graphics2D buf = buffer.createGraphics();
    buf.setPaint(label.getBackground());
    buf.fill(textRect);
    buf.dispose();

    return shimBuf;
  }

  private void paintShimmerBuffer(BufferedImage shimBuf) {
    Graphics2D buf = buffer.createGraphics();
    buf.setComposite(AlphaComposite.SrcOver);
    buf.drawImage(shimBuf, 0, 0, null);
    buf.dispose();
  }
}