Обрезать изображения с динамической подписью

Я обещал подход с параллельным потоком. Вот.

Необрезанное изображение (png): http://assets-cdn.github.com/images/modules/open_graph/github-mark.png

public static void main(String[] args) throws Exception {
    BufferedImage img = ImageIO.read(new URL("http://assets-cdn.github.com/images/modules/open_graph/github-mark.png"));

    BufferedImage sigImage = cropSignature(img);

    SwingUtilities.invokeLater(() -> {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        f.setContentPane(new JLabel(new ImageIcon(sigImage)));
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    });
}

private static BufferedImage cropSignature(BufferedImage img) {
    // Iterating over height has better CPU cache performance.
    Rectangle sigRect = IntStream.range(0, img.getHeight())
                                 .parallel()
                                 .mapToObj(y -> getHorizontalSpan(img, y))
                                 .collect(() -> new Rectangle(0, 0, -1, -1),
                                          (r1, r2) -> r1.add(r2),
                                          (r1, r2) -> r1.add(r2));

    return img.getSubimage(sigRect.x, sigRect.y, sigRect.width, sigRect.height);
}

private static Rectangle getHorizontalSpan(BufferedImage img, int y) {
    // Don't parallelize. The 'combiner' is therefore not necessary and may be null.
    return IntStream.range(0, img.getWidth())
                    .filter(x -> isSignatureColor(img.getRGB(x, y)))
                    .collect(() -> new Rectangle(0, 0, -1, -1),
                             (r, x) -> r.add(x, y),
                             null);
}

private static boolean isSignatureColor(int rgb) {
    // Easiest criteria, but fails on JPG files because it doesn't use a tolerance. 
    return rgb != -1;
}