BufferedImageOpで画像にモザイク処理を適用する

class MosaicImageFilter implements BufferedImageOp {
  private final int blockSize;

  protected MosaicImageFilter(int blockSize) {
    this.blockSize = blockSize;
  }

  @Override public BufferedImage filter(
      BufferedImage src, BufferedImage dst) {
    int width = src.getWidth();
    int height = src.getHeight();
    WritableRaster srcRaster = src.getRaster();
    BufferedImage img = dst == null
        ? createCompatibleDestImage(src, null)
        : dst;
    WritableRaster dstRaster = img.getRaster();
    int[] pixels = new int[blockSize * blockSize];
    for (int y = 0; y < height; y += blockSize) {
      for (int x = 0; x < width; x += blockSize) {
        int w = Math.min(blockSize, width - x);
        int h = Math.min(blockSize, height - y);
        srcRaster.getDataElements(x, y, w, h, pixels);
        updatePixels(w, h, pixels);
        dstRaster.setDataElements(x, y, w, h, pixels);
      }
    }
    return img;
  }

  public static int getBlockRgb(int w, int h, int[] pixels) {
    int r = 0;
    int g = 0;
    int b = 0;
    for (int by = 0; by < h; by++) {
      for (int bx = 0; bx < w; bx++) {
        int argb = pixels[bx + by * w];
        r += (argb >> 16) & 0xFF;
        g += (argb >> 8) & 0xFF;
        b += argb & 0xFF;
      }
    }
    int size = w * h;
    return (r / size) << 16 | (g / size) << 8 | (b / size);
  }

  public static void updatePixels(int w, int h, int[] pixels) {
    int rgb = getBlockRgb(w, h, pixels);
    for (int by = 0; by < h; by++) {
      for (int bx = 0; bx < w; bx++) {
        int i = bx + by * w;
        pixels[i] = (pixels[i] & 0xFF_00_00_00) | rgb;
      }
    }
  }

  @Override public Rectangle2D getBounds2D(BufferedImage src) {
    return null;
  }

  @Override public BufferedImage createCompatibleDestImage(
      BufferedImage src, ColorModel dstCm) {
    return new BufferedImage(
      src.getWidth(), src.getHeight(), BufferedImage.TYPE_INT_ARGB);
  }

  @Override public Point2D getPoint2D(Point2D srcPt, Point2D dstPt) {
    return null;
  }

  @Override public RenderingHints getRenderingHints() {
    return new RenderingHints(Collections.emptyMap());
  }
}