Preserving alpha while converting images to grayscale

Question

I am trying to convert images into grayscale.
I use this code
public static BufferedImage grayScale(BufferedImage im) {
        BufferedImage image = new BufferedImage(im.getWidth(), im.getHeight(), BufferedImage.TYPE_BYTE_GRAY);
        Graphics g = image.getGraphics();
        g.drawImage(im, 0, 0, null);
        g.dispose();
        return image;
    }but transparent parts become black. Is there a simple way to preserve opacity while converting to grayscale.

Answer 1

Since you want to preserve the alpha channel you'll need to get and set the pixels yourself. And you'll have to use a BufferedImage type that
supports an alpha channel. TYPE_BYTE_GRAY won't do...
public static BufferedImage grayScale(BufferedImage im) {
     BufferedImage grayImage = new BufferedImage(im.getWidth(), im.getHeith(), BufferedImage.TYPE_INT_ARGB);
     for(int x = 0; x < im.getWidth(); x++)
          for(int y = 0; y < im.getHeight(); y++){
               int argb = im.getRGB(x,y);
               int a = (argb >> 24) & 0xff;
               int r = (argb >> 16) & 0xff;
               int g = (argb >>  8) & 0xff;
               int b = (argb      ) & 0xff;
               int l= (int) (.299 * r + .587 * g + .114 * b) //luminance
               grayImage.setRGB(x,y, (a << 24) + (l << 16) + (l << 8) + l);
     return grayImage;
}