Set BufferedImage alpha mask in Java

I have two BufferedImages I loaded in from pngs. The first contains an image, the second an alpha mask for the image.

I want to create a combined image from the two, by applying the alpha mask. My google-fu fails me.

I know how to load/save the images, I just need the bit where I go from two BufferedImages to one BufferedImage with the right alpha channel.

I'm too late with this answer, but maybe it is of use for someone anyway. This is a simpler and more efficient version of Michael Myers' method:

public void applyGrayscaleMaskToAlpha(BufferedImage image, BufferedImage mask)
{
    int width = image.getWidth();
    int height = image.getHeight();

    int[] imagePixels = image.getRGB(0, 0, width, height, null, 0, width);
    int[] maskPixels = mask.getRGB(0, 0, width, height, null, 0, width);

    for (int i = 0; i < imagePixels.length; i++)
    {
        int color = imagePixels[i] & 0x00ffffff; // Mask preexisting alpha
        int alpha = maskPixels[i] << 24; // Shift blue to alpha
        imagePixels[i] = color | alpha;
    }

    image.setRGB(0, 0, width, height, imagePixels, 0, width);
}

It reads all the pixels into an array at the beginning, thus requiring only one for-loop. Also, it directly shifts the blue byte to the alpha (of the mask color), instead of first masking the red byte and then shifting it.

Like the other methods, it assumes both images have the same dimensions.

I played recently a bit with this stuff, to display an image over another one, and to fade an image to gray.
Also masking an image with a mask with transparency (my previous version of this message!).

I took my little test program and tweaked it a bit to get the wanted result.

Here are the relevant bits:

TestMask() throws IOException
{
    m_images = new BufferedImage[3];
    m_images[0] = ImageIO.read(new File("E:/Documents/images/map.png"));
    m_images[1] = ImageIO.read(new File("E:/Documents/images/mapMask3.png"));
    Image transpImg = TransformGrayToTransparency(m_images[1]);
    m_images[2] = ApplyTransparency(m_images[0], transpImg);
}

private Image TransformGrayToTransparency(BufferedImage image)
{
    ImageFilter filter = new RGBImageFilter()
    {
        public final int filterRGB(int x, int y, int rgb)
        {
            return (rgb << 8) & 0xFF000000;
        }
    };

    ImageProducer ip = new FilteredImageSource(image.getSource(), filter);
    return Toolkit.getDefaultToolkit().createImage(ip);
}

private BufferedImage ApplyTransparency(BufferedImage image, Image mask)
{
    BufferedImage dest = new BufferedImage(
            image.getWidth(), image.getHeight(),
            BufferedImage.TYPE_INT_ARGB);
    Graphics2D g2 = dest.createGraphics();
    g2.drawImage(image, 0, 0, null);
    AlphaComposite ac = AlphaComposite.getInstance(AlphaComposite.DST_IN, 1.0F);
    g2.setComposite(ac);
    g2.drawImage(mask, 0, 0, null);
    g2.dispose();
    return dest;
}

The remainder just display the images in a little Swing panel.
Note that the mask image is gray levels, black becoming full transparency, white becoming full opaque.

Although you have resolved your problem, I though I could share my take on it. It uses a slightly more Java-ish method, using standard classes to process/filter images.
Actually, my method uses a bit more memory (making an additional image) and I am not sure it is faster (measuring respective performances could be interesting), but it is slightly more abstract.
At least, you have choice! :-)