Set BufferedImage alpha mask in Java

Question

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.

Answer 1

Actually, I've figured it out. This is probably not a fast way of doing it, but it works:

for (int y = 0; y < image.getHeight(); y++) {
    for (int x = 0; x < image.getWidth(); x++) {
     Color c = new Color(image.getRGB(x, y));
     Color maskC = new Color(mask.getRGB(x, y));
     Color maskedColor = new Color(c.getRed(), c.getGreen(), c.getBlue(),
      maskC.getRed());
     resultImg.setRGB(x, y, maskedColor.getRGB());
    }
}

Answer 2

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! :-)

Answer 3

Your solution could be improved by fetching the RGB data more than one pixel at a time(see http://java.sun.com/javase/6/docs/api/java/awt/image/BufferedImage.html), and by not creating three Color objects on every iteration of the inner loop.

final int width = image.getWidth();
int[] imgData = new int[width];
int[] maskData = new int[width];

for (int y = 0; y < image.getHeight(); y++) {
    // fetch a line of data from each image
    image.getRGB(0, y, width, 1, imgData, 0, 1);
    mask.getRGB(0, y, width, 1, maskData, 0, 1);
    // apply the mask
    for (int x = 0; x < width; x++) {
        int color = imgData[x] & 0x00FFFFFF; // mask away any alpha present
        int mask = (maskData[x] & 0x00FF0000) << 8; // shift red into alpha bits
        color |= mask;
        imgData[x] = color;
    }
    // replace the data
    image.setRGB(0, y, width, 1, imgData, 0, 1);
}