Java: extract Alpha Channel from BufferedImage

Question

I would like to extract the Alpha Channel from a bufferedimage and paint it in a separate image using greyscale. (like it is displayed in photoshop)

Answer 1

Not tested but contains the main points.

public Image alpha2gray(BufferedImage src) {

    if (src.getType() != BufferedImage.TYPE_INT_ARGB)
        throw new RuntimeException("Wrong image type.");

    int w = src.getWidth();
    int h = src.getHeight();

    int[] srcBuffer = src.getData().getPixels(0, 0, w, h, (int[]) null);
    int[] dstBuffer = new int[w * h];

    for (int i=0; i<w*h; i++) {
        int a = (srcBuffer[i] >> 24) & 0xff;
        dstBuffer[i] = a | a << 8 | a << 16;
    }

    return Toolkit.getDefaultToolkit().createImage(new MemoryImageSource(w, h, pix, 0, w));
}

Answer 2

I don't believe there's a single method call to do this; you would have to get all the image data and mask off the alpha byte for each pixel. So for example, use getRGB() to get the ARGB pixels for the image. The most significant byte is the alpha value. So for each pixel in the array you get from getRGB(),

int alpha = (pixel >> 24) & 0xFF; 

Answer 3

You could grab the Raster from the BufferedImage, and then create a child Raster of this one which contains only the band you're interested in (the bandList parameter). From this child raster you can create a new BufferedImage with a suitable ColorModel which would only contain the grayscale aplpha mask.

The benefit of doing it this way instead of manually iterating over the pixels is that the runtime has chance to get an idea of what you are doing, and thus this might get accelerated by exploiting the hardware capabilities. Honestly I doubt it will be accelerated with current JVMs, but who knows what the future brings?