Image handling and processing - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 375

Similarly, you can pack separate component values into an integer pixel variable:
int color = (alpha << 24) | (red << 16) | (green << 8) | blue;
The alpha, or transparency, factor is used when an image is overlaid on a back-
ground color or another image. This can be useful for various situations such
as when placing an icon on a button. For example, you may want to allow the
background color of the button to show through the blank areas of the icon.
11.8.2 PixelGrabber
The Image class does not offer direct access to its pixel array. Instead, you use the
class java.awt.image.PixelGrabber ,which, as its name implies, grabs
the pixel data for you. The following code shows how to put the pixel values of
an image into an array:
...
int[] pixels = new int[width * height];
boolean got - pixels = false;
PixelGrabber grabber =
new PixelGrabber (img, x0, y0, width, height, pixels, 0,
scan - Width);
try {
grabber.grabPixels ();
}
catch (InterruptedException e) {
got - pixels = false;
return;
}
if ((grabber.getStatus () & ImageObserver.ALLBITS)! = 0) {
got - pixels = false;
return;
}
First an array big enough to hold the pixels is created. The first parameter of the
PixelGrabber constructor is the image reference. The pixels to be grabbed
come from a rectangular section of the image as specified by the top left corner
at (x0, y0) and by the width and height parameters. The pixel array reference
is passed in the next parameter, and the last two parameters include an offset
into the array to indicate where to begin putting the pixels (here set to 0) and the
scan - Width value, which specifies the number of pixels per row. To capture a
whole image in the pixels array, set the x0, y0 values to zero, use the width,
height for the image and set scan - Width equal to the image width.