Get RGB of a BufferedImage

Question

Can anybody explain how to get an array of rgb value from a BufferedImage?

I have a grey scale image in a BufferedImage and need to extract an array of 0 to 255 values that describe the image.
I know the BufferedImage is correct because I can save it to PNG. However, if I use int[] dataBuffInt = ((DataBufferInt) heightMap.getDataBuffer()).getData(); I get a bunch of huge negative numbers.

I have searched for a while and seen some references to shifting some values (post) but don't really understand what they are saying.

Basically I want to go from a BufferedImage to an array of 0 to 255 RBG values.
I'm not sure I explained myself properly, plaese ask for more details is needed.

Edit:
@Garbage Thanks for the tip. I tried int[] dataBuffInt = heightMap.getRGB(0, 0, heightMap.getWidth(), heightMap.getHeight(), null, 0, heightMap.getWidth()); But get the same result.
@Greg Kopff The result is 2 and it was set to TYPE_INT_ARGB

Answer 1

You get negative numbers since the int value you get from one of the pixels are composed by red, green, blue and alpha. You need to split the colors to get a value for each color component.

The simplest way to do this is to create a Color object and use the getRed, getGreen and getBlue (aswell as getAlpha) methods to get the components:

public static void main(String... args) throws Exception {

    BufferedImage image = ImageIO.read(
         new URL("http://upload.wikimedia.org/wikipedia/en/2/24/Lenna.png"));

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

    int[] dataBuffInt = image.getRGB(0, 0, w, h, null, 0, w); 

    Color c = new Color(dataBuffInt[100]);

    System.out.println(c.getRed());   // = (dataBuffInt[100] >> 16) & 0xFF
    System.out.println(c.getGreen()); // = (dataBuffInt[100] >> 8)  & 0xFF
    System.out.println(c.getBlue());  // = (dataBuffInt[100] >> 0)  & 0xFF
    System.out.println(c.getAlpha()); // = (dataBuffInt[100] >> 24) & 0xFF
}

Outputs:

173
73
82
255