reading black/white image in java with TYPE_USHORT_GRAY

Question

I have the following code to read a black-white picture in java.

imageg = ImageIO.read(new File(path));    
BufferedImage bufferedImage = new BufferedImage(image.getWidth(null), image.getHeight(null),  BufferedImage.TYPE_USHORT_GRAY);
        Graphics g = bufferedImage.createGraphics();
        g.drawImage(image, 0, 0, null);
        g.dispose();
        int w = img.getWidth();
        int h = img.getHeight();
        int[][] array = new int[w][h];
        for (int j = 0; j < w; j++) {
            for (int k = 0; k < h; k++) {
                array[j][k] = img.getRGB(j, k);
                System.out.print(array[j][k]);
            }
        }

As you can see I have set the type of BufferedImage into TYPE_USHORT_GRAY and I expect that I see the numbers between 0 and 255 in the two D array mattrix. but I will see '-1' and another large integer. Can anyone highlight my mistake please?

It's due the `getRGB` method. If you interested in binary(pure B/W) image have a look at http://stackoverflow.com/questions/5925426/java-how-would-i-load-a-black-and-white-image-into-binary — blackSmith, Sep 29 '14 at 06:23
And if your intention is to read as GrayScale you'll need a conversion formula. http://stackoverflow.com/questions/687261/converting-rgb-to-grayscale-intensity — blackSmith, Sep 29 '14 at 06:29

score 0 · Answer 1 · answered Sep 29 '14 at 07:24

A BufferedImage of type TYPE_USHORT_GRAY as its name says stores pixels using 16 bits (size of short is 16 bits). The range 0..255 is only 8 bits, so the colors may be well beyond 255.
And BufferedImage.getRGB() does not return these 16 pixel data bits but quoting from its javadoc:

Returns an integer pixel in the default RGB color model (TYPE_INT_ARGB) and default sRGB colorspace.

getRGB() will always return the pixel in RGB format regardless of the type of the BufferedImage.

Icza, Thanks also for your explanation about TYPE_SHORT_GRAY. I thaught it is 8 bit. — Mohammadreza, Oct 05 '14 at 01:05

score 0 · Accepted Answer · answered Sep 29 '14 at 08:52

As already mentioned in comments and answers, the mistake is using the getRGB() method which converts your pixel values to packed int format in default sRGB color space (TYPE_INT_ARGB). In this format, -1 is the same as ´0xffffffff`, which means pure white.

To access your unsigned short pixel data directly, try:

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

DataBufferUShort buffer = (DataBufferUShort) img.getRaster().getDataBuffer(); // Safe cast as img is of type TYPE_USHORT_GRAY 

// Conveniently, the buffer already contains the data array
short[] arrayUShort = buffer.getData();

// Access it like:
int grayPixel = arrayUShort[x + y * w] & 0xffff;

// ...or alternatively, if you like to re-arrange the data to a 2-dimensional array:
int[][] array = new int[w][h];

// Note: I switched the loop order to access pixels in more natural order
for (int y = 0; y < h; y++) {
    for (int x = 0; x < w; x++) {
        array[x][y] = buffer.getElem(x + y * w);
        System.out.print(array[x][y]);
    }
}

// Access it like:
grayPixel = array[x][y];

PS: It's probably still a good idea to look at the second link provided by @blackSmith, for proper color to gray conversion. ;-)

@DanielMårtensson You can always cast to a more specific type at compile time. Given that OPs image is `TYPE_USHORT_GRAY`, that is a safe cast also at run time. If it doesn’t work for you, your image probably has a different type. — Harald K, May 14 '20 at 16:47
@DanielMårtensson I see from one of your questions that you probably have a `TYPE_BYTE_GRAY` image. In that case, the cast must be to `DataBufferByte` and your array will be `byte[]`. With values in the 0...255 range. — Harald K, May 14 '20 at 16:53

reading black/white image in java with TYPE_USHORT_GRAY

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