grayscale bitmap into 2d array

Question

Hi everyone i have problems in converting GrayScale bmp images into integer 2D-array (with values 0-255) in Java.

I have a pmb image that could be seen as an integer(0-255) 2D-array and i want to see that 2D-array in a Java data structure

i tried this way:

Image image = ImageIO.read(new File("my_img.bmp"));
BufferedImage img = new BufferedImage(image.getWidth(null), image.getHeight(null), BufferedImage.TYPE_BYTE_GRAY);
Graphics g = img.createGraphics();
g.drawImage(image, 0, 0, null);
g.dispose();

Then with my BufferedImage i create int[][] this way:

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);
    }
}

But now all the 2D-array is full of number like "-9211021" or similar.

i think that the problem is in getRGB(j,k) but i don't know if it's possible to solve it.

edit:

i know RGB is not grayscale, so how can i get the grayscale value of a single pixel from a grayscale BufferedImage?

RGB is not greyscale, take a look at docs.oracle.com/javase/6/docs/api/java/awt/image/… — arynaq
– arynaq, Commented Jun 24, 2013 at 15:35

Zac · Accepted Answer · 2014-04-18 01:18:30Z

1

In a grayscale image, BufferedImage.getPixel(x,y) wont give values within the [0-255] range. Instead, it returns the corresponding value of a gray level(intensity) in the RGB colorspace. That's why you are getting values like "-9211021".

The following snippet should solve your problem :

Raster raster = image.getData();
for (int j = 0; j < w; j++) {
    for (int k = 0; k < h; k++) {
        array[j][k] = raster.getSample(j, k, 0);
    }
}

where image is the created BufferedImage. The 0 in the getSample indicates that we are accessing the first byte/band(setting it to a greater value will throw a ArrayOutOfBoundException in grayscale images).

edited Apr 18, 2014 at 1:18

Zac

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answered Jul 10, 2013 at 12:54

blackSmith

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2 Comments

drstein Over a year ago

thanks for the reply, but i solved in one other way, by extracting a single channer from the RGB value in this way: array[i][j] = (bmp.getRGB(i, j) >> 16) & 0xFF;

blackSmith Over a year ago

Then you used RED channel(>>16). If that suffices your need, it's okay. However, to obtain graylevel from RGB, usually all three channel values are extracted and converted as : I = .299 * R + .587 * G + .114 * B only ~34% of the R value is considered for the intensity.

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