Reading pixels from images using Java

Question

I'm trying to convert from RGB to GrayScale Image.

The method that does this task is the following:

public BufferedImage rgbToGrayscale(BufferedImage in) { int width = in.getWidth(); int height = in.getHeight(); BufferedImage grayImage = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY); WritableRaster raster = grayImage.getRaster(); int [] rgbArray = new int[width * height]; in.getRGB(0, 0, width, height, rgbArray, 0, width); int [] outputArray = new int[width * height]; int red, green, blue, gray; for(int i = 0; i < (height * width); i++) { red = (rgbArray[i] >> 16) & 0xff; green = (rgbArray[i] >> 8) & 0xff; blue = (rgbArray[i]) & 0xff; gray = (int)( (0.30 * red) + (0.59 * green) + (0.11 * blue)); if(gray < 0) gray = 0; if(gray > 255) gray = 255; outputArray[i] = (gray & 0xff); } } raster.setPixels(0, 0, width, height, outputArray); return grayImage; }

I have a method that saves the pixels value in a file:

public void writeImageValueToFile(BufferedImage in, String fileName) { int width = in.getWidth(); int height = in.getHeight(); try { FileWriter fstream = new FileWriter(fileName + ".txt"); BufferedWriter out = new BufferedWriter(fstream); int [] grayArray = new int[width * height]; in.getRGB(0, 0, width, height, grayArray, 0, width); for(int i = 0; i < (height * width); i++) { out.write((grayArray[i] & 0xff) + "\n"); } out.close(); } catch (Exception e) { System.err.println("Error: " + e.getMessage()); } }

The problem that I have is that, the RGB value I get from my method, is always bigger than the expected one.

I created an image and I filled it with color 128, 128, 128. According to the first method, if I print the outputArray's data, I get:
r, g, b = 128, 128, 128. Final = 127 ---> correct :D

However, when I called the second method, I got the RGB value 187 which is incorrect.

Any suggestion?

Thanks!!!

Why not create another BufferedImage that is grayscale, draw the original one into it, and then grab the pixel values from the graycale bufferedimage — MeBigFatGuy
– MeBigFatGuy, Commented Aug 2, 2011 at 0:58

Joshua · Accepted Answer · 2011-08-01 23:43:19Z

Take a look at javax.swing.GrayFilter, it uses the RBGImageFilter class to accomplish the same thing and has very similar implementation. It may make your life simpler.

Charles Goodwin · Accepted Answer · 2011-08-01 22:16:07Z

I'm not an expert at these things but aren't RGB values stored as hex (base16)? If so, theproblem lies in your assumption that the operation & 0xff will cause your int to be stored/handled as base16. It is just a notation and default int usage in strings will always be base10.

 int a = 200; a = a & 0xff; System.out.println(a); // output 200

You need to use an explicit base16 toString() method.

 System.out.println(Integer.toHexString(200)); // output c8

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