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!!!

Answer 1

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.

Answer 2

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