int[][] representing a grayscale image, how to convert from signed to unsigned values

Question

EDIT 3:

    int rRows = result.length;
    int rColums = result[0].length;

    BufferedImage img = new BufferedImage(rColums, rRows, BufferedImage.TYPE_BYTE_GRAY);
    for (int r = 0; r < rRows; r++) {
        for (int t = 0; t < result[r].length; t++) {
            img.setRGB(t, r, result[r][t]);   

EDIT2:

Created the image like so....

 BufferedImage img = new BufferedImage(rColums, rRows,BufferedImage.TYPE_BYTE_GRAY)  


private static int[][] convertToArray(BufferedImage inputImage) {

final byte[] pixels = ((DataBufferByte) inputImage.getRaster().getDataBuffer()).getData(); 


final int width = inputImage.getWidth();

final int height = inputImage.getHeight();  
System.out.println("height" + height + "width");

int[][] result = new int[height][width];

for (int pixel = 0, row = 0, col = 0; pixel < pixels.length; pixel++)     {

 int argb = 0;

  argb = (int) pixels[pixel];


result[row][col] = argb;

col++;

if (col == width) {

 col = 0; 

 row++;

 }

 }

 return result;

                                                                                            

Edit:

I've realized what I'm trying to ask is how to go from signed grayscale to unsigned grayscale, as I said adding 256 didnt work for me and also that would still seem to leave the image to dark as it wont raise the value of the +127 signed values to 256 unsigned.(hopefully I've expressed that correctly.

As per title I've an int[][] extracted from a buffered image via

 ((DataBufferByte) inputImage.getRaster().getDataBuffer()).getData()

The array ranges from -128 to 127. The problem is that when I attempt to reconstruct the image based on the int[][] by passing it to BufferedImage, it comes out too dark, and more like a black and white(mostly black) image.

I saw suggestion to add 256 to each sub zero value of the byte[] produced by the DataBufferByte, in the process of converting byte[] to in[][], but this actually produces a totally black image and I dont really get the logic of it, Like wouldnt you want to shift the entire scale over by 128, rather than just the negative numbers??

Answer 1

When you're writing your conversion. You have a signed byte, and you need to convert it to an ARGB int.

int unsignedByte = pixels[pixel]&0xff;

Now that isn't quite finished because we need it to be argb/grayscale.

argb = ( unsignedByte << 16 ) + ( unsignedByte << 8 ) + unsignedByte;

I've ignored the A part and just added rgb components. There is quite some documentation on this though.

Here is a complete example you can play with.

import java.awt.image.*;
import java.awt.*;
import javax.swing.*;
import java.util.HashSet;
import java.util.Set;
public class GrayScales{
    public static void main(String[] args){
        BufferedImage img = new BufferedImage( 256, 256, BufferedImage.TYPE_BYTE_GRAY );
        Graphics g = img.createGraphics();
        g.setColor( new Color( 128, 128, 128) );
        g.fillOval( 32, 32, 192, 192 );
        g.setColor( new Color( 255, 255, 255) );
        g.fillOval( 96, 96, 64, 64 );
        g.dispose();
        BufferedImage dup = new BufferedImage( 256, 256, BufferedImage.TYPE_BYTE_GRAY );
        byte[] pixels = ((DataBufferByte) img.getRaster().getDataBuffer()).getData();
        Set<Integer> ints = new HashSet<>();
        for(int i = 0; i<256; i++){
            for(int j = 0; j<256; j++){
                int px = (pixels[j*256 + i])&0xff;
                ints.add(px);
                int rgb = (px << 16) + (px << 8) + (px);
                dup.setRGB(i, j, rgb);
            }
        }
        System.out.println(ints);
        JFrame frame = new JFrame("compare");
        JLabel a = new JLabel( new ImageIcon( img ) );
        JLabel b = new JLabel( new ImageIcon(dup) );
        
        frame.add(a, BorderLayout.EAST);
        frame.add(b, BorderLayout.WEST);
        frame.pack();
        frame.setVisible(true);
    }
}