How to convert a BufferedImage to black and white?

Question

How can I convert an existing colored BufferedImage to monochrome? I want the image to be completely split between only two rgb codes, black and white. So if there's a border around the image which is a lighter or darker shade of the background, and the background is being converted to white, then I want the border to be converted to white as well, and so on.

How can I do this?

If its necessary to save the image / load it from disk, I can do that if necessary.

Edit: Code to test this:

package test;

import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.util.HashSet;
import javax.imageio.ImageIO;

public class Test
{
    public static void main(String[] args)
    {
        try
        {
            BufferedImage img = ImageIO.read(new File("https://i.sstatic.net/yhCnH.png") );
            BufferedImage gray = new BufferedImage(img.getWidth(), img.getHeight(),
                    BufferedImage.TYPE_BYTE_GRAY);

            Graphics2D g = gray.createGraphics();
            g.drawImage(img, 0, 0, null);

            HashSet<Integer> colors = new HashSet<>();
            int color = 0;
            for (int y = 0; y < gray.getHeight(); y++)
            {
                for (int x = 0; x < gray.getWidth(); x++)
                {
                    color = gray.getRGB(x, y);
                    System.out.println(color);
                    colors.add(color);
                }
            }

            System.out.println(colors.size() );
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

Image used:

Using this, I get the output of 24 as the last output, i.e there were 24 colors found in this image, when there should be 2.

Fascicule

Like this? – Outlandish

@AndrewThompson Yes, like the grayscale example in that one! Will it completely convert all colors to gray & white? – Fascicule

@AndrewThompson Well, I've tried it, but other colors within the image remain, its not completely gray / white. Any other suggestions? – Fascicule

@AndrewThompson Its resolved now anyway. If you want to post an answer with what you've mentioned and TYPE_BYTE_BINARY , I can accept as it was with your help that I found it. – Fascicule

@AndrewThompson But you were the one who pointed me to it, I won't have gotten it without your help. So, you can post your answer and I'll delete mine and accept yours. :) – Fascicule

@AndrewThompson I didn't come up with it independently, I found it within the answer that you linked! – Fascicule

@AndrewThompson All right, I'll just accept mine when it lets me :) – Fascicule

@AndrewThompson If you might be able to answer this one, it'd be great. #18942281 – Fascicule

Answer 1

The solution was to use BufferedImage.TYPE_BYTE_BINARY as the type rather than TYPE_BYTE_GRAY. Using BINARY causes a pure black & white copy of the image to be created, with no colors except b & w. Not pretty looking, but perfect for things like OCR where the colors need to be exact.

Fascicule

You should consider using an advanced binarization method like Sauvola's, if you have poor quality scans. fiji.sc/wiki/index.php/Auto_Local_Threshold – Wattenberg