如何将彩色图像转换为纯黑白图像（0-255格式）

Question

public class BlackWhite {

    public static void main(String[] args) 
    {
        try 
        {
         BufferedImage original = ImageIO.read(new File("colorimage"));
         BufferedImage binarized = new BufferedImage(original.getWidth(), original.getHeight(),BufferedImage.TYPE_BYTE_BINARY);

         int red;
         int newPixel;
         int threshold =230;

            for(int i=0; i<original.getWidth(); i++) 
            {
                for(int j=0; j<original.getHeight(); j++)
                {

                    // Get pixels
                  red = new Color(original.getRGB(i, j)).getRed();

                  int alpha = new Color(original.getRGB(i, j)).getAlpha();

                  if(red > threshold)
                    {
                        newPixel = 0;
                    }
                    else
                    {
                        newPixel = 255;
                    }
                    newPixel = colorToRGB(alpha, newPixel, newPixel, newPixel);
                    binarized.setRGB(i, j, newPixel);

                }
            } 
            ImageIO.write(binarized, "jpg",new File("blackwhiteimage") );
         }
        catch (IOException e) 
        {
                e.printStackTrace();
        }    
    }

     private static int colorToRGB(int alpha, int red, int green, int blue) {
            int newPixel = 0;
            newPixel += alpha;
            newPixel = newPixel << 8;
            newPixel += red; newPixel = newPixel << 8;
            newPixel += green; newPixel = newPixel << 8;
            newPixel += blue;

            return newPixel;
        }
}

I got a black and white output image, but as I zoomed the image I discovered some gray area. I want the output image to only contain the colors black or white.

Please let me know if I am correct or incorrect in my current approach? And if I am, please suggest another way.

Answer 1

You are converting the image correctly from color to black and white; however, when you save the output as JPEG some colors are created as a result of the lossy compression .

Simply save the output to PNG (or anything other than JPEG ), and the output will be only black and white, as you had expected.

ImageIO.write(binarized, "png",new File("blackwhiteimage") );

For example, if you have a binary image that saved as PNG the histogram can be something like (strictly black and white pixels only):

And for the same image if you saved as JPEG , you can see that in the histogram some pixels near the white and black color start to appear