Parsing Image in Java with different actions for different pixel colours

Question

I want to colour every black pixel in my image with the color the previous pixel had. And if there are 2 or more consecutive black pixels, the colour of the last non-black pixel is taken. I figured out how to iterate through pixels but the pixels won't change their colour. I think I miss the "save changes" line.

Here the code:

public static void iteratePixels() throws IOException {
    File file = new File("C:\\blackDots.png");
    BufferedImage image = ImageIO.read(file);
    int lastNotBlack = -1;
    int actualColour = 0;

    for (int x = 0; x < image.getHeight(); x++) {
        for (int y = 0; y < image.getWidth(); y++) {

            int black = -16777216;

            try {
                actualColour = image.getRGB(x, y);

            } catch (Exception e) {
                continue;
            }
            if(image.getRGB(x, y)==black){
                image.setRGB(x, y, lastNotBlack);

                System.out.println("black pixel at: " +x +" "+y);
            }
            if (actualColour != black){
                lastNotBlack= actualColour;
            }

        }
    }
}

So how do I appy the changes ? Or is there another mistake?

Answer 1

You're changing the pixels only in the in-memory image, but you need to write those pixels back to a file:

ImageIO.write(image, "png", new File("C:\\blackDots_modified.png"));

(to be called after all pixels have been modified)

See also: https://docs.oracle.com/javase/tutorial/2d/images/saveimage.html