CODEWITHSUNDEEP
javaswingcolorspixelbufferedimage
Saiyan
Saiyan

Reputation: 197

Get colour of pixel on a BufferedImage

I want to get the colour of a pixel on a BufferedImage. I set the background of the BufferedImage to white and I draw a line from (100, 100) to (100, 200) on the BufferedImage. Then, I draw the BufferedImage onto a JPanel. There is the line but the background is not white. Why?

Also, the getRGB method returns 0 for R, G and B, even if it is not getRGB(100, 100). What is wrong?

The code:

public class PixelColour extends JPanel{

    public void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        BufferedImage bi = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB);
        Graphics2D gbi = bi.createGraphics();
        gbi.setColor(Color.black);
        gbi.setBackground(Color.white);
        gbi.drawLine(100, 100, 100, 200);
        g2.drawImage(bi, null, 0, 0);
        int rgb = bi.getRGB(100, 100);
        int red = (rgb >> 16) & 0xFF;
        int green = (rgb >> 8) & 0xFF;
        int blue = (rgb & 0xFF);
        System.out.println(red + " " + green + " " + blue);
    }

    public static void main(String[] args) throws IOException{
        PixelColour pc = new PixelColour();
        JFrame frame = new JFrame("Pixel colour");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(pc);
        frame.setSize(500,500);
        frame.setVisible(true);
    }
}

Upvotes: 2

Views: 365

Answers (1)

Karl
Karl

Reputation: 437

After gbi.setBackground(Color.white) add gbi.clearRect(0,0,bi.getWidth(), bi.getHeight());

clearRect() paints the background color onto the image. If you just set the a new background color it doesn't change the image.

public void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    BufferedImage bi = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB);
    Graphics2D gbi = bi.createGraphics();
    gbi.setColor(Color.black);
    gbi.setBackground(Color.white);

    // here
    gbi.clearRect(0, 0, bi.getWidth(), bi.getHeight());

    gbi.drawLine(100, 100, 100, 200);
    g2.drawImage(bi, null, 0, 0);
    int rgb = bi.getRGB(50, 50);    // off the black line
    int red = (rgb >> 16) & 0xFF;
    int green = (rgb >> 8) & 0xFF;
    int blue = (rgb & 0xFF);
    System.out.println(red + " " + green + " " + blue);
}

This prints

255 255 255
255 255 255

Upvotes: 4

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