如何在 JLabel 中绘制现有图像？

Question

I am trying to draw a grid over an existing image which is already displayed in a JLabel.

However, my code does not seem to do anything.

This is the method I wrote:

public void drawGrid(final JLabel label, final int rowAmount, final int columnAmount) {
    ImageIcon icon = (ImageIcon)label.getIcon();
    BufferedImage tempImage = new BufferedImage(icon.getIconWidth(), icon.getIconHeight(), BufferedImage.TYPE_INT_ARGB);
    Graphics2D g2d = tempImage.createGraphics();
    g2d.setColor(Color.GREEN);
    BasicStroke bs = new BasicStroke(2);
    g2d.setStroke(bs);
    int width = tempImage.getWidth();
    int height = tempImage.getHeight();
    System.out.println("Width: " + width + ", Height: " + height);
    int x1 = 0;
    int y1;
    int x2 = width;
    int y2;
    for (int i = 1; i < rowAmount; i++) {
        y1 = (height * i) / rowAmount;
        y2 = y1;
        g2d.drawLine(x1, y1, x2, y2);
        System.out.println("Drawing line from (" + x1 + ", " + y1 + ") to (" + x2 + ", " + y2 + ")");
    }
    y1 = 0;
    y2 = height;
    for (int i = 1; i < columnAmount; i++) {
        x1 = (width * i) / columnAmount;
        x2 = x1;
        g2d.drawLine(x1, y1, x2, y2);
        System.out.println("Drawing line from (" + x1 + ", " + y1 + ") to (" + x2 + ", " + y2 + ")");
    }
    icon.paintIcon(label, g2d, 0,0);
    g2d.dispose();
    label.setIcon(new ImageIcon(tempImage));
}

When calling drawGrid(label, 10, 10); the prints show the correct coordinates, but nothing happens.

I already checked whether a manual label.repaint() at the end would do the trick, but Label.setIcon(ImageIcon) already repaints on call and this did not fix it.

Can anyone explain to me what went wrong and how to solve this issue?

Also, I think that the conversions and casts between ImageIcon , Icon , BufferedImage and Graphics2D may be inefficient or faulty, but I am unexperienced with those.

Answer 1

I realize that the question now kind of has been answered already, but here is a fixed and improved version of your method:

public void drawGrid(JLabel lbl, int rows, int columns) {
        Image img = ((ImageIcon) lbl.getIcon()).getImage();
        Graphics2D g2d = (Graphics2D) img.getGraphics();
        g2d.setStroke(new BasicStroke(2));
        g2d.setColor(Color.GREEN);

        int width = img.getWidth(null) / columns;
        int height = img.getHeight(null) / rows;

        for (int i = 1; i < columns; i++) {
            g2d.drawLine(width * i, 0, width * i, img.getHeight(null));
        }
        for (int i = 1; i < rows; i++) {
            g2d.drawLine(0, height * i, img.getWidth(null), height * i);
        }
    }

Oh and if your image has a different size than your label, the lines will be centered on the image and therefore appear offset on the label. In that case just change lines 7 and 8 to the following:

int width = lbl.getWidth() / columns;
int height = lbl.getHeight() / rows;