How to Efficiently Scale Images in a JLabel to Fit or Fill its Dimensions?

Automatic Image Scaling for JLabel

When displaying images within a JFrame using a JLabel, you may encounter situations where the image size is inconsistent. To uniformly scale and fit the images within the JLabel, you can leverage the following approach.

Avoid Using getScaledInstance

While Image.getScaledInstance is a common choice, it's not the most efficient or highest quality option. Instead, a custom component can be created to enhance both speed and image clarity.

Determine the Scaling Method

You have the flexibility to choose the scaling method, whether you want to maintain the aspect ratio with "toFit" or to completely fill the available space with "toFill."

Custom Component for Scalable Images

The following custom component, ScalablePane, allows you to set the scaling mode and automatically adjust the image size based on the label's dimensions.

public class ScalablePane extends JPanel {
    // Scaled image
    private Image scaled;
    // Scaling mode: fit or fill
    private boolean toFit;

    // Constructor with image and scaling mode
    public ScalablePane(Image master, boolean toFit) {
        this.toFit = toFit;
        generateScaledInstance(); // Initial scaling
    }

    // Override getPreferredSize to return master image size
    @Override
    public Dimension getPreferredSize() {
        return master == null ? super.getPreferredSize() : new Dimension(master.getWidth(this), master.getHeight(this));
    }

    // Override paintComponent to draw the scaled image
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(scaled, 0, 0, this);
    }

    // Generate the scaled image based on the scaling mode
    protected void generateScaledInstance() {
        if (isToFit()) {
            scaled = getScaledInstanceToFit(master, getSize());
        } else {
            scaled = getScaledInstanceToFill(master, getSize());
        }
    }

    // Generate toFit image
    protected Image getScaledInstanceToFit(Image master, Dimension size) {
        Dimension masterSize = new Dimension(master.getWidth(this), master.getHeight(this));
        return getScaledInstance(master, getScaleFactorToFit(masterSize, size));
    }

    // Generate toFill instance
    protected Image getScaledInstanceToFill(Image master, Dimension size) {
        Dimension masterSize = new Dimension(master.getWidth(this), master.getHeight(this));
        return getScaledInstance(master, getScaleFactorToFill(masterSize, size));
    }

    // Actual scaling logic
    protected Image getScaledInstance(Image master, double scaleFactor) {
        BufferedImage img = toBufferedImage(master);
        return getScaledInstance(img, scaleFactor, RenderingHints.VALUE_INTERPOLATION_BILINEAR, true);
    }
    // ... additional helper methods and utility functions omitted for brevity
}

How to Use

To use this approach, create a ScalablePane instance with your desired image and scaling mode, add it to your label, and then set the preferred size of the label. The custom component will automatically scale and display the image as necessary.

ImageIcon img = new ImageIcon("res.png");
JLabel label = new JLabel(img);
label.setPreferredSize(new Dimension(200, 100));
label.add(new ScalablePane(img, true));

By leveraging this custom component, you can achieve automatic and efficient image scaling to match the dimensions of your JLabel, ensuring consistent image representation regardless of its original size.

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