I am designing a GUI with Java Swing and AWT (Java 8) and am struggling with the icons I use.

I load a large PNG image and scale it to 18x18px and then use it in a button or label. It works well in all resolutions when the operating system does not zoom in.

However, with the advent of large screen resolutions (hidpi), it is common practice to use operating system settings to zoom in on user interface controls, including buttons and such things in Java applications. For example, on Windows I use a 150% or 200% scaling of user elements with my 4K resolution to ensure the user interface is still usable. I imagine many users will do so as well.

When that is the case, however, the icons are merely increased in size after already scaling them down to 18x18px. That is, I first scale them down and then the operating system tries to scale them up again with the little information that is still left in the image.

Is there any way to design image icons in Java that are based on a higher resolution when the zooming/scaling capabilities of the operating system are used in order to avoid them appearing blurred?

Here is a working example:

import java.awt.Container;
import java.awt.Image;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

@SuppressWarnings("serial")
class Example extends JFrame {

    public static void main(String[] args) {
        new Example();
    }
    
    public Example() {
        Container c = getContentPane();
        JPanel panel = new JPanel();
        ImageIcon icon = new ImageIcon(new ImageIcon(getClass().getResource("tabler-icon-beach.png")).getImage().getScaledInstance(18, 18, Image.SCALE_SMOOTH));
        JButton button = new JButton("Test button", icon);
        panel.add(button);
        c.add(panel);
        this.pack();
        this.setLocationRelativeTo(null);
        this.setVisible(true);
    }
}

You can find the icon here. All icons are available as PNG or SVG files.

To illustrate the problem, let me first show you two screenshots in the normal 100% screen resolution:

On Linux with 100% zoom:

On Windows with 100% zoom:

And now when I set Windows 7 to have a 200% magnification of layout elements, it's obviously just the 18x18px version stretched out, which becomes blurred:

Is there any way to provide a higher-resolution image icon that is used when the operating system uses a scaling that is larger than 100%? Moreover, you can see that even at 100% the image quality is not perfect; is there any way to improve that as well?

Java 8 does not support High DPI, the UI gets scaled up by Windows. You should use Java 11 or a later version which support per-monitor High DPI settings.

If your goal is to make the icons look crisp, prepare a set of icons for different resolutions using BaseMultiResolutionImage (the basic implementation of MultiResolutionImage) to provide higher resolution alternatives. (These are not available in Java 8.)

You say that you scaled down the original image (240×240) to 18×18px. If the UI needs a higher resolution according to the system setting, all it has now is your small icon (18×18) which will be scaled up, which results in poor quality. You should use a MultiResolutionImage or paint the original image into the required size, letting Graphics to scale it down for you.

No Down-Scale

This is the simplest way I came up with to make the icon 18×18 without downscaling the original image:

private static final String IMAGE_URL =
        "https://tabler-icons.io/static/tabler-icons/icons-png/beach.png";

private static ImageIcon getIcon() {
    return new ImageIcon(Toolkit.getDefaultToolkit()
                                .getImage(new URL(IMAGE_URL))) {
        @Override
        public int getIconWidth() {
            return 18;
        }
        @Override
        public int getIconHeight() {
            return 18;
        }

        @Override
        public synchronized void paintIcon(Component c, Graphics g,
                                           int x, int y) {
            g.drawImage(getImage(), x, y, 18, 18, null);
        }
    };
}

I left out the exception handling code for MalformedURLException which can be thrown from the URL constructor.

In this case, the painted image gets down-scaled each time it's painted, which is ineffective. Yet the quality is better. Well, for the standard resolution screen, it's nearly the same as if you down-scaled the image when loading. But in High DPI case, it looks better. It's because for 200% UI Scale, the image will be rendered to 36×36 pixels and these pixels will be created from the source of 240×240 rather than up-scaling the down-scaled version which lost its quality.

To get even better results, I recommend using MultiResolutionImage.

MultiResolutionImage

The app below loads the images from base64-encoded strings (for simplicity so that there are no external dependencies). There are three variants provided: 24×24 (100%, 96dpi), 36×36 (150%, 144dpi), 48×48 (200%, 192dpi).

If the current scale factor is set to any of the provided resolutions, the image will be rendered as is. If 125% or 175% are used, the larger image will be scaled down; if the scale is greater than 200%, then the image for 200% will be scaled up. You can add more resolutions if needed.

The app doesn't compile in Java 8 because MultiResolutionImage is not available there. To compile it with JDK 11, you have to replace text blocks with regular String concatenation.

import java.awt.Image;
import java.awt.image.BaseMultiResolutionImage;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.util.Arrays;
import java.util.Base64;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class BeachIconButton {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(BeachIconButton::new);
    }

    private BeachIconButton() {
        JPanel panel = new JPanel();
        ImageIcon icon = getIcon();
        JButton button = new JButton("Test button", icon);
        panel.add(button);

        JFrame frame = new JFrame("Beach Icon Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(panel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private static ImageIcon getIcon() {
        return new ImageIcon(
               new BaseMultiResolutionImage(
               Arrays.stream(new String[] { BEACH_100, BEACH_150, BEACH_200})
                     .map(BeachIconButton::loadImage)
                     .toArray(Image[]::new)));
    }

    private static Image loadImage(String base64) {
        try {
            return ImageIO.read(new ByteArrayInputStream(
                                Base64.getMimeDecoder().decode(base64)));
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }

    private static final String BEACH_100 = """
            iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAYAAADgdz34AAAACXBIWXMAAA7DAAAO
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    private static final String BEACH_150 = """
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    private static final String BEACH_200 = """
            iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAYAAABXAvmHAAAACXBIWXMAAB2HAAAd
            hwGP5fFlAAAAGXRFWHRTb2Z0d2FyZQB3d3cuaW5rc2NhcGUub3Jnm+48GgAAA5FJ
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}