Clipping is jagged despite anti-aliasing

Question

I am trying to clip my Graphics2D canvas using RoundRectangle2D.Double, but the clipping is very jagged and not smooth. I have the following code to anti-alias:

Graphics2D g = (Graphics2D)graphics;
g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);

I know it is working because when I draw a RoundRectangle2D.Double using Graphics2D.fill() the smoothing is fine. How do I make the clipping smooth?

Note: I am aware of this post, but this pertains to JPanels and images, but I am not dealing with either of those. I am just trying to smoothly clip a section of the drawing area.

Thanks in advance for any help.

Example.java

import java.awt.*;
import java.awt.geom.RoundRectangle2D;

import javax.swing.*;

public class Example extends JPanel {
    public Example() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setLocation(10, 10);
        setPreferredSize(new Dimension(400, 400));
        setBackground(Color.BLACK);
        Container container = frame.getContentPane();
        container.add(this);
        frame.pack();
        frame.setVisible(true);
    }

    public void paintComponent(Graphics graphics) {
        super.paintComponent(graphics);
        Graphics2D g = (Graphics2D)graphics;
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        RoundRectangle2D clippingArea = new RoundRectangle2D.Double(50, 50, getWidth() - 100, getHeight() -100, 40, 40);
        g.setClip(clippingArea);

        g.setColor(Color.WHITE);
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(Color.BLACK);
        String s = "area for me to draw on where";
        g.drawString(s, getWidth()/2 - g.getFontMetrics().stringWidth(s)/2, getHeight()/2 - g.getFontMetrics().getHeight());
        s = "the roundrectangle should be anti-aliased";
        g.drawString(s, getWidth()/2 - g.getFontMetrics().stringWidth(s)/2, getHeight()/2);
    }

    public static void main(String[] args) {
        new Example();
    }
}

Answer 1

So, based on your example, the "simple" answer is, cheat.

RoundRectangle2D clippingArea = new RoundRectangle2D.Double(50, 50, getWidth() - 100, getHeight() - 100, 40, 40);
// Make the clipping space rectangular
g.setClip(clippingArea.getBounds2D());

g.setColor(Color.WHITE);
g.fill(clippingArea);
g.setColor(Color.BLACK);

Basically all this does it makes the clipping area rectangular and then fills it using the RoundRectangle2D shape.

Because most of your issues revolve around the edges, this eliminates the core problem, the round edges.

There are different ways you "might" solve the issue, using intermediate BufferedImages is one (so you can constraint the content to the specified area, but still apply "soft clipping"), but for this context, this would be the immediately simplest solution available to you

Answer 2

There are two ideas for the problem you raised.

The first idea
Set the clip with the setClip() method and draw in a specific range using the methods of the Graphics class. This was an idea that you also proposed.

The second idea
Create a BufferedImage object that has a graphicsObj field of type Graphics. Set a Rendering Hint for the Graphics object like below:

graphicsObj.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

Draw an oval with the fillOval() method. Then, using the setComposite() method, set an AlphaComponent object for the Graphics object as follows:

AlphaComposite alphaComposite = AlphaComposite.getInstance(AlphaComposite.SRC_IN);
graphicsObj.setComposite(alphaComposite);

Then draw something using a method like drawImage or any other method. this BufferedImage is what you want. Just don't forget to use the following Rendering Hint wherever you want to draw the BufferedImage:

setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);

Example:

public void paint(Graphics g) {
            super.paint(g);

            BufferedImage bufferedImage = new BufferedImage(50, 50, BufferedImage.TYPE_INT_ARGB);

            Graphics2D bg2 = bufferedImage.createGraphics();

            bg2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            bg2.fillOval(0, 0, bufferedImage.getWidth(), bufferedImage.getHeight());

            AlphaComposite alphaComposite = AlphaComposite.getInstance(AlphaComposite.SRC_IN);
            bg2.setComposite(alphaComposite);

            Image img = null;
            
            try {
                img = ImageIO.read(new File("some-image.png"));
            } catch (IOException e) {
                e.printStackTrace();
            }
            
            if (img == null) {
                // TODO something
            } else {
                bg2.drawImage(img, 0, 0, bufferedImage.getWidth(), bufferedImage.getHeight(), null);
            }

            Graphics2D g2 = (Graphics2D) g;
            
            g2.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);

                g2.drawImage(bufferedImage, 0, 0, getWidth(), getHeight(), null);
        }
    }

The second idea result: The second idea result