Java Graphics2D floating point accurate drawOval a

Question

So I am trying to draw an arc and put a circle around its round endpoint, but I am having issues due to rounding to the nearest pixel. This is visible in some but not all cases.

Is there a way to draw circles using floating points and anti-aliasing to eliminate this rounding error?

You can run this code to see the problem. I have drawn arcs of 0 length (appearing as large dots) instead of full arcs for clarity.

import java.awt.*;
import javax.swing.*;

public class Example extends JFrame {

    private int CENTER = 200;
    private static int WINDOW = 400;

    private int LEFT = 50;
    private int TOP = 50;
    private int DIM = 300;
    private int DIAMETER = 26;

    public Example () {
        super();
    }

    public void paint (Graphics g) {
        Graphics2D g2 = (Graphics2D) g;

        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        g2.setStroke(new BasicStroke(16, BasicStroke.CAP_ROUND, BasicStroke.JOIN_MITER));
        g2.setColor(new Color(0, 0, 255));

        g2.drawArc(LEFT, TOP, DIM, DIM, 0, 0);
        g2.drawArc(LEFT, TOP, DIM, DIM, 32, 0);
        g2.drawArc(LEFT, TOP, DIM, DIM, 115, 0);
        g2.drawArc(LEFT, TOP, DIM, DIM, 200, 0);
        g2.drawArc(LEFT, TOP, DIM, DIM, 331, 0);

        this.drawCircle(g2, 0);
        this.drawCircle(g2, 32);
        this.drawCircle(g2, 115);
        this.drawCircle(g2, 200);
        this.drawCircle(g2, 331);

        g2.setStroke(new BasicStroke(1));
        g2.setColor(new Color(0, 0, 0));
        g2.drawLine(0, CENTER, DIM * 2, CENTER);
        g2.drawLine(CENTER, 0, CENTER, DIM * 2);
    }

    private void drawCircle(Graphics2D g, int angle) {
        g.setStroke(new BasicStroke(3));
        g.setColor(new Color(0, 0, 255));
        g.drawOval(
            Math.round(CENTER + (float)(Math.cos(Math.toRadians(angle)) * (DIM/2)) - DIAMETER/2),
            Math.round(CENTER - (float)(Math.sin(Math.toRadians(angle)) * (DIM/2)) - DIAMETER/2),
            DIAMETER,
            DIAMETER
        );
    }

    public static void main (String args[]) {
        Example e = new Example();
        e.setSize(WINDOW, WINDOW);
        e.setVisible(true);
        e.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

Answer 1

As an alternative, consider Ellipse2D with suitable RenderingHints. Typical usage is shown here.

Ellipse2D circle = new Ellipse2D.Float(…);
Graphics2D g2d = (Graphics2D) g;
g2d.setRenderingHints(…);
g2d.fill(circle);

Because various RenderingHints are implementation dependent, the example below will let you evaluate the effects individually.

import java.awt.BasicStroke;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

/**
 * @see https://stackoverflow.com/a/38669048/230513
 */
public class Test {

    private void display() {
        JFrame f = new JFrame("Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new JPanel() {
            private static final int N = 8;
            private final Ellipse2D ellipse = new Ellipse2D.Float();

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                Graphics2D g2d = (Graphics2D) g;
                g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                    RenderingHints.VALUE_ANTIALIAS_ON);
                g2d.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL,
                    RenderingHints.VALUE_STROKE_PURE);
                g2d.setStroke(new BasicStroke(N));
                ellipse.setFrame(N, N, getWidth() - 2 * N, getHeight() - 2 * N);
                g2d.draw(ellipse);
            }

            @Override
            public Dimension getPreferredSize() {
                return new Dimension(320, 240);
            }
        });
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Test()::display);
    }
}