Set gradient direction in swing

Question

I have been trying to draw lines in swing and get some laser-like effects using gradients. I want to apply the gradient on the line's width (ex: a red line core fading to orange on the edges). The problem is when I draw at an angle, I want to somehow apply the same angle to the gradient.

public void paintComponent(Graphics g) {
        Graphics2D en = (Graphics2D) g;
        GradientPaint gp = new GradientPaint(25, 25, Color.red, 15, 25,
                Color.orange, true);
        en.setPaint(gp);
        en.setStroke(new BasicStroke(4.0F));
        en.drawLine(10, 10, 800, 600);
}

Answer 1

One easy trick to get away with this, is to call setTransform() on the Graphics2D. This will do for you the rotation of both the line and the gradient at once. If you don't want to do this, then you will have to rotate the gradient itself (so basically pt1 and pt2 of the Gradient manually, ie, you need to compute them according to the rotation you have applied to your line).

Here is a small example illustrating my first idea. Just slide the ticker at the bottom to watch the line (and the gradient) rotate around the center of the panel:

import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.SwingUtilities;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class TestPanel extends JPanel {

    private double angle = 0;

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D en = (Graphics2D) g;
        AffineTransform tf = AffineTransform.getTranslateInstance(-getWidth() / 2, -getHeight() / 2);
        tf.preConcatenate(AffineTransform.getRotateInstance(Math.toRadians(angle)));
        tf.preConcatenate(AffineTransform.getTranslateInstance(getWidth() / 2, getHeight() / 2));
        en.setTransform(tf);
        GradientPaint gp = new GradientPaint(25, 25, Color.red, 15, 25, Color.orange, true);
        en.setPaint(gp);
        en.setStroke(new BasicStroke(4.0F));
        en.drawLine(400, 400, 600, 600);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(800, 800);
    }

    public void setAngle(double angle) {
        this.angle = angle;
        repaint();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                final TestPanel panel = new TestPanel();
                final JSlider slider = new JSlider(0, 360);
                slider.addChangeListener(new ChangeListener() {

                    @Override
                    public void stateChanged(ChangeEvent e) {
                        panel.setAngle(slider.getValue());
                    }
                });
                slider.setValue(0);
                frame.add(panel);
                frame.add(slider, BorderLayout.SOUTH);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}