How to Calculate the angle of gradient? With atan2()????

Question

Hi,
my Problem is I have 2 Points and I have to calculate the angle of gradient of the line between those two points.
I tried it like this:
Point2D.Double p = new Point2D.Double(5.0,4.0),                               q = new Point2D.Double(2.0,2.0);          
     double angle = Math.atan2((p.y - q.y), (p.x - q.x));
     double calc=Math.toDegrees(angle);
     System.out.println("calc: "+calc);
but if I switching the values of the two points the Result is a other but I need a Method that returns me always the angle of gradient.
Can anyone help me?
Thanks

Answer 1

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.text.*;
import javax.swing.*;
public class TrigTest
    public static void main(String[] args)
        TrigPanel trigPanel = new TrigPanel();
        JFrame f = new JFrame("Click on the panel");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(trigPanel.getTopPanel(), "North");
        f.getContentPane().add(trigPanel);
        f.getContentPane().add(trigPanel.getBottomPanel(), "South");
        f.setSize(400,300);
        f.setLocation(400,300);
        f.setVisible(true);
class TrigPanel extends JPanel
    Point originP, endP;
    double degrees;
    JLabel
        slopeLabel,       // slope = rise/run
        angleLabel,       // anti-clockwise angle from abcissa (3 o'clock)
        relativeLabel;    // angle relative to horizontal
    NumberFormat nf;
    public TrigPanel()
        originP = new Point();
        endP = new Point();
        nf = NumberFormat.getInstance();
        nf.setMaximumFractionDigits(1);
        setBackground(Color.white);
        addMouseListener(new PointSelector());
    public void paintComponent(Graphics g)
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int width = getWidth();
        int height = getHeight();
        int dia = Math.min(width, height)*3/5;
        originP.x = width/2;
        originP.y = height/2;
        // abcissa
        g2.draw(new Line2D.Double(originP.x, originP.y, width*4/5, originP.y));
        // line from origin to selected point
        g2.draw(new Line2D.Double(originP, endP));
        // angle arc from abcissa to selected line
        Ellipse2D e = new Ellipse2D.Double(originP.x - dia/2,
                                           originP.y - dia/2,
                                           dia, dia);
        g2.draw(new Arc2D.Double(e.getBounds(), 0, degrees, Arc2D.OPEN));
        // arrow head
        if(degrees > 10)
            double x = originP.x + (dia/2)*Math.cos(Math.toRadians(degrees));
            double y = originP.y - (dia/2)*Math.sin(Math.toRadians(degrees));
            //g2.fill(new Ellipse2D.Double(x - 1, y - 1, 2, 2));  // tip marker
            double x1 = x + 5*Math.cos(Math.toRadians(degrees - 60));
            double y1 = y - 5*Math.sin(Math.toRadians(degrees - 60));
            double x2 = x + 5*Math.cos(Math.toRadians(degrees - 120));
            double y2 = y - 5*Math.sin(Math.toRadians(degrees - 120));
            g2.draw(new Line2D.Double(x, y, x1, y1));
            g2.draw(new Line2D.Double(x, y, x2, y2));
        // origin
        g2.setPaint(Color.red);
        g2.fill(new Ellipse2D.Double(originP.x - 1, originP.y - 1, 2, 2));
    private class PointSelector extends MouseAdapter
        public PointSelector()
            endP = originP;
        public void mousePressed(MouseEvent e)
            endP = e.getPoint();
            computeSlope();
            computeAngles();
            repaint();
    private void computeSlope()
        double deltaY = endP.y - originP.y;
        double deltaX = originP.x - endP.x;
        double slope = deltaY / deltaX;
        slopeLabel.setText(nf.format(slope));
    private void computeAngles()
        double y = originP.y - endP.y;
        double x = endP.x - originP.x;
        // angle from abcissa
        double atan2 = Math.atan2(y, x);
        degrees = Math.toDegrees(atan2);
        if(degrees < 0)
            degrees += 360;
        angleLabel.setText(nf.format(degrees));
        // angle relative to horizontal - unsigned
        double atan = Math.atan(y/x);
        double relative = Math.abs(Math.toDegrees(atan));
        relativeLabel.setText(nf.format(relative));
    public JPanel getTopPanel()
        slopeLabel = new JLabel("");
        angleLabel = new JLabel("");
        setLabelSize(slopeLabel);
        setLabelSize(angleLabel);
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        gbc.weightx = 1.0;
        addLabels(new JLabel("slope"), slopeLabel, panel, gbc);
        addLabels(new JLabel("angle"), angleLabel, panel, gbc);
        return panel;
    public JPanel getBottomPanel()
        relativeLabel = new JLabel("");
        setLabelSize(relativeLabel);
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        addLabels(new JLabel("angle relative to horizontal"),
                              relativeLabel, panel, gbc);
        return panel;
    private void setLabelSize(JLabel label)
        label.setPreferredSize(new Dimension(50, 25));
    private void addLabels(JLabel label1, JLabel label2,
                           JPanel panel, GridBagConstraints gbc)
        gbc.anchor = gbc.EAST;
        panel.add(label1, gbc);
        gbc.anchor = gbc.WEST;
        panel.add(label2, gbc);
}