Re: Find point perpendicular to line

From:

"Jeff Higgins" <oohiggins@yahoo.com>

Newsgroups:

comp.lang.java.help

Date:

Sun, 22 Jul 2007 20:41:14 -0400

Message-ID:

<OOSoi.41$t87.36@newsfe04.lga>

Joshua Cranmer wrote:

On Sun, 22 Jul 2007 14:13:22 -0400, Jeff Higgins wrote:

Lew wrote:

Jeff Higgins wrote:

I would like to find a third point that is:

on a line perpendicular to lineAB,

and passes throught point A

and
double distanceC = 3d;
distant from A

I belive this is your criteria:
C
| -- distance 3d.
B-----A

If the line AB is at an angle theta to the origin line, then theta =
arctan (Ay-By)/(Ax-Bx). The angle that line AC makes with the origin is
phi, which is theta + pi/2 in this case. Translating the origin to point
A, we have that Cx is r*cos phi and Cy is r*sin phi.

phi should also be equal to arctan (Bx-Ax)/(Ay-By), and reducing, we get
the Cx = 3d / sqrt (1+x^2) + Ax and Cy = 3d * x /sqrt(1+x^2) + Ay where x
is (Bx-Ax)/(Ay-By), except if Ay=By (i.e., a horizontal line), where Cx =
Ax and Cy = Ay+3d.

Note: only checked for slope AB = 0, 1, and infinity.

Joshua,
  Thanks very much.
Appreciative,
JH

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

public class HitTestBoundsTest
{
  static class GraphicPanel extends JPanel
  {
    Point2D.Double getMyPoint_PerpendicularToALine(
        Point2D.Double source,
        Point2D.Double target,
        double width)
    {
      // TODO test for horizontal line
      double x = ((target.x - source.x) / (source.y - target.y));
      double cx = (width / 2) / Math.sqrt(1 + Math.pow(x, 2)) + source.x;
      double cy = (width / 2) * x / Math.sqrt(1 + Math.pow(x, 2)) +
source.y;
      Point2D.Double ret = new Point2D.Double(cx, cy);
      return ret;
    }

    public void paintComponent( Graphics g )
    {
      Graphics2D g2 = (Graphics2D)g;
      Path2D.Double line = new Path2D.Double();
      line.moveTo(source.x, source.y);
      line.lineTo(target.x, target.y);
      Point2D.Double C =
        getMyPoint_PerpendicularToALine(source, target, width);
      Point2D.Double D =
        getMyPoint_PerpendicularToALine(target, source, width);
      Path2D.Double CD = new Path2D.Double();
      Path2D.Double EF = new Path2D.Double();
      Path2D.Double CE = new Path2D.Double();
      Path2D.Double DF = new Path2D.Double();
      CD.moveTo(C.x, C.y);
      CD.lineTo(D.x, D.y);
      EF.moveTo(C.y, C.x);
      EF.lineTo(D.y, D.x);
      CE.moveTo(C.x, C.y);
      CE.lineTo(C.y, C.x);
      DF.moveTo(D.x, D.y);
      DF.lineTo(D.y, D.x);
      g2.draw(CD);
      g2.draw(EF);
      g2.draw(CE);
      g2.draw(DF);
      g2.setColor(Color.RED);
      g2.draw(line);
    }

    public GraphicPanel(){}

    Point2D.Double source =
      new Point2D.Double(100d, 100d);
    Point2D.Double target =
      new Point2D.Double(200d, 200d);
    double width = 3d;
  }

  JFrame frame;
  JPanel panel;

  HitTestBoundsTest()
  {
    frame = new JFrame("HitTestBoundsTest");
    panel = new GraphicPanel();
    frame.setSize(600, 480);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(panel);
    frame.setVisible(true);
  }

  public static void main(String[] args)
  {
    HitTestBoundsTest test = new HitTestBoundsTest();
  }
}