How to create a curve between 2 points in 2D and get back Points that makes that curve every d distance?

Question

I'm not good in math.

I have 2 points, A(x1, y1) and B(x2, y2) in 2D.

I need to create a virtual path from point A to B curved at R(radius), and then return an array of points which are describing this curved path, not all maybe every D(distance) from each other.

In Java I need a method like this:

private ArrayList<PointF> generateCurve(PointF pFrom,PointF pTo,float pRadius,float pMinDistance){

    ArrayList<PointF> pOutPut = new ArrayList<PointF>();
    // ...generate result to pOutPut 

    return pOutPut;
}

How to do this ?

Answer 1

This works:

private static double GetAngle(Point2D x, Point2D o, double R){
    double cosa = (x.getX()-o.getX())/R;
    double sina = (x.getY()-o.getY())/R;

    double angle = Math.acos(cosa);

    return Math.sin(angle)*sina >= 0 ? angle : 2*Math.PI - angle;
}

private static ArrayList<Point2D> generateCurve(Point2D pFrom,Point2D pTo,float pRadius,float pMinDistance){

    ArrayList<Point2D> pOutPut = new ArrayList<Point2D>();

    double dist = pFrom.distance(pTo);
    double h = Math.sqrt(pRadius * pRadius - (dist * dist / 4.0));
    double angleStep = pMinDistance/pRadius;

    if(2*pRadius <= dist)
        throw new Error("Radius is too small");

    //find center
    double x1 = pFrom.getX(), x2 = pFrom.getY();
    double y1 = pTo.getX(), y2 = pTo.getY();
    double m1 = (x1+y1)/2, m2 = (x2+y2)/2;
    double u1 = - (y2-x2)/dist, u2 = (y1-x1)/dist;
    double o1 = m1 + h * u1, o2 = m2 + h * u2;
    Point2D o = new Point2D.Double(o1, o2);

    double startAngle = GetAngle(pFrom, o, pRadius);
    double endAngle = GetAngle(pTo, o, pRadius);

    if(endAngle < startAngle)
        endAngle += 2 * Math.PI;        

    for(double a = startAngle; a < endAngle; a+=angleStep){
        pOutPut.add(new Point2D.Double(o1+pRadius*Math.cos(a), o2+pRadius*Math.sin(a)));
    }

    pOutPut.add(pTo);

    return pOutPut;
}

Here is what I get when I call it like this: generateCurve(new Point2D.Double(10,10), new Point2D.Double(400, 400), 300, 15)

Answer 2

I didn't gave up and I've been working on it for a few more hours. And here is the result:

I created a method where you can specify if you want the shortest of the longest arc between the points.

Here are some calls to it, with the produced output:

generateCurve(pFrom, pTo, 100f, 7f, false, false);

---

generateCurve(pFrom, pTo, 100f, 7f, true, false);

---

generateCurve(pFrom, pTo, 100f, 7f, false, true);

---

generateCurve(pFrom, pTo, 100f, 7f, true, true);

As you can see, it is working like a charm. Here is the code:

package curve;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import javax.imageio.ImageIO;

/**
 *
 * @author martijn
 */
public class Main
{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException
    {
        PointF pFrom = new PointF(-10f, 30.0f);
        PointF pTo = new PointF(-100f, 0.0f);
        List<PointF> points = generateCurve(pFrom, pTo, 100f, 7f, true, true);

        System.out.println(points);

        // Calculate the bounds of the curve
        Rectangle2D.Float bounds = new Rectangle2D.Float(points.get(0).x, points.get(0).y, 0, 0);
        for (int i = 1; i < points.size(); ++i) {
            bounds.add(points.get(i).x, points.get(i).y);
        }
        bounds.add(pFrom.x, pFrom.y);
        bounds.add(pTo.x, pTo.y);

        BufferedImage img = new BufferedImage((int) (bounds.width - bounds.x + 50), (int) (bounds.height - bounds.y + 50), BufferedImage.TYPE_4BYTE_ABGR_PRE);
        Graphics2D g = img.createGraphics();
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        g.translate(25.0f - bounds.getX(), 25.0f - bounds.getY());
        g.setStroke(new BasicStroke(1.0f));


        g.setColor(Color.DARK_GRAY);
        g.drawLine(-1000, 0, 1000, 0);
        g.drawLine(0, -1000, 0, 1000);

        g.setColor(Color.RED);
        for (int i = 0; i < points.size(); ++i) {
            if (i > 0) {
                Line2D.Float f = new Line2D.Float(points.get(i - 1).x, points.get(i - 1).y, points.get(i).x, points.get(i).y);
                System.out.println("Dist : " + f.getP1().distance(f.getP2()));
//                g.draw(f);
            }

            g.fill(new Ellipse2D.Float(points.get(i).x - 0.8f, points.get(i).y - 0.8f, 1.6f, 1.6f));

        }
        g.setColor(Color.BLUE);
        g.fill(new Ellipse2D.Float(pFrom.x - 1, pFrom.y - 1, 3, 3));
        g.fill(new Ellipse2D.Float(pTo.x - 1, pTo.y - 1, 3, 3));

        g.dispose();

        ImageIO.write(img, "PNG", new File("result.png"));
    }

    static class PointF
    {

        public float x, y;

        public PointF(float x, float y)
        {
            this.x = x;
            this.y = y;
        }

        @Override
        public String toString()
        {
            return "(" + x + "," + y + ")";
        }
    }

    private static List<PointF> generateCurve(PointF pFrom, PointF pTo, float pRadius, float pMinDistance, boolean shortest, boolean side)
    {

        List<PointF> pOutPut = new ArrayList<PointF>();

        // Calculate the middle of the two given points.
        PointF mPoint = new PointF(pFrom.x + pTo.x, pFrom.y + pTo.y);
        mPoint.x /= 2.0f;
        mPoint.y /= 2.0f;
        System.out.println("Middle Between From and To = " + mPoint);


        // Calculate the distance between the two points
        float xDiff = pTo.x - pFrom.x;
        float yDiff = pTo.y - pFrom.y;
        float distance = (float) Math.sqrt(xDiff * xDiff + yDiff * yDiff);
        System.out.println("Distance between From and To = " + distance);

        if (pRadius * 2.0f < distance) {
            throw new IllegalArgumentException("The radius is too small! The given points wont fall on the circle.");
        }

        // Calculate the middle of the expected curve.
        float factor = (float) Math.sqrt((pRadius * pRadius) / ((pTo.x - pFrom.x) * (pTo.x - pFrom.x) + (pTo.y - pFrom.y) * (pTo.y - pFrom.y)) - 0.25f);
        PointF circleMiddlePoint = new PointF(0, 0);
        if (side) {
            circleMiddlePoint.x = 0.5f * (pFrom.x + pTo.x) + factor * (pTo.y - pFrom.y);
            circleMiddlePoint.y = 0.5f * (pFrom.y + pTo.y) + factor * (pFrom.x - pTo.x);
        } else {
            circleMiddlePoint.x = 0.5f * (pFrom.x + pTo.x) - factor * (pTo.y - pFrom.y);
            circleMiddlePoint.y = 0.5f * (pFrom.y + pTo.y) - factor * (pFrom.x - pTo.x);
        }
        System.out.println("Middle = " + circleMiddlePoint);

        // Calculate the two reference angles
        float angle1 = (float) Math.atan2(pFrom.y - circleMiddlePoint.y, pFrom.x - circleMiddlePoint.x);
        float angle2 = (float) Math.atan2(pTo.y - circleMiddlePoint.y, pTo.x - circleMiddlePoint.x);

        // Calculate the step.
        float step = pMinDistance / pRadius;
        System.out.println("Step = " + step);

        // Swap them if needed
        if (angle1 > angle2) {
            float temp = angle1;
            angle1 = angle2;
            angle2 = temp;

        }
        boolean flipped = false;
        if (!shortest) {
            if (angle2 - angle1 < Math.PI) {
                float temp = angle1;
                angle1 = angle2;
                angle2 = temp;
                angle2 += Math.PI * 2.0f;
                flipped = true;
            }
        }
        for (float f = angle1; f < angle2; f += step) {
            PointF p = new PointF((float) Math.cos(f) * pRadius + circleMiddlePoint.x, (float) Math.sin(f) * pRadius + circleMiddlePoint.y);
            pOutPut.add(p);
        }
        if (flipped ^ side) {
            pOutPut.add(pFrom);
        } else {
            pOutPut.add(pTo);
        }

        return pOutPut;
    }
}

Enjoy!
PS: I created two questions on Mathematics to solve your question:

• Analytic Geometry: Point coordinates, same distance from two points.
• Trigonometry: Solve (1−cosα)2+sin2α=d2 for α