c# getting a sporadic results

Question

So I've literally been stuck on this for hours and it's quite frustrating. I am getting sporadic results. the program is working sometime, but at other times, it can't find one of the "tangents" I'm looking for, even though mathematically there is guaranteed to be one.

I'm really new to c# so, if anyone had some time to help a noob, it would be much appreciated.

private List<PointF> SplitAndSolve(Graphics g, List<PointF> pointList)
    {
        if (pointList.Count < 4)
            return pointList;
        List<List<PointF>> leftAndRight = SplitByX(pointList);
        List<PointF> left = leftAndRight[0];
        List<PointF> right = leftAndRight[1];
        drawPolygon(g, left);// just a way to visually assess the correctness
        drawPolygon(g, right);// same
        left = SplitAndSolve(g, left);
        right = SplitAndSolve(g, right);
        Combine(g, left, right);
        return pointList;
    }
    private List<PointF> Combine(Graphics g, List<PointF> left, List<PointF> right)
    {
        //find tangents
        List<PointF> topTangents = GetTangents(g, left, right, TOP);
        drawPoint(g, topTangents[0]);//visual debug
        drawPoint(g, topTangents[1]);//visual debug
        List<PointF> botTangents = GetTangents(g, left, right, BOT);
        drawPoint(g, botTangents[0]);//visual debug
        drawPoint(g, botTangents[1]);//""
        // get a new polygon

        return left;// just a place holder so I don't get errors for the time being
    }
    private List<PointF> GetTangents(Graphics g, List<PointF> left, List<PointF> right, float topOrBot)
    {
        List<PointF> tangents = new List<PointF>();
        foreach (PointF leftAnchor in left)
        {
            foreach (PointF rightAnchor in right)
            {
                double lax = leftAnchor.X;
                double lay = leftAnchor.Y;
                double rax = rightAnchor.X;
                double ray = rightAnchor.Y;
                double m = (lay - ray) / (lax - rax);
                double b = (-1 * m * lax) + lay;
                bool isTangent = true;
                foreach (PointF lpoi in left)
                {
                    if ((topOrBot == TOP) && (Test(m, b, lpoi) > 0))
                    {
                        isTangent = false;

                    }
                    if ((topOrBot == BOT) && (Test(m, b, lpoi) < 0))
                    {
                        isTangent = false;

                    }
                }

                foreach (PointF rpoi in right)
                {
                    if ((topOrBot == TOP) && (Test(m, b, rpoi) > 0))
                    {
                        isTangent = false;

                    }
                    if ((topOrBot == BOT) && (Test(m, b, rpoi) < 0))
                    {
                        isTangent = false;

                    }
                }
                if (isTangent)
                {
                    tangents.Add(leftAnchor);
                    tangents.Add(rightAnchor);
                    return tangents;
                }

            }

        }
        return null;
    }

    /*  Test, test to see the location of a point in relation to a line
     *  @float m slope of the line
     *  @float b the constast of the y intercept form
     *  @Pointf r is the point to be tested against the line
     * 
     *  returns some k > 0 if point is below the line
     *  returns some k < 0 if point is above the line
     *  returns 0 if point is found along the line
     */
    private double Test(double m, double b, PointF r)
    {
        return (m*(double)r.X) + b - (double)r.Y;
    }

so I'm somewhat convinced it's a programming error, because I've been over it and over it, though I may be mistaken.

I'm sorry if this is an inappropriate post, I really am just stuck, I don't mean to be bothersome to anyone, if I am misusing the forum please let me know.

I am finding the line that crosses two vertices, one from each subgroup, that all other lines are either below(top tangent) or above(bottom tangent). The error is that with the algorithm, it should always iterative loops should never reach the return null. Yet occasionally it does. I guessing it is a precision error.

Answer 1

Could this just be precision issues with double? Maybe you should make your Test function "fuzzy".

Like this:

  double result = (m*(double)r.X) + b - (double)r.Y;

  if ((result > -0.0001) && (result < 0.0001))
     return 0.0;
  else
     return result;