CODEWITHSUNDEEP
c#unity-game-enginemathbezier
Zizo
Zizo

Reputation: 31

Cubic bezier curve arc length is always zero

I've written the following code to calculate the length of a cubic bezier curve. I got the idea from Calculate the arclength, curve length of a cubic bezier curve. Why is not working?. The problem is it always produces a length of zero.

public Vector2 SegmentAtPoint(int segmentIndex, float t)
{
    t = Mathf.Clamp01(t);
    float oneMinusT = 1f - t;

    return
        oneMinusT * oneMinusT * oneMinusT * points[segmentIndex * 3] + 
        3f * oneMinusT * oneMinusT * t * points[segmentIndex * 3 + 1] +
        3f * oneMinusT * t * t * points[segmentIndex * 3 + 2] +
        t * t * t * points[segmentIndex * 3 + 3];

}

public float SegmentLength(int segmentIndex)    {
    var steps = 10;
    var t = 1 / steps;
    var sumArc = 0.0f;
    var j = 0.0f;
    var a = new Vector2(0.0f, 0.0f);
    var b = points[segmentIndex * 3];

    var dX = 0.0f;
    var dY = 0.0f;
    var dS = 0.0f;

    for (int i = 0; i < steps; j = j + t)
    {
        a = SegmentAtPoint(segmentIndex, j);

        dX = a.x - b.x;
        dY = a.y - b.y;
        dS = Mathf.Sqrt((dX * dX) + (dY * dY));

        sumArc = sumArc + dS;
        b.x = a.x;
        b.y = a.y;
        i++;
    }

    return sumArc;

}

Upvotes: 0

Views: 241

Answers (1)

MBo
MBo

Reputation: 80325

Code var t = 1 / steps; makes integer division, so result t is zero

Also note that j = j + t is executed after every loop, so at the first iteration j==0

This flaw causes such problem: both b and a are equal at the first iteration because j still remains =0. So you calculate segment lengths on intervals: 0-0, 0-0.1, 0.1-0.2...0.7-0.8,0.8-0.9 - ignoring 0.9-1.0 interval

Upvotes: 3

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