Hermite Interpolation

Question

I am trying to interpolate between 4 points using a Hermite spline. However my spline seems to always start on the second point and only interpolate to the 3rd point. I have tried this with several differnt calculations and keep getting the same result.

Can anyone give me insight on this? Here is my code.

public ControlPoint Get(float t)
{
    //speed multiplyer
    //t = t * 10;

    return new ControlPoint(
        new Vector3(Hermite(points[0].pos.x,  points[1].pos.x, points[2].pos.x, points[3].pos.x, t)
        , Hermite(points[0].pos.y, points[1].pos.y, points[2].pos.y, points[3].pos.y, t)
        , Hermite(points[0].pos.z, points[1].pos.z, points[2].pos.z, points[3].pos.z, t)
        ),
        new Quaternion(Hermite(points[0].rot.x, points[1].rot.x, points[2].rot.x, points[3].rot.x, t)
        , Hermite(points[0].rot.y, points[1].rot.y, points[2].rot.y, points[3].rot.y, t)
        , Hermite(points[0].rot.z, points[1].rot.z, points[2].rot.z, points[3].rot.z, t)
        , Hermite(points[0].rot.w, points[1].rot.w, points[2].rot.w, points[3].rot.w, t)
        )
        );
}

float Hermite(
    float y0, float y1,
    float y2, float y3,
    float mu,
    float tension = 0,
    float bias = 0)
{
    float m0, m1, mu2, mu3;
    float a0, a1, a2, a3;

    mu2 = mu * mu;
    mu3 = mu2 * mu;
    m0 = (y1 - y0) * (1 + bias) * (1 - tension) / 2;
    m0 += (y2 - y1) * (1 - bias) * (1 - tension) / 2;
    m1 = (y2 - y1) * (1 + bias) * (1 - tension) / 2;
    m1 += (y3 - y2) * (1 - bias) * (1 - tension) / 2;
    a0 = 2 * mu3 - 3 * mu2 + 1;
    a1 = mu3 - 2 * mu2 + mu;
    a2 = mu3 - mu2;
    a3 = -2 * mu3 + 3 * mu2;

    return (a0 * y1 + a1 * m0 + a2 * m1 + a3 * y2);
}

Answer 1

I'm not an expert Hermite Splines by any stretch of the imagination, but from what I've seen is that the expected behavior would be to interpolate between the second and third point. It looks to me like you just hardcoded in each coordinate to your Get function, so it makes sense that you only get a single interpolation when a Hermite Spline is a function. Think of the second and third points as the two points you want to interpolate between, and the first and fourth points just help to create a better curve.

Since it appears you only have four points total, to interpolate between the first and second points, and third and fourth points, try repeating your first coordinates and last coordinates.

//Interpolate between 1st and 2nd points' x coord Hermite(points[0].pos.x, points[0].pos.x, points[1].pos.x, points[2].pos.x);

//Interpolate between 3rd and 4th points' x coord Hermite(points[2].pos.x, points[3].pos.x, points[4].pos.x, points[4].pos.x);

To interpolate between the first and second points points[0] is repeated twice because there is no points[-1]. For interpolation between the third and fourth points, points[4] is repeated because there is no points[5].

To reiterate, do not hardcode in coordinates unless you only want a single interpolation. You'll have to modify your Get function and call it a few times to adjust for the behavior you want. Check out how Snea implemented a Hermite Spline in his DrawGraph function, it helped me to better understand Hermite Spline behavior: Cubic Hermite Spline behaving strangely