Function cubicDerive

Engine/Interpolation.cpp:97–108

derive at t

// derive at t
static double
cubicDerive(double /*c0*/,
            double c1,
            double c2,
            double c3,
            double t)
{
    const double t2 = t * t;
    assert(t == t && t2 == t2 && c1 == c1 && c2 == c2 && c3 == c3);

    return c1 + (c2 ? 2 * c2 * t : 0.) + (c3 ? 3 * c3 * t2 : 0.);
}

#define EQN_EPS 1e-9