OGCubicBezierTangent.c

CGPoint OGCubicBezierTangent(CGFloat t, CGPoint P[static 4])
{
    CGFloat a = -3 * (1 - t) * (1 - t),
            b =  3 * (t - 1) * (3 * t - 1),
            c =  3 * t * (2 - 3t),
            d =  3 * t * t;

    return (CGPoint) {
        (a * P[0].x) + (b * P[1].x) + (c * P[2].x) + (d * P[3].x),
        (a * P[0].y) + (b * P[1].y) + (c * P[2].y) + (d * P[3].y)
    };
}