For $z\in S^2$ a unit vector in $\R^3$, the function $zf\colon
M\to\R$ given by
$$zf(p) = z\cdot f(p)$$ is the height function on $M$ in the direction of $z$.
Theorem: $f$ is tight if, and only if, every
nondegenerate height function on $M$ has exactly
one maximum and one minimum.
Consequence: All the positive curvature of $M$ is on the surface of
the convex hull of $f(M)$, denoted $\dHf$.
