  The Brunn-Minkowski Theorem asserts that $\mu_d(A+B)^{1/d}\geq
\mu_d(A)^{1/d}+\mu_d(B)^{1/d}$ for convex bodies $A,\,B\subseteq \R^d$, where
$\mu_d$ denotes the $d$-dimensional Lebesgue measure. It is well-known that
equality holds if and only if $A$ and $B$ are homothetic, but few
characterizations of equality in other related bounds are known. Let $H$ be a
hyperplane. Bonnesen later strengthened this bound by showing $$\mu_d(A+B)\geq
(M^{1/(d-1)}+N^{1/(d-1)})^{d-1}(\frac{\mu_d(A)}{M}+\frac{\mu_d(B)}{N}),$$ where
$M=\sup\{\mu_{d-1}((\mathbf x+H)\cap A)\mid \mathbf x\in \R^d\}$ and
$N=\sup\{\mu_{d-1}((\mathbf y+H)\cap B)\mid \mathbf y\in \R^d\}$. Standard
compression arguments show that the above bound also holds when
$M=\mu_{d-1}(\pi(A))$ and $N=\mu_{d-1}(\pi(B))$, where $\pi$ denotes a
projection of $\mathbb R^d$ onto $H$, which gives an alternative generalization
of the Brunn-Minkowski bound. In this paper, we characterize the cases of
equality in this later bound, showing that equality holds if and only if $A$
and $B$ are obtained from a pair of homothetic convex bodies by `stretching'
along the direction of the projection, which is made formal in the paper. When
$d=2$, we characterize the case of equality in the former bound as well.
