  Nonnegative matrix factorization (NMF) is a widely used linear dimensionality
reduction technique for nonnegative data. NMF requires that each data point is
approximated by a convex combination of basis elements. Archetypal analysis
(AA), also referred to as convex NMF, is a well-known NMF variant imposing that
the basis elements are themselves convex combinations of the data points. AA
has the advantage to be more interpretable than NMF because the basis elements
are directly constructed from the data points. However, it usually suffers from
a high data fitting error because the basis elements are constrained to be
contained in the convex cone of the data points. In this letter, we introduce
near-convex archetypal analysis (NCAA) which combines the advantages of both AA
and NMF. As for AA, the basis vectors are required to be linear combinations of
the data points and hence are easily interpretable. As for NMF, the additional
flexibility in choosing the basis elements allows NCAA to have a low data
fitting error. We show that NCAA compares favorably with a state-of-the-art
minimum-volume NMF method on synthetic datasets and on a real-world
hyperspectral image.
