  In this report, we discuss a simple model for RGB color and polarization
images under a unified framework of quaternion nonnegative matrix factorization
(QNMF) and present a hierarchical nonnegative least squares method to solve the
factor matrices. The convergence analysis of the algorithm is discussed as
well. We test the proposed method in the polarization image and color facial
image representation. Compared to the state-of-the-art methods, the
experimental results demonstrate the effectiveness of the hierarchical
nonnegative least squares method for the QNMF model.
