“…In this paper, several representative Riemannian manifold learning-based visual classification methods are selected to better evaluate the effectiveness of the proposed approach, which can be grouped into the following four categories: 1) general methods for SPD matrix learning, including Log-Euclidean Metric Learning (LEML) [10] and SPD Manifold Learning (SPDML) [12]; 2) general methods for linear subspace learning, including Projection Metric Learning (PML) [38], Graph Embedding Projection Metric Learning (GEPML) [39], and Graph Embedding Multi-Kernel Metric Learning (GEMKML) [40]; 3) multi-order statistical learning methods, containing Hybrid Euclidean-and-Riemannian Metric Learning (HERML) [11] and Multiple Riemannian Manifolds Metric Learning (MRMML) [41]; 4) Riemannian deep learning methods, containing Grassmannian Neural Network (GrNet) [42], SPD Neural Network (SPDNet) [7], SPDNet embedded with Riemannian Batch Normalization (SPDNetBN) [8], Lightweight SPD Neural Network (SymNet) [5], Manifoldvalued Deep Network (ManifoldNet) [3], and our baseline model, i.e., Deep SPDNet (DSPDNet) [6].…”