“…For example, several studies [40], [41] used a manifold-based neural network to extract manifold features from SPD matrix input and a logarithm map to transform the manifold feature back to its Euclidean tangent space for subsequent classification. Another group of work [42], [43] aims to fuse SPD matrix input and regular Euclidean vector input for feature extraction. They thus used the logarithm map to transform the SPD matrix input into the Euclidean tangent space as a mapped feature.…”