In this case the feature has 31 dimensions. HOG exists in many variants. VLFeat supports two: the UoCTTI variant (used by default) and the original Dalal-Triggs variant (with 2×2 square HOG blocks for normalization). The main difference is that the UoCTTI variant computes bot directed and undirected gradients as well as a four dimensional texture-energy feature, but projects the result down to 31 dimensions. Dalal-Triggs works instead with undirected gradients only and does not do any compression, for a total of 36 dimension. The Dalal-Triggs variant can be computed as