DBoW2 is an improved version of the DBow library, an open source C++ library for
indexing and converting images into a bag-of-word representation. It implements
a hierarchical tree for approximating nearest neighbours in the image feature
space and creating a visual vocabulary. DBoW2 also implements an image database
with inverted and direct files to index images and enabling quick queries and
feature comparisons.

WWW: https://github.com/dorian3d/DBoW2