improved const-correctness#23

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Commits on Mar 18, 2019

improved const-correctness of internal Block structure as well as API interface
Benedikt Führer
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Commits on Mar 20, 2019

fix: shadowing of outer template parameter
Benedikt Führer
committed

Commits on May 16, 2019

ported cluster initialization algorithm from DBoW2 since the fbow one sometimes assigns the same visual word as center point leading to empty/meaningless visual words

Benedikt Führer
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jbfuehrer
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