What values does the algorithm parametre take in OpenCV's FlannBasedMatcher constructor?

Question

In python, OpenCV's FlannBasedMatcher constructor takes a dictionary of parametres, the first of which is algorithm. The python tutorial suggests that one can specify different values of algorithm by passing differently named variables, e.g. FLANN_INDEX_KDTREE and FLANN_INDEX_LSH, but the variable name cannot, of course, convey anything to the constructor. One might think that these names should instead be passed as strings, but in the example that follows, FLANN_INDEX_KDTREE is initialised as 0, and so it is entirely unclear how the algorithm parametre works.

OpenCV doesn't really have any python documentation. In C++, the constructor doesn't take a generic dictionary, but an object that instantiates IndexClass, where each subclass of IndexClass corresponds to a different algorithm.

Answer 1

algorithm takes an integer, what is missing from the tutorial is the initialisation of FLANN_INDEX_KDTREE and FLANN_INDEX_LSH with different values. (The upper case should have been a hint that these are meant as descriptive labels of fixed integer values.)

The C++ source code has such a list of initialisations, from which it appears that the LSH algorithm corresponds to the value 6. It also initialises FLANN_INDEX_KDTREE as 1, which means that either the python tutorial, which has 0, is wrong, or that the source code uses conflicting definitions in different places (that I missed), which would be unfortunate.

These are all the values in the source code:

FLANN_INDEX_LINEAR = 0
FLANN_INDEX_KDTREE = 1
FLANN_INDEX_KMEANS = 2
FLANN_INDEX_COMPOSITE = 3
FLANN_INDEX_KDTREE_SINGLE = 4
FLANN_INDEX_HIERARCHICAL = 5
FLANN_INDEX_LSH = 6
FLANN_INDEX_SAVED = 254
FLANN_INDEX_AUTOTUNED = 255

EDIT: I filed a pull request fixing both issues that has now been accepted.