The Mask does not work as the documentation states: ( docs.opencv.org/3.1.0/db/d39/classcv_1_1DescriptorMatcher.html#a8737756123c4aba88ae5fc15982e0dee ) Optional mask (or masks) can be passed to specify which query and training descriptors can be matched. Namely, queryDescriptors[i] can be matched with trainDescriptors[j] only if mask.at<uchar>(i,j) is non-zero.
But the implementation only lets you choose which queryDescriptors shall be matched (against all trainDescriptors, no choice against which of them).
GitHubCode: github.com/opencv/opencv_contrib/blob/master/modules/line_descriptor/src/binary_descriptor_matcher.cpp#L197
Am I mistaken or is the documentation inconsistent with the implementation. The documentations states what would actually be desirable here.