Use of feature matcher in stiching module c++

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I have three images from left right and front cameras i want to match the features of left and front then front and right to find the camera parameters as a controlled matching using the function

void detail::FeaturesMatcher::operator()(const std::vector<ImageFeatures>& features, std::vector<MatchesInfo>& pairwise_matches, const Mat& mask=Mat()

here is the link to docs

Parameters:

features – Features of the source images
pairwise_matches – Found pairwise matches
mask – Mask indicating which image pairs must be matched

my doubt is how the mask is created for this matching if it gets created the i can specify the pairs kindly help me