Sometimes when I see the message “Please specify at least 6 correspondences for further processing”, I notice that measurement results can change quite a bit with each additional correspondence added.
That is true. When you see the warning message, the algorithm will refine all measurement results with each added correspondence. Drastic changes in existing measurements do occur if measured objects are far away or the depth variation is large (this can happen for outdoor scenes). To ensure high metrology accuracy, you should specify significantly more correspondences (a dozen or more or until the measurements stabilize). Another possibility is to retake pictures with a larger baseline.
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