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I've tried both methods and I believe that the circle pattern is more accurate than the square one, judging the rms error after calibration. Should I need to explain it, I would bet on the pattern morphology itself. A square is more likely to be distorted while moving around than a circle due to camera focus. A circle tends to stay pretty much the same, regardless of camera focus, whereas a square will loose its corner sharpness. I prefer using the circles pattern, which yields better results on stereo correspondance.