We propose a probabilistic algorithm able to detect the curves that are unexpectedy smooth in a set of digital curves. The only parameter is a false alarme rate, influencing the detection only by its logarithm. We experiment the good continuation criterion on image level lines. One of the conclusion is that, accordingly to Gestalt Theory, one can detect egdes in a way that is widely independent of contrast. We also use the same kind of method to detect corners and junctions.