First intelligent scissors. The user can use a pointer, the mouse for example, and can draw a sketch around the region of interest which is shown in white here, and then that white thing goes into yellow contour because that is the nearest border of the object. In this case the cube border is being selected which is this one. This white goes into the yellow border, and you can have different selections, you can draw different white sketches and get the yellow ones an so on.

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