The invention relates to a method and to a related numerical control machine for automatically cutting sheets of pliable material, for example for shoe, leather, clothing components and the like, not necessarily made of leather. The method of the invention is applied to a numerical control machine (7) provided with an electronic control system (20) having a memory (21) comprising at least one reference image (32) of the shape of the component to be cut, the machine being designed to receive at its inlet (5) material in the form of sheets (2) so as to define on said sheets (2) a cutting perimeter or trimming and to transfer cut portions (3) to its outlet (13). The method comprises at least the steps of:
- storing in the memory (21) at least one second reference image (16) representing a qualitative aspect of the surface (24) of the sheet (2) or of the portion (3) to be cut;
- defining by means of selection, via a user interface (30) of the electronic control system (20), at least one first sample portion (18) and at least one second sample portion (19) of said at least one second reference image (16), obtaining a criterion for comparative classification of said sample portions (18, 19) of the image;
- acquiring at least one image (12) of the sheet (2) to be cut, by means of image detectors (10) associated with the machine (7);
- classifying pixels of the acquired image (12) of the sheet (2) to be cut, assigning thereto a percentage value or weight based on said classification criterion, by means of an image processing unit of said electronic control system (20), obtaining a filtered image (28) based on this criterion;
- further processing said filtered image (28) and the reference image of the shape of the component to be cut, so as to define the perimeter (29) of a cutting area of the portion (3) of the sheet (2).