1. An automatic process of predictive determination, as a digital simulation, of a position and movements of skin of a human or animal subject in a zone of interest located at a level of a thorax or an abdomen, with said subject being stretched on said subject's back and breathing, said process comprising: in a preliminary phase, acquiring or determining multiple positions and configurations of a profile of the skin in predetermined, consecutive axial planes that are essentially perpendicular to the craniocaudal axis and distributed along the craniocaudal axis at determined locations, at given successive times, in different respiratory positions, during inhalation and exhalation phases of the subject, during a preliminary treatment phase, constructing at least one independent deformable digital model of the profile of the skin in each said axial plane (Pa) from data pertaining to different skin profiles ( 3 ) that are acquired or determined previously, noting, in a repetitive manner, an actual position of a point or a specific point zone ( 4 ) on the skin ( 2 ) of the subject ( 1 ) at the level of each of said axial planes (Pa), whose position is modified in a during the inhalation and exhalation phases, and providing, essentially in real time, a simulation ( 3 ′) of the profile of the skin in each said axial plane (Pa), as a function of the actual position noted for each point or specific point zone ( 4 ) and by exploiting the deformable digital model of the skin profile in the corresponding axial plane (Pa), and, an evolving three-dimensional representation ( 2 ′) of the skin ( 2 ) at the level of the zone of interest, this on the basis of a three-dimensional model for overall simulation of the skin composed of plural deformable and independent elementary two-dimensional axial models established in the axial planes (Pa) and by interpolation between said axial planes (Pa), wherein the points or specific point zones ( 4 ), whose actual position is monitored over time, are distributed along the craniocaudal axis by being aligned or not, with their number being at least equal 10 and their spacing of at most 5 cm in the direction of the craniocaudal axis.