1. A method for calculating a region of interest in a second image using an initial region of interest in a first image, the first and second images being linked by a spatial transformation function, the method comprising: generating a first polygonal mesh enclosing the initial region of interest in the first image; computing a second polygonal mesh in the second image by applying the spatial transformation function to said first polygonal mesh; identifying image elements within the second image that belong to an area enclosed by said second polygonal mesh; computing intersections of the second polygonal mesh with a 2D second image, so as to generate a 2D contour image; identifying image elements of said 2D second image that belong to at least one area enclosed by contours of the 2D contour image; applying a connected components detection to the 2D contour image so as to obtain a 2D image, where 2D areas located outside any closed shape are marked as outside; creating a depth map image in a form of a buffer associating to pixels of said 2D image one of the following depth values: complimentary values inside, or outside, or undefined; initializing in said depth map image the pixels previously identified as outside by the connected components detection to outside value; and filling said depth map image by iteratively identifying areas enclosed by contours from an outermost to an innermost contour, each contour separating outside pixels and inside pixels, said inside pixels defining 2D regions of interest.