Polygonal representations of digital sets with the same convexity properties allow a simple decomposition of digital boundaries into convex and concave parts. Representations whose vertices are boundary points, i.e. are integer numbers, attract most attention. The existing linear Algorithm UpPolRep computes polygonal representations with some uncorresponding parts. However, the algorithm is unable to decide if a corresponding polygonal representation still exists and in the case of existence it is unable to compute the representation. Studying situations where uncorrespondences appear we extended the algorithm. The extention does not change the time complexity. If a digital set possesses a corresponding representation then it detects this representation. Otherwise, it recognizes that such representation does not exist.