A cellular algorithm for extraction of isolines from 2D image of 3D surface is presented. The algorithm is divided into two stages: secondary quantization and extraction of isolines. Realization of each stage as one-time iteration on the base of the Parallel Substitution Algorithm is obtained. Results of the algorithm simulation in a parallel substitution simulation system ALT are presented for an example of a 256-colours grayscale surface image processing. Cellular structures for extraction of two types (dense and minimal) isolines are also suggested.