1 ) A method comprising: a) receiving, into a provided memory readable and writable by a provided CPU, one or more sets of digital data for presentation in two-dimensions each set comprising i) at least two data values per data point organized in sequential addresses or at set intervals of addresses, ii) the number of data values per data point, iii) a specified starting address, iv) a specified number of data points, called the width, in the horizontal or x-direction, v) a specified number of data points, called the height, in the vertical or y-direction, b) the computation of a numerical similarity signature, referred to as the equivalence signature, for a set of two-dimensional digital data, as the Second Homotopy Group's invariant for the values of said data interpreted as a map from the two-dimensional presentation space into the space of the data, and c) the computation of a similarity distance between any two sets of two-dimensional digital data as the absolute value of the difference of their equivalence signatures.