1. A method comprising: a) receiving, into a provided memory readable and writable by a provided CPU, one or more sets of digital data which can be presented in two-dimensions, each set comprising: 1) data values organized in sequential addresses or at set intervals of addresses, 2) the number of data values per data point, 3) a specified starting address, and 4) a specified number of data points; b) computing of a numerical similarity signature, referred to as the equivalence signature, with value given by the action for the motion of a string which sweeps out a sheet specified by the values of the digital data, and c) computing of a similarity distance between any two sets of digital data as the absolute value of the difference of their equivalence signatures; wherein, for images, graphics, video frames, and the like, that may be presented in two-dimensions, computing an equivalence signature as the action for the motion of a string which sweeps out a sheet specified by the values of the normalized digital data and a configurable metric on the space of data, further comprises looping over each Y-position, from the first Y-position to the second to last Y-position and, for each Y-position, looping over each X-position, from the first X-position to the second to last X-position, and for each (X, Y)-position, a) performing a loop over each plane and then a second loop over each plane, inner to the first loop over each plane, b) for the plane of the outer loop over planes, computing the difference of the data value at said X-position in the plane and the data value at said X-position shifted by one in the X-direction in the plane, c) for the plane of the outer loop over planes, computing the difference of the data value at said Y-position in the plane and the data value at said Y-position shifted by one in the Y-direction in the plane, d) for the plane of the inner loop over planes, computing the difference of the data value at said X-position in the plane and the data value at said X-position shifted by one in the X-direction in the plane, e) for the plane of the inner loop over planes, computing the difference of the data value at said Y-position in the plane and the data value at said Y-position shifted by one in the Y-direction in the plane, f) multiplying the result from the latter step with the result from the latter step and adding then multiplying the result from the latter step and the result from the latter step and the add the resulting products, g) evaluating the component, with indices specified by the counters of the two loops over the planes, of the metric on the space of data for the data values at said at said (X,Y)-position, h) forming the functional density at said (X,Y)-position as the product of the results from the latter step with the result from the latter step, i) adding the result for the functional density from the previous step to the result for the functional from the previous roll of this innermost loop; if this is the first roll of said innermost loop over each plane, then the result for the functional is set to the value of the latter functional density, and j) upon completion of said loops over the (X,Y)-positions assigning the result for the functional as the value of the equivalence signature.