Corneal geometry: preprocessing the map of curvature before compressing

An algorithm to obtain a concise description of a map of corneal curvature is described in [Lenarduzzi, 2012]; here we enter into the details of its computational cost. In the current note we describe how to preprocess the map in order to correct the colors of some pixels: such pixels are mixture of colors from the palette (empty of pure black and pure white) with the pure white. The algorithm assigns one color of the palette to each one of such pixels.