Chromaticity contour map of the RGB cube: A simple algorithm

A simple algorithm (amenable to spreadsheets) is described for plotting a luminance contour map of the color gamut of an additive-primary display given the chromaticities of its primaries and screen white. Each contour of the plot represents (in either r,g or x,y coordinates) the polygon of intersection of the RGB cube with a plane of constant Y. Each vertex on a contour is an intersection of 1 of the 12 edges of the RGB cube with the constant-Y plane. Feasibility of an edge as a vertex of the polygon is tested through comparison of the computed r,g coordinates with functions of the luminosity coefficients of the primaries. By ordering the RGB-cube edges in a particular way prior to scanning them for feasibility, one is guaranteed to retrieve the vertices of any constant-Y polygon in convex order. This fact facilitates plotting of the polygons.