Schematic illustration of the reduced matching procedure. (a) The line of sight is fixed along the z-axis, while the principal axis of the three-dimensional density field is rotated to representative directions in the positive xz plane. (b) Four-image configurations related by translation, rotation, or reflection are treated as geometrically equivalent. In the actual calculation, all six pairwise distances among the four images are used, including the two diagonal distances not explicitly labeled in the schematic figure. See Section A.2 for further discussion.