The present disclosure relates generally to signal encoding for printed objects. One implementation selects an embed direction based on a minimal visibility axis of a 1 JND ellipse at a certain color center. One claim recites an apparatus comprising: memory for storing chromatic contrast sensitivity data representing multiple color encoding angles; one or more multi-core processors configured for: using the stored chromatic contrast sensitivity data, generating an ellipse around a first color center; and from the ellipse, determining a signal encode direction, the signal encode direction comprising an angle α representing a negative angle between the ‘a*’ axis in an CIELAB space and a direction of minimum sensitivity of an encoded signal, in which the angle α comprises −9 degrees≥α≥−25 degrees. Other technology described.