This document describes a biometric authentication system including a biometric authentication manager is configured to receive user input at a touch-sensitive display, determine a shape of the user input, and alter a luminosity and/or color of one or more portions of a region of the touch-sensitive display. Through such a technique, the biometric authentication manager can implement spatially, temporally, and/or chromatically variable illumination patterns to produce a fingerprint reflection, receive the fingerprint reflection, and then analyze the fingerprint reflection for authenticity and/or for liveness characteristics effective to detection spoof fingerprints.