1. An apparatus comprising, in combination: (a) an active light source; (b) a time-of-flight camera which is configured to take measurements of phase and amplitude of incident light at each pixel, respectively, in a set of pixels of the camera, which incident light comprises amplitude-modulated continuous-wave light that (i) is emitted by the active light source and reflects from an object, and then (ii) travels to a diffusely reflective surface, and then (iii) reflects from the diffusely reflective surface and travels to the time-of-flight camera; and (c) a computer that is programmed to perform a calculation that computes, for each respective point in a set of points on a surface of the object, a 3D position of the respective point, wherein the calculation involves solving a system of linear equations that represents the measurements.