1. A method for processing data, comprising: receiving a temporal sequence of depth maps of a scene containing a humanoid form, the depth maps comprising a matrix of pixels having respective pixel depth values; using a digital processor, processing at least one of the depth maps so as to find three-dimensional (3D) medial axes of limbs of the humanoid form; identifying joints in the limbs based on the 3D medial axes; and tracking movements of the humanoid form over the sequence using the medial axes and the joints of the limbs.