1. A system, comprising: a support; a pendular element; an elastic restraint coupling the pendular element to the support and operable to apply to the pendular element a restraining force that opposes movement of the pendular element in at least a first direction; an image sensor operable to capture images of the pendular element; and a processing system operable to determine one or more movement measures describing movement of the support from comparisons of ones of the images captured by the image sensor.