1. A 3D sensing system comprising: a stereo camera, the stereo camera including an infra-red camera; a light source; and a 3D sensing module arranged to track a retroreflector during a time that an object partially occluding the retroreflector is tracked by the 3D sensing module using image pairs captured by the stereo camera, and to detect a touch event or determine a pressure value based at least in part on a depression of the retroreflector; the 3D sensing module further comprising a synchronization module arranged to generate a trigger pulse to trigger capture of a pair of images by the stereo camera and simultaneous illumination by the light source and an analysis module arranged to analyze the captured images to simultaneously track both the retroreflector and the object partially occluding the retroreflector, the analysis module further arranged to analyze the captured images to generate a 3D contour of each of the retroreflector and the object by extracting a 2D contour of the retroreflector and a 2D contour of the object from each image in an image pair and performing stereo matching between the 2D contours from each image in the image pair; and the 3D sensing module further arranged to remove distortion from each image in the image pair, rectify each image in the image pair, and identify parts of each image to not consider parts of the scene not contained in the other image in the image pair in the analysis.