Velocity-Specific Representation

Low-pass and band-pass prefiltering are common precursors to many existing techniques for measuring image velocity (reviewed in Chapter 5). Depending on the particular approach, the main objectives of such filtering are noise reduction and/or the isolation of different types of image structure (e.g. zero-crossing contours, or different scales for coarse-fine analysis). More recently, it has been recognized that the filters themselves can be tuned to relatively narrow ranges of speed and orientation, as well as scale [Morgan, 1980; Fahle and Poggio, 1981; Adelson and Bergen, 1985; Fleet and Jepson, 1985; Watson and Ahumada, 1985; Heeger, 1988]. Since translating image patches can be viewed as oriented intensity structure in space-time (in much the same way as 1-d profiles can be oriented in space) they might therefore be detected using 3-d orientation-tuned filters. This intuition, in addition to the more formal analysis described in Chapter 3, leads to the design of velocity-tuned filters.