Spatio-temporal image processing involves an extra dimension of information in addition to spatial ones, i.e. temporal information. It is known that temporal information, usually addressed in the context of motion detection, can provide extra cues about the contents, structure, and other high or low level information present in a scene. This belief is strongly supported by experiments on species with relatively primitive visual system, but very capable of performing visual tasks. These creatures are insects. Insects heavily rely on motion detection in avoiding obstacles, landing, tracking, estimating range, and so on.
In spatio-temporal image processing the intensity values should be considered across the spatial and temporal axes. Hence, past values of image intensity should somehow be stored or delayed to be used in processing. As already mentioned through the words ``store'' and ``delay'', two methods can be used for involving past image information, sampled-data and continuous. In sampled-data methods the image is sampled and stored in analog memory elements. In continuous method analog delay elements are used. A challenging part of designing spatio-temporal vision chips arises from the problems associated with analog memory and delay elements.
Fundamentally, an analog storage element can consist of either a capacitance holding charge or an inductor holding current. With the latter being infeasible in standard VLSI processes the first method is the only choice. Due to the leakage existing in any capacitive node, large capacitances should be used to increase the so called ``charge retention time''. There are two main categories for charge storage, a DRAM style structure, and a floating gate structure. DRAM type memories can hold the charge only up to a few seconds, but floating gate devices can achieve retention times of several hundred years.
Delaying a signal in analog domain again requires a capacitive node, which can hold the information. However, in continuous delay elements charge is continuously injected to the capacitor and read out.
It is necessary to point out the fact that detecting intensity variations over time for single pixels cannot be regarded as motion detection in any sense. The term ``motion'' has a spatial as well as temporal component built into it.
In the following sections circuits utilized for storing and delaying charge are described.