Time-derivative adaptive silicon photoreceptor array

We designed and tested a two-dimensional silicon receptor array constructed from pixels that temporally highpass filter the incident image. There are no surround interactions in the array; all pixels operate independently except for their correlation due to the input image. The high-pass output signal is computed by sampling the output of an adaptive, high-gain, logarithmic photoreceptor during the scanout of the array. After a pixel is sampled, the output of the pixel is reset to a fixed value. An interesting capacitive coupling mechanism results in a controllable high-pass filtering operation. The resulting array has very low offsets. The computation that the array performs may be useful for time-domain image processing, for example, motion computation. 1. TIME-DOMAIN IMAGE PROCESSING Real-time image processing is expensive. Much of the computational load involved in computing motion parallax, optical flow, and object tracking lies in the image preprocessing, before any sophisticated global vision algorithms are applied. Specialized parallel digital processors like the PIPE machine have been used to do real-time, time-domain, image processing. These machines are reprogrammable and flexible in their applications, and have been used to implement algorithms developed by the machine vision community,2 and also to model biological visual function.11 Since they use a large number of high speed, high power, digital chips, they are of limited usefulness in terms of teaching us how to build vision systems that map naturally onto silicon. One of the fundamental preprocessing operations is high-pass filtering, or temporal differentiation. This operation is useful for all types of image motion computation. In this paper, we describe a circuit that computes a pure high-pass, temporally filtered version of the incident image. This time-derivative operation is accomplished with a pixel measuring 86 microns on a side, and the entire core of the chip, consisting of 68 by 43 pixels, consumes about 4 mW of power (1.4μW per pixel). The pixels have zero DC response, and the offsets between pixels are very small. Many models of motion discrimination rely on inputs with strictly high-pass, or time-derivative, characteristics. Much of the previous work on silicon retinas has focused on the spatial aspects of image processing, and in those models, time is treated largely as a dimension over which to adapt.5, 8, 9 In more recent work, the time-domain has been treated more centrally. Mahowald4 reported a design which elegantly integrates both time and space into the same pixel. Her pixels have responses with both high-pass and low-pass characteristics, and they incorporate feedback from