As in every other system, nonlinearity can introduce functional capabilities not available by linear elements. Basically all active resistors that have been used so far for implementing the resistive elements are nonlinear, although the nonlinearity has not been a major goal in these networks. The problem with designing nonlinear resistive elements is two-folded with respect to linear resistor networks. Firstly, a model should exist which utilizes a nonlinear resistive element, and of course illustrates improvements. Secondly, a proper resistive element should be found which realizes the required nonlinearity. Considering that even designing a linear resistor to operate under various condition is a great challenge, the difficulty in designing networks using nonlinear elements becomes obvious.
There is only one form of nonlinear element that has been reported and used for vision chips, i.e. the resistive fuse. As described in 2.15 the resistive fuse provides smoothing only when the image contrast is small, but when the contrast is large no smoothing is performed. Therefore, edges with large contrast which include useful information remain safe from being smoothed out.