1. A method of processing an input digital image containing an array of input pixels having associated digital input signals which are to be processed to produce digital output signals representative of the values of output pixels of an enhanced output digital image, said method comprising the steps of: (a) providing an adaptive spatial filter mechanism which selectively performs spatial high pass filtering of successive portions of an input digital signal associated with a respective input pixel of an input digital image in accordance with a first prescribed relationship between a variation in spatial activity content within an image window containing said input pixel and an adjustable threshold level; (b) applying a digital input signal to the adaptive filter mechanism provided in step (a), so as to produce a digital output signal associated with a respective output pixel of said output digital image; and wherein said adaptive spatial filter mechanism is operative to selectively adjust said adjustable threshold level to a new threshold level for each of said successive portions of said input digital image in accordance with the contents of a respective portion of said digital input image being processed, and wherein, for each successive portion of said input digital image to be processed, said adaptive spatial filter mechanism is operative to generate a respective prospective threshold level in accordance with the contents of said each successive portion of said digital input image being processed, and wherein, for a respective one of said successive portions of said input digital image, said adaptive spatial filter mechanism is operative to selectively employ either its prospective threshold level or the previous threshold level as said adjustable threshold level in dependence upon a prescribed difference in contents of said respective one of said successive portions of said image and another of said successive portions of said image.