Abstract: A technique for reducing images that provides useful information about the image and allows fast computation. Using threshold values near the extreme possible values for the convolution window size and using large subsampling tiles nevertheless allows extraction of the information about the typical textures that exist in the document image: text words, text lines, rules, and halftones. In a particular embodiment, 16.times.16 tiles are used for subsampling, 16.times.1 and 1.times.16 windows are used for the convolution, and threshold values of 1 and 16 are used. If the horizontal windows in tiles are aligned with 16-bit boundaries in the computer, the implementation is particularly efficient. For the 16.times.1 horizontal window, a threshold convolution with T=1 can be done on any of the sixteen 16-bit words in the tile by checking whether the word is zero or non-zero. For a 1.times.

Abstract: A programmable delay for the gate signal output of the differentiator in a data analyzing circuit is provided. A data signal is produced by a low pass filter, and a differentiator (high pass filter) produces a gate signal. The cutoff frequency of the low pass filter is controlled by a first control circuit, and a second control circuit is provided to separately control the cutoff frequency of the high pass filter. The second control circuit produces a variation from the base frequency determined by the first control circuit. Thus, the cutoff frequency of the high pass filter will always be proportional to that of the low pass filter.