Image comparison systems

The invention is for comparing a master image, stored by means of binary signals, with an image under test, which is stored by means of corresponding binary signals. Both sets of binary signals are fed out into an EXCLUSIVE OR gate. This will therefore produce output signals defining any differences between the image under test and the master image. In addition, however, the gate output binary signals due to any misalignment or digitization errors between the images stored in the two stores. To reduce or eliminate the latter, the master image is fed into an edge detector which produces binary signals representing a peripheral edge region or "frame" around the master image. This peripheral edge region is "dilated" (that is, the peripheral edge region is thickened to a predetermined extent). The signals representing the stored and dilated edge region are inverted and ANDed with the output of the EXCLUSIVE OR gate. Provided that the peripheral edge region has been sufficiently dilated, it will cancel out any binary signals produced by the EXCLUSIVE OR gate as a result of peripheral misalignment. The AND gate will therefore only output signals corresponding to the true errors between the two stored images.