Abstract: An edge detection mechanism for an imagery signal compression and transmission system processes histogram and variance data to identify the presence of a uniform area with a block of transform signals. The presence of an edge within an image block, for which corresponding transform coefficients are obtained, is identified by initially generating a histogram of the image block. If the maximum value of the histogram exceeds a first threshold value, a uniform area is inferred, and adaptive quantization that would otherwise be carried out prior to executing a minimum redundancy encoding scheme is bypassed. If the maximum value of the histogram does not exceed the first threshold value, a set of variance calculations is carried out for use with a further sequence of edge detection decisions. The total variance and low frequency variance of the block are determined in accordance with the transform coefficients associated with the image block.