1. A halftone image data signal processing apparatus comprising: memory means for receiving and storing half tone image data signals corresponding to a plurality of pixels, each pixel corresponding to an N-valued data value; first evaluation means for evaluating a contrast level of a target pixel stored in said memory means as one of said image data signals by weighting said target pixel by a first predetermined amount and gathering data of pixels represented by further ones of said image data signals surrounding said target pixel, and for generating a first evaluation signal indicative of contrast evaluation of said target pixel, said first evaluation signal being M-valued data, wherein M is greater than N; second evaluation means for evaluating a contrast level of said target pixel stored in said memory means as said one of said image data signals by weighting said target pixel by a second predetermined amount, less than said first predetermined amount, and gathering data of pixels represented by said further ones of said image data signals surrounding said target pixel, and for generating a second evaluation signal indicative of contrast evaluation of said target pixel, said second evaluation signal corresponding to an M-valued data value; feature extraction means for locating pixels corresponding to image features at an area surrounding said target pixel, for receiving and evaluating image data signals corresponding to said pixels to detect whether or not an image formed by said pixels at said surrounding area contains an edge component, and for generating an edge appearance signal indicative of degree of appearance of the image to be an edge component at said surrounding area; and mixer means for receiving said edge appearance signal and for receiving and mixing said first and second evaluation signals at a ratio determined by said edge appearance signal such that said first evaluation signal is mixed at a higher rate than said second evaluation signal as the edge appearance signal increases to generate a plurality of processed half tone image data signals, said mixer means further comprises quantizing means for quantizing the mixed data having a value between zero and M to a further data value having a value between zero and L, wherein L is equal to or greater than M.