Method and apparatus for processing image signal

A method for processing image signals comprising the steps of:

• determining a mask size in accordance with a pitch of a periodic pattern in original image information corresponding to said image signals, respectively;
• smoothing a readout of said original image information with reference to said mask size to obtain a first unsharpness signal;
• smoothing said first unsharpness signal to obtain a second unsharpness signal;
• determining the difference between said first unsharpness signal and said second unsharpness signal;
• comparing said difference with a predetermined threshold value;
• sharpening said first unsharpness signal to obtain an output image signal only when the absolute value of said difference is greater than said threshold value;
• and
• halftoning said output image signal.

An apparatus for processing image signals comprising:

• a mask size computing circuit that determines a mask size in accordance with a pitch of periodic pattern in original image information corresponding to said image signals, respectively;
• a first smoothing circuit that smoothes a readout of said original image information with reference to said mask size to obtain a first unsharpness signal;
• a second smoothing circuit that smoothes said first unsharpness signal to obtain a second unsharpness signal;
• a differential circuit that determines the difference between said first and second unsharpness signals;
• a comparator circuit that compares the absolute value of said difference with a predetermined threshold value;
• a sharpening circuit that sharpens said first unsharpness signal to obtain an output image signal when the absolute value of said difference is greater than said threshold value;
• a circuit that directly delivers one of said first unsharpness signal and said second unsharpness signal as said output image signal when the absolute value of said difference is at most equal to said threshold value;
• and
• a halftoning circuit that halftones said output image signal.