This invention relates to an image processing device for converting a scanned image into a monochrome image, and more particularly relates to an image processing device for processing a half tone between a black level and a white level.
A conventional device converts a scanned image into a black and white image in order to process and to transmit the scanned image easily. Japanese laid open patent application Ser. No. 62-277857 published on Dec. 2, 1987 discloses a circuit for converting the scanned image into a black and white image. The circuit samples and holds a peak level of the scanned image signal in order to determine a threshold level. The circuit compares the image signal with the threshold level and generates a corresponding image signal having only black and white levels.
However, the conventional device cannot process half tones between black and white levels, because the half tone levels fall into one of the black and the white levels. Additional circuits for determining double or triple threshold levels are required in order to process the half tone levels.