Binarizing system of picture image signals

A system for preprocessing an analog picture image signal representing a characteristic of a picture image obtained by scanning a document along successive main scanning lines, said system comprising:

• quantizing means for sequentially quantizing said analog picture image signals to form a first number of picture element signals for each of the main scanning lines, each of the picture element signals representing a different position of said picture image along one of the main scanning lines;
• a line memory for temporarily storing a plurality of storage signals corresponding to picture element signals for main scanning lines preceding the main scanning line whose picture element signals are currently being formed by said quantizing means;
• output means coupled to said line memory for receiving from said line memory output signals corresponding to ones of the storage signals for positions of the picture elements currently being formed by said quantizing means;
• comparator means, coupled to said quantizing means and said output means, for forming comparison signals representing a comparison of the picture element signals and the output signals;
• first calculating means for adjusting values of the output signals toward a first level of the picture image characteristic thereby to produce adjusted picture element signals;
• selection means, coupled to said first calculating means and said quantizing means and responsive to the comparison signals, for selecting the adjusted picture element signals as reference signals when the picture element signals have values closer to the first level than to values of the adjusted picture element signals, and otherwise for selecting the picture element signals as the reference signals;
• writing means for writing the reference signals into said line memory;
• and
• second calculating means coupled to said selection means and said quantizing means for modifying the picture element signals toward the first level when the picture element signals have values closer to the first level than do said reference signals.