Bi-level image display signal processing apparatus

Claim 1

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, including error memory means (101) for holding an accumulated error (S XY ) for an object picture element in one of a set of memory locations respectively corresponding to said object picture element and to a set of picture elements which have not yet been processed and are peripherally adjacent to said object picture element, input correction means (105) for adding an input level value of said object picture element to said accumulated error (S XY ) for said object picture element to thereby produce a corrected input level value, bi-level conversion means (108) for comparing said corrected input level value with a predetermined threshold value to thereby determine a bi-level value (P xy) for said object picture element, and difference computation means (109) for obtaining a bi-level error (E XY ) which is a difference between said corrected input level value and said bi-level value determined for said object picture element, characterized in: density modulation means (928) for adding to said input level values (I XY ) respective ones of successively generated values which vary in amplitude and polarity independently of said input level values, to produce successive modulated input level values (I' XY ), and for supplying said modulated input level values to said input correction means (105); apportionment factor generating means (910) for generating apportionment factors for apportioning said bi-level error among said set of peripherally adjacent set of picture elements error apportionment and updating means (920) for computing error apportionment values respectively corresponding to said peripherally adjacent set of picture elements, based on a value of bi-level error read out from said bi-level error register and said set of apportionment factors produced from said apportionment factor generating means, adding said error apportionment values to respective values of accumulated error which have been stored previously at locations in said error memory means respectively corresponding to said set of peripheral picture elements, and storing said respective addition results in said error memory means as updated accumulated error values.