Bi-level image display signal processing apparatus

Claim 2

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory mans for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined level value to thereby determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value determined for the object picture element; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; error apportionment value computing means for computing, based on the bi-level error of the object picture element and on the apportionment factors, a set of error apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; residual error computing means for computing the sum total of the error apportionment values and a residual error which is the difference between the bi-­level value produced from the difference computing means and the sum total, and for apportioning the residual error as a set of residual error apportionment values by utilizing respective factors which have been predetermined for respective ones of the set of unprocessed picture elements positioned at the periphery of the object picture element; apportionment value computing means for computing respective apportionment values for the set of unprocessed picture elements positioned at the periphery of the object picture element, based on the error apportionment values and the residual error apportionment values; and error updating means for adding the apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing results of the additions in the error memory means as updated accumulated error values.

Claim 3

An image signal processing apparatus according to claim 1, in which the apportionment factor generating means generates the apportionment factors as a combination of the ratios 8/16, 4/16, 2/16 and 2/16, in which the bi-level error is obtained as a binary value comprising more than three bits, and in which the residual error computation means determines respective residual apportionment values corresponding to the set of unprocessed picture elements positioned peripheral to the object picture element in accordance with respective logic levels of three least significant bits of the bi-level error.

Claim 4

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; error apportionment value computing means for computing, based on the bi-level error from the error computation means and the apportionment factors, error apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; residual error computation means for deriving the sum total of the error apportionment values and computing a residual error which is the difference between the bi-level error and the sum total; and error updating means for adding a predetermined one of the error apportionment values to the residual error, and for adding the remaining ones of the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and storing results of each of these additions in the error memory means as updated accumulated error values.

Claim 5

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; error apportionment value computing means for computing, based on the bi-level error from the error computation means and the apportionment factors, error apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; residual error computation means for deriving the sum total of the error apportionment values and computing a residual error which is the difference between the bi-level error and the sum total; and error updating means for adding a predetermined one of the error apportionment values to the residual error, and for adding the remaining ones of the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and storing results of each of these additions in the error memory means as updated accumulated error values.

Claim 6

An image signal processing apparatus according to claim 3 in which, during a first processing interval the residual error computation means derives the sum of the error apportionment values corresponding to the set of unprocessed picture elements positioned peripheral to the object picture element, computes the residual error as a difference between the sum of the error apportionment values and the bi-level error, and temporarily stores the residual error which is thus obtained, to be read out during a processing interval immediately succeeding the first processing interval.

Claim 7

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error E xy which is the difference between the corrected input level value and the bi-level value of the object picture element; bi-level error apportionment means for extracting the n least significant bits of the bi-level error E xy , where n is a predetermined fixed integer, and producing these bits as a bi-level error E xyl , and for setting each of these n least significant bits of the bi-level error E xy to zero and outputting the result as a bi-­level error E xyu ; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error E xyu among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; apportionment value computing means for computing, based on the bi-level error E xyu and the apportionment factors from the apportionment factor generating means, apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; and error updating means for adding a predetermined one of the error apportionment values to the bi-level error E xyl and for adding the remaining error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing addition results obtained thereby in the error memory means as respective updated accumulated error values.

Claim 8

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error E xy which is the difference between the corrected input level value and the bi-level value of the object picture element; bi-level error apportionment means for extracting the n least significant bits of the bi-level error E xy , where n is a predetermined fixed integer, and producing these bits as a bi-level error E xyl , and for setting each of these n least significant bits of the bi-level error E xy to zero and outputting the result as a bi-­level error E xyu ; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error E xyu among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; apportionment value computing means for computing, based on the bi-level error E xyu and the apportionment factors from the apportionment factor generating means, apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; and error updating means for adding a predetermined one of the error apportionment values to the bi-level error E xyl and for adding the remaining error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing addition results obtained thereby in the error memory means as respective updated accumulated error values.

Claim 9

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; error computation means for computing an amended error based on the bi-level error and a factor which is a function of the threshold level; apportionment factor generating means for generating apportionment factors for apportioning the amended error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, the apportionment factors being generated by selection from a plurality of sets of apportionment factors at intervals having a predetermined alteration period, and; error apportionment and updating means for computing error apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element, based on the correction error and the apportionment factors, adding the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing addition results obtained thereby in the error memory means as updated accumulated error values.

Claim 10

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding an input level value of the object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; error computation means for computing an amended error based on the bi-level error and a factor which is a function of the threshold level; apportionment factor generating means for generating apportionment factors for apportioning the amended error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, the apportionment factors being generated by selection from a plurality of sets of apportionment factors at intervals having a predetermined alteration period, and; error apportionment and updating means for computing error apportionment values respectively corresponding to the set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element, based on the correction error and the apportionment factors, adding the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing addition results obtained thereby in the error memory means as updated accumulated error values.

Claim 11

An image signal processing apparatus according to claim 6, in which the error computation means derives the correction error as a difference between a maximum value of the bi-level value and one-half of the threshold level value.

Claim 12

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: density modulation means for adding to the input level values respective ones of successively generated values which vary in amplitude and polarity independently of the input level values, to produce successive modulated input level values; error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding a modulated input level value of an object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, the apportionment factors being generated by randomly altering mutual correspondences between picture element positions and apportionment factors of a single set of apportionment factors, the random alteration being executed at predetermined periodic intervals; and error apportionment and updating means for computing error apportionment values res corresponding to the set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element, based on the bi-level error and the apportionment factors, adding the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing results of the additions in the error memory means as updated accumulated error values.

Claim 13

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: density modulation means for adding to the input level values respective ones of successively generated values which vary in amplitude and polarity independently of the input level values, to produce successive modulated input level values; error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding a modulated input level value of an object picture element to an accumulated error which has been previously stored at a location in the error memory means corresponding to the object picture element position, to thereby produce a corrected input level value; bi-level conversion means for comparing the corrected input level value with a predetermined value to determine a bi-level value for the object picture element; difference computation means for obtaining a bi-­level error which is the difference between the corrected input level value and the bi-level value of the object picture element; apportionment factor generating means for generating apportionment factors for apportioning the bi-level error among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, the apportionment factors being generated by randomly altering mutual correspondences between picture element positions and apportionment factors of a single set of apportionment factors, the random alteration being executed at predetermined periodic intervals; and error apportionment and updating means for computing error apportionment values res corresponding to the set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element, based on the bi-level error and the apportionment factors, adding the error apportionment values to respective accumulated error values which have been stored previously at respectively corresponding locations in the error memory means, and for storing results of the additions in the error memory means as updated accumulated error values.

Claim 14

An image signal processing apparatus according to claim 8, in which the density modulation means superimposes successive density levels upon respective input level values of successive picture elements by cyclically generating predetermined sets of density levels, the density levels being generated in synchronism with successive picture element processing intervals.

Claim 15

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding the input level of an object picture element to an accumulated error which has been previously stored at a position within the error memory means corresponding to the object picture element, to thereby produce a corrected input level; difference computation means for subtracting the corrected input level from the first and second predetermined output levels to produce a first difference level value D₁ and a second difference level value D₂ respectively, prior to obtaining the bi-level value for the object picture element, and storing the first and second difference level values temporarily in respective difference level registers; bi-level conversion means for respectively adding the difference level values D₁ and D₂, read out from the difference level registers, to a bi-level error stored during a preceding picture element processing operation to thereby obtain errors E₁ and E₂ respectively, comparing the error E₁ with a predetermined threshold value to thereby obtain a bi-­level value for the object picture element, selecting error E₁ or error E₂ in accordance with the bi-level value thus obtained, to thereby obtain a bi-level error for the object picture element, and storing the bi-­level error temporarily in a bi-level error register for use during a succeeding picture element processing operation; apportionment factor generating means operating once in each of predetermined picture element processing intervals for generating a set of apportionment factors, by selection from a plurality of sets of apportionment factors, for apportioning the bi-level error of the object picture element among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; and error apportionment and updating means for computing respective error apportionment values corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, based on a value of bi-level error read out from the bi-level error register and the set of of apportionment factors produced from the apportionment factor generating means, adding the error apportionment values to respective values of accumulated error which have been stored previously at locations in the error memory means respectively corresponding to the set of peripheral picture elements, and storing the respective addition results in the error memory means as updated accumulated error values.

Claim 16

An image signal processing apparatus for sequentially processing input level values representing respective density levels of picture elements to produce corresponding bi-level values, comprising: error memory means for storing a bi-level error of an object picture element in a set of memory locations respectively corresponding to a set of picture elements which have not yet been processed and are peripherally adjacent to the object picture element; input correction means for adding the input level of an object picture element to an accumulated error which has been previously stored at a position within the error memory means corresponding to the object picture element, to thereby produce a corrected input level; difference computation means for subtracting the corrected input level from the first and second predetermined output levels to produce a first difference level value D₁ and a second difference level value D₂ respectively, prior to obtaining the bi-level value for the object picture element, and storing the first and second difference level values temporarily in respective difference level registers; bi-level conversion means for respectively adding the difference level values D₁ and D₂, read out from the difference level registers, to a bi-level error stored during a preceding picture element processing operation to thereby obtain errors E₁ and E₂ respectively, comparing the error E₁ with a predetermined threshold value to thereby obtain a bi-­level value for the object picture element, selecting error E₁ or error E₂ in accordance with the bi-level value thus obtained, to thereby obtain a bi-level error for the object picture element, and storing the bi-­level error temporarily in a bi-level error register for use during a succeeding picture element processing operation; apportionment factor generating means operating once in each of predetermined picture element processing intervals for generating a set of apportionment factors, by selection from a plurality of sets of apportionment factors, for apportioning the bi-level error of the object picture element among the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element; and error apportionment and updating means for computing respective error apportionment values corresponding to the set of picture elements which have not yet been processed and are positioned at the periphery of the object picture element, based on a value of bi-level error read out from the bi-level error register and the set of of apportionment factors produced from the apportionment factor generating means, adding the error apportionment values to respective values of accumulated error which have been stored previously at locations in the error memory means respectively corresponding to the set of peripheral picture elements, and storing the respective addition results in the error memory means as updated accumulated error values.

Claim 17

An image signal processing apparatus according to claim 10, in which during a first processing interval in which a bi-level value corresponding to a first picture element is derived by the bi-level conversion means, difference level values D₁ and D₂ corresponding to a second picture element are respectively derived by the difference computation means and temporarily stored to be read out and transferred to the bi-level conversion means during a second processing interval following the first processing interval, and a corrected input value corresponding to a third picture element is derived by the input correction means and temporarily stored to be read out to the difference computation section during the second processing interval.