Multilevel value output device

A multilevel value output device, comprising:

• a relative density value storage portion that prestores therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined by normalizing the at least three multilevel output values based on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the at least three relative density values including a middle relative density value, a higher relative density value higher than the middle relative density value, and a lower relative density value lower than the middle relative density value, the at least three multilevel output values including a middle multilevel output value, a higher multilevel output value higher than the middle multilevel output value, and a lower multilevel output value lower than the middle multilevel output value, the higher, middle, and lower relative density values corresponding to the higher, middle, and lower multilevel output values, respectively;
• an input portion that receives an input value indicative of density of a pixel in an input image;
• a corrected value calculation portion that calculates a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• an output value generation portion that compares the corrected value with at least one of the at least two threshold values, the at least two threshold values including a higher threshold value and a lower threshold value that is lower than the higher threshold value, the output value generation portion converting the corrected value into one of the at least three multilevel values based on the compared results and outputting the resultant one multilevel output value, the output value generation portion converting the corrected value into the higher multilevel output value when the corrected value is greater than the higher threshold value, the output value generation portion converting the corrected value into the middle multilevel output value when the corrected value is between the higher threshold value and the lower threshold value, the output value generation portion converting the corrected value into the lower multilevel output value when the corrected value is smaller than the lower threshold value;
• a relative density setting portion that refers to the threshold value storage portion and that sets one relative density value that corresponds to the multilevel output value generated by the output value generation portion;
• an error value calculation portion that calculates a difference between the corrected value and the relative density value set by the relative density setting portion, and that sets the calculated result as an error value for the subject pixel;
• and
• a threshold setting portion that sets, upon receipt of the input value, the higher and lower threshold values in a manner that the higher and lower threshold values become close to each other when the input value becomes close to the middle relative density value.

A multilevel value output method using a relative density value storage portion that prestores therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined dependently on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the method comprising:

• receiving an input value indicative of density of a pixel in an input image;
• calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• comparing the corrected value with at least one of the at least two threshold values, converting the corrected value into one of the at least three multilevel values based on the compared results, outputting a resultant multilevel output value, referring to the threshold value storage portion, setting one relative density value that corresponds to the output value, calculating a difference between the corrected value and the relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• reducing, when the corrected value is close to each of at least one of the at least three relative density values, a frequency, at which the corrected value is converted into one multilevel output value that corresponds to the subject relative density value, thereby reducing a frequency at which the error value for the subject pixel becomes close to zero.

A multilevel value output method using a relative density value storage portion that prestores therein at least three relative density values for at least three multilevel output values in one-to-one correspondence With each other, the at least three relative density values being defined by normalizing the at least three multilevel output values based on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the at least three relative density values including a middle relative density value, a higher relative density value higher than the middle relative density value, and a lower relative density value lower than the middle relative density value, the at least three multilevel output values including a middle multilevel output value, a higher multilevel output value higher than the middle multilevel output value, and a lower multilevel output value lower than the middle multilevel output value, the higher, middle, and lower relative density values corresponding to the higher, middle, and lower multilevel output values, respectively, the method comprising:

• receiving an input value indicative of density of a pixel in an input image;
• calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• comparing the corrected value with at least one of the at least two threshold values, the at least two threshold values including a higher threshold value and a lower threshold value that is lower than the higher threshold value, thereby converting the corrected value into one of the at least three multilevel values based on the compared results and outputting the resultant one multilevel output value, the comparing step converting the corrected value into the higher multilevel output value when the corrected value is greater than the higher threshold value, the comparing step converting the corrected value into the middle multilevel output value when the corrected value is between the higher threshold value and the lower threshold value, the comparing step converting the corrected value into the lower multilevel output value when the corrected value is smaller than the lower threshold value;
• referring to the threshold value storage portion and setting one relative density value that corresponds to the multilevel output value generated;
• calculating a difference between the corrected value and the set relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• setting, upon receipt of the input value, the higher and lower threshold values in a manner that the higher and lower threshold values become close to each other when the input value becomes close to the middle relative density value.

A multilevel value output program to be executed by a computer that has a relative density value storage portion storing therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined dependently on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the program comprising:

• a program receiving an input value indicative of density of a pixel in an input image;
• a program calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• a program comparing the corrected value with at least one of the at least two threshold values, converting the corrected value into one of the at least three multilevel values based on the compared results, outputting a resultant multilevel output value, referring to the threshold value storage portion, setting one relative density value that corresponds to the output value, calculating a difference between the corrected value and the relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• a program reducing, when the corrected value is close to each of at least one of the at least three relative density values, a frequency, at which the corrected value is converted into one multilevel output value that corresponds to the subject relative density value, thereby reducing a frequency at which the error value for the subject pixel becomes close to zero.

A recording medium storing a multilevel value output program and readable by a computer that has a relative density value storage portion storing therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined dependently on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the multilevel value output program comprising:

• a program receiving an input value indicative of density of a pixel in an input image;
• a program calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• a program comparing the corrected value with at least one of the at least two threshold values, converting the corrected value into one of the at least three multilevel values based on the compared results, outputting a resultant multilevel output value, referring to the threshold value storage portion, setting one relative density value that corresponds to the output value, calculating a difference between the corrected value and the relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• a program reducing, when the corrected value is close to each of at least one of the at least three relative density values, a frequency, at which the corrected value is converted into one multilevel output value that corresponds to the subject relative density value, thereby reducing a frequency at which the error value for the subject pixel becomes close to zero.

A multilevel value output program to be executed by a computer that has a relative density value storage portion storing therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined by normalizing the at least three multilevel output values based on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the at least three relative density values including a middle relative density value, a higher relative density value higher than the middle relative density value, and a lower relative density value lower than the middle relative density value, the at least three multilevel output values including a middle multilevel output value, a higher multilevel output value higher than the middle multilevel output value, and a lower multilevel output value lower than the middle multilevel output value, the higher, middle, and lower relative density values corresponding to the higher, middle, and lower multilevel output values, respectively, the program comprising:

• a program of receiving an input value indicative of density of a pixel in an input image;
• a program of calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• a program of comparing the corrected value with at least one of the at least two threshold values, the at least two threshold values including a higher threshold value and a lower threshold value that is lower than the higher threshold value, thereby converting the corrected value into one of the at least three multilevel values based on the compared results and outputting the resultant one multilevel output value, the comparing program converting the corrected value into the higher multilevel output value when the corrected value is greater than the higher threshold value, the comparing program converting the corrected value into the middle multilevel output value when the corrected value is between the higher threshold value and the lower threshold value, the comparing program converting the corrected value into the lower multilevel output value when the corrected value is smaller than the lower threshold value;
• a program of referring to the threshold value storage portion and setting one relative density value that corresponds to the multilevel output value generated;
• a program of calculating a difference between the corrected value and the set relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• a program of setting, upon receipt of the input value, the higher and lower threshold values in a manner that the higher and lower threshold values become close to each other when the input value becomes close to the middle relative density value.

A recording medium storing a multilevel value output program and readable by a computer that has a relative density value storage portion storing therein at least three relative density values for at least three multilevel output values in one-to-one correspondence with each other, the at least three relative density values being defined by normalizing the at least three multilevel output values based on a predetermined maximum density that is defined for a highest relative density value among the at least three relative density values, the at least three relative density values including a middle relative density value, a higher relative density value higher than the middle relative density value, and a lower relative density value lower than the middle relative density value, the at least three multilevel output values including a middle multilevel output value, a higher multilevel output value higher than the middle multilevel output value, and a lower multilevel output value lower than the middle multilevel output value, the higher, middle, and lower relative density values corresponding to the higher, middle, and lower multilevel output values, respectively, the program comprising:

• a program of receiving an input value indicative of density of a pixel in an input image;
• a program of calculating a corrected value by adding to the input value at least a part of an error value that has been generated by at least one pixel near to the subject pixel;
• a program of comparing the corrected value with at least one of the at least two threshold values, the at least two threshold values including a higher threshold value and a lower threshold value that is lower than the higher threshold value, thereby converting the corrected value into one of the at least three multilevel values based on the compared results and outputting the resultant one multilevel output value, the comparing program converting the corrected value into the higher multilevel output value when the corrected value is greater than the higher threshold value, the comparing program converting the corrected value into the middle multilevel output value when the corrected value is between the higher threshold value and the lower threshold value, the comparing program converting the corrected value into the lower multilevel output value when the corrected value is smaller than the lower threshold value;
• a program of referring to the threshold value storage portion and setting one relative density value that corresponds to the multilevel output value generated;
• a program of calculating a difference between the corrected value and the set relative density value, and setting the calculated result as an error value for the subject pixel;
• and
• a program of setting, upon receipt of the input value, the higher and lower threshold values in a manner that the higher and lower threshold values become close to each other when the input value becomes close to the middle relative density value.