Generating a threshold matrix to be compared with an original image for performing halftoning of the original image
An image obtained by shrinking an original image in a row direction is halftoned, and this halftone dot image is enlarged in the row direction and recorded at double speed. In order to determine a threshold value of a threshold matrix used for halftoning an image, prepared is a matrix space in accordance with writing positions used for actual recording of an image. In the matrix space, writing elements are set alternately with non-writing elements both in the row direction and in a column direction. In the matrix space, turn-on numbers are allocated to all the writing elements in avoidance of the non-writing elements. Then, in accordance with the turn-on numbers, respective threshold values for the writing elements are determined. By shrinking the threshold matrix in the row direction with omission of the non-writing elements, a threshold matrix for double speed is obtained.
1. A threshold matrix generation method of generating a threshold matrix to be compared with an original image of multi-gradation in performing halftoning of said original image in an image recording apparatus which is capable of recording an image at single speed and at N-fold speed where N is an integer not less than 2, comprising:
a) preparing a matrix space which is a set of matrix elements arranged both in a column direction which corresponds to a recording direction in said image recording apparatus and in a row direction perpendicular to said column direction;
b) setting writing elements which are matrix elements corresponding to writing positions on a recording object so that one writing element is positioned at intervals of (N−1) matrix elements in said column direction and one writing element is positioned at intervals of (N−1) matrix elements or two writing elements are positioned at intervals of (2N−2) matrix elements in said row direction in a whole of said matrix space and setting the other matrix elements as non-writing elements always evenly in the whole area of said matrix space in said operation a) or after said operation a);
c) giving the first turn-on number to one writing element, being thereafter changed to a determined element, and then repeating a process of specifying a writing element which is farthest away from all determined elements and giving a next turn-on number to said writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of an original image, to thereby allocate turn-on numbers to all writing elements; and
d) obtaining a threshold matrix for N-fold speed by determining a threshold value of each writing element in accordance with said turn-on numbers.
2. The threshold matrix generation method according to claim 1 , wherein said N is 2.
3. The threshold matrix generation method according to claim 1 , further comprising:
after said operation c),
e) giving a turn-on number next to the turn-on number to one non-writing element, being thereafter changed to a determined element, and then repeating a process of specifying a non-writing element which is farthest away from all determined elements and giving a next turn-on number to said non-writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of said original image, to thereby allocate turn-on numbers also to all non-writing elements; and
f) obtaining a threshold matrix for single speed by determining a threshold value of each writing element and each non-writing element in accordance with said turn-on numbers.
4. The threshold matrix generation method according to claim 1 , further comprising:
shrinking said threshold matrix for N-fold speed in said row direction by omitting said non-writing elements after said operation d).
5. The threshold matrix generation method according to claim 1 , wherein said matrix space is a virtual matrix space which is virtually set,
a matrix space which is said virtual matrix space shrunken in said row direction by omitting said non-writing elements is a real matrix space to be set in a memory device, and
said operation c) is executed with conversion of coordinates of said real matrix space into coordinates of said virtual matrix space.
6. An image data generation method of generating image data in an image recording apparatus which is capable of recording an image at single speed and at N-fold speed where N is an integer not less than 2, comprising:
preparing a threshold matrix; and
generating halftone dot image data in which an original image of multi-gradation is halftoned, by comparing said original image with said threshold matrix,
wherein said operation of preparing said threshold matrix comprises:
a) preparing a matrix space which is a set of matrix elements arranged both in a column direction which corresponds to a recording direction in said image recording apparatus and in a row direction perpendicular to said column direction;
b) setting writing elements which are matrix elements corresponding to writing positions on a recording object so that one writing element is positioned at intervals of (N−1) matrix elements in said column direction and one writing element is positioned at intervals of (N−1) matrix elements or two writing elements are positioned at intervals of (2N−2) matrix elements in said row direction in a whole area of said matrix space and setting the other matrix elements as non-writing elements always evenly in the whole area of said matrix space in said operation a) or after said operation a);
c) giving the first turn-on number to one writing element, being thereafter changed to a determined element, and then repeating a process of specifying a writing element which is farthest away from all determined elements and giving a next turn-on number to said writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of an original image, to thereby allocate turn-on numbers to all writing elements; and
d) obtaining a threshold matrix for N-fold speed by determining a threshold value of each writing element in accordance with said turn-on numbers.
7. The image data generation method according to claim 6 , wherein said N is 2.
8. The image data generation method according to claim 6 , wherein said operation of preparing said threshold matrix further comprises:
after said operation c),
e) giving a turn-on number next to the turn-on number to one non-writing element, being thereafter changed to a determined element, and then repeating a process of specifying a non-writing element which is farthest away from all determined elements and giving a next turn-on number to said non-writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of said original image, to thereby allocate turn-on numbers also to all non-writing elements; and
f) obtaining a threshold matrix for single speed by determining a threshold value of each writing element and each non-writing element in accordance with said turn-on numbers.
9. The image data generation method according to claim 6 , wherein said operation of preparing said threshold matrix further comprises:
shrinking said threshold matrix for N-fold speed in said row direction by omitting said non-writing elements after said operation d).
10. The image data generation method according to claim 6 , wherein
said matrix space is a virtual matrix space which is virtually set,
a matrix space which is said virtual matrix space shrunken in said row direction by omitting said non-writing elements is a real matrix space to be set in a memory device, and
said operation c) is executed with conversion of coordinates of said real matrix space into coordinates of said virtual matrix space.
11. An image recording apparatus capable of recording an image at single speed and at N-fold speed where N is an integer not less than 2, comprising:
a head part having a plurality of dot output elements for writing dots into a plurality of writing positions, respectively, which are arranged in a width direction perpendicular to a recording direction on a recording object;
a moving mechanism for relatively moving said plurality of writing positions on said recording object with respect to said recording object in said recording direction;
a storage part for storing a threshold matrix;
an image data processing part for generating halftone dot image data in which an original image of multi-gradation is halftoned, by comparing said original image with said threshold matrix; and
a control part for controlling outputs of said plurality of dot output elements on the basis of said halftone dot image data concurrently with a relative movement of said plurality of writing positions on said recording object with respect to said recording object,
wherein said threshold matrix is generated by the operations of:
a) preparing a matrix space which is a set of matrix elements arranged both in a column direction which corresponds to a recording direction in said image recording apparatus and in a row direction perpendicular to said column direction;
b) setting writing elements which are matrix elements corresponding to writing positions on a recording object so that one writing element is positioned at intervals of (N−1) matrix elements in said column direction and one writing element is positioned at intervals of (N−1) matrix elements or two writing elements are positioned at intervals of (2N−2) matrix elements in said row direction in a whole area said matrix space and setting the other matrix elements as non-writing elements always evenly in the whole area of said matrix space in said operation a) or after said operation a);
c) giving the first turn-on number to one writing element, being thereafter changed to a determined element, and then repeating a process of specifying a writing element which is farthest away from all determined elements and giving a next turn-on number to said writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of an original image, to thereby allocate turn-on numbers to all writing elements; and
d) obtaining a threshold matrix for N-fold speed by determining a threshold value of each writing element in accordance with said turn-on numbers.
12. The image recording apparatus according to claim 11 , wherein
said non-writing elements correspond to non-writable positions on said recording object in recording an image at N-fold speed.
13. The image recording apparatus according to claim 11 , wherein
said head part passes only once over each position on said recording object, to thereby record an image.
14. A non-transitory computer-readable recording medium recorded therein a program to cause a computer to generate a threshold matrix to be compared with an original image of multi-gradation in performing halftoning of said original image in an image recording apparatus which is capable of recording an image at single speed and at N-fold speed where N is an integer not less than 2,
said program being executed by a computer to cause said computer to perform the operations of:
a) preparing a matrix space which is a set of matrix elements arranged both in a column direction which corresponds to a recording direction in said image recording apparatus and in a row direction perpendicular to said column direction;
b) setting writing elements which are matrix elements corresponding to writing positions on a recording object so that one writing element is positioned at intervals of (N−1) matrix elements in said column direction and one writing element is positioned at intervals of (N−1) matrix elements or two writing elements are positioned at intervals of (2N−2) matrix elements in said row direction in a whole area of said matrix space and setting the other matrix elements as non-writing elements always evenly in the whole area of said matrix space in said operation a) or after said operation a);
c) giving the first turn-on number to one writing element, being thereafter changed to a determined element, and then repeating a process of specifying a writing element which is farthest away from all determined elements and giving a next turn-on number to said writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of an original image, to thereby allocate turn-on numbers to all writing elements; and
d) obtaining a threshold matrix for N-fold speed by determining a threshold value of each writing element in accordance with said turn-on number.
15. The recording medium according to claim 14 , wherein said N is 2.
16. The recording medium according to claim 14 , wherein said program is executed by said computer to cause said computer to further perform the operations of:
after said operation c),
e) giving a turn-on number next to the turn-on number to one non-writing element, being thereafter changed to a determined element, and then repeating a process of specifying a non-writing element which is farthest away from all determined elements and giving a next turn-on number to said non-writing element, being thereafter changed to a determined element, in consideration of repetitive application of said threshold matrix in performing halftoning of said original image, to thereby allocate turn-on numbers also to all non-writing elements; and
f) obtaining a threshold matrix for single speed by determining a threshold value of each writing element and each non-writing element in accordance with said turn-on numbers.
17. The recording medium according to claim 14 , wherein
said program is executed by said computer to cause said computer to further perform the operation of:
shrinking said threshold matrix for N-fold speed in said row direction by omitting said non-writing elements after said operation d).
18. The recording medium according to claim 14 , wherein
said matrix space is a virtual matrix space which is virtually set,
a matrix space which is said virtual matrix space shrunken in said row direction by omitting said non-writing elements is a real matrix space to be set in a memory device, and
said operation c) is executed with conversion of coordinates of said real matrix space into coordinates of said virtual matrix space.