Image forming device, image forming method, computer program, and recording medium

• US 20030151773A1
• Filed: 01/21/2003
• Published: 08/14/2003
• Est. Priority Date: 01/24/2002
• Status: Active Grant

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Takahashi, Hiroshi, Ogawa, Takeshi
• Granted Patent: US 7,564,588 B2
• Time in Patent Office: Days
• US Class Current: 358/3.03
• CPC Class Codes: H04N 1/40087 Multi-toning, i.e. converting a continuous-tone signal for reproduction with more than two discrete brightnesses or optical densities, e.g. dots of grey and black inks on white paper

Abstract

An image forming device includes a quantization unit which quantizes each pixel of an M-level gradation input image into one of N levels (M>N≧2) through an error diffusion process. An operation unit performs a predetermined operation according to a position of a target pixel to a gradation value of the target pixel before quantization when the gradation value of the target pixel is equal to a specific concentration.

Claims

1. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of N levels (M>

N≧

3) through an error diffusion process; and

an operation unit performing a predetermined operation according to a position of a target pixel to a gradation value of the target pixel when the gradation value of the target pixel is equal to a specific concentration.

2. The image forming device according to claim 1 wherein the operation unit is provided to perform addition or subtraction of a predetermined data to the gradation value of the target pixel before quantization.

5. The image forming device according to claim 2 wherein a pixel position where the predetermined data is added or subtracted is determined through a predetermined calculation based on the position of the target pixel.

6. The image forming device according to claim 5 wherein the operation unit comprises a memory that stores a mask in a matrix formation, the memory being accessed when the predetermined data is added or subtracted.

7. The image forming device according to claim 5 wherein the operation unit comprises a memory that stores a plurality of masks each in a matrix formation, one of the plurality of masks, stored in the memory, being selected and accessed when the predetermined data is added or subtracted.

8. The image forming device according to claim 2 wherein the operation unit determines a pixel position where the predetermined data is added or subtracted by using a random number.

9. The image forming device according to claim 2 wherein the predetermined data which is added to or subtracted from the gradation value of the target pixel is different from an N-level quantization output value corresponding to the specific concentration of the target pixel.

10. The image forming devices according to claim 2 wherein the addition or subtraction of the predetermined data generates a frequency lower than a spatial frequency of a neighboring gradation value of the input image.

3. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of N levels (M>

N≧

3) through an error diffusion process by using (N−

1) threshold values; and

an operation unit performing addition or subtraction of a predetermined data according to a position of a target pixel to a gradation value of the target pixel before quantization, when the gradation value of the target pixel is equal to a specific concentration.

4. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of two levels-through an error diffusion process; and

an operation unit performing addition or subtraction of a predetermined data according to a position of a target pixel to a gradation value of the target pixel before quantization, when the gradation value of the target pixel is equal to a specific concentration.

11. The image forming device according to claim 4 wherein the addition or subtraction of the predetermined data generates a dot neighboring on a highlight portion of an output image.

12. The image forming device according to claim 4 wherein the addition or subtraction of the predetermined data generates a blank dot neighboring on a solidly shaded portion of an output image.

13. An image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N≧

2) through an error diffusion process, comprising the steps of;

determining a threshold value according to a gradation value of a target pixel of the input image before quantization;

determining whether the gradation value of the target pixel is equal to a specific concentration;

performing addition of a predetermined data according to a position of the target pixel to the gradation value of the target pixel when the gradation value of the target pixel is equal to the specific concentration; and

performing the quantization through the error diffusion process by using the threshold value and the gradation value of the target pixel to which the predetermined data is added.

14. A computer program embodied therein for causing a computer to execute an image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N≧

2) through an error diffusion process, the method comprising the steps of;

determining a threshold value according to a gradation value of a target pixel of the input image before quantization;

determining whether the gradation value of the target pixel is equal to a specific concentration;

performing addition of a predetermined data according to a position of the target pixel to the gradation value of the target pixel when the gradation value of the target pixel is equal to the specific concentration; and

performing the quantization through the error diffusion process by using the threshold value and the gradation value of the target pixel to which the predetermined data is added.

15. A computer-readable storage medium storing a computer program embodied therein for causing a computer to execute an image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N≧

2) through an error diffusion process, the method comprising the steps of;

determining a threshold value according to a gradation value of a target pixel of the input image before quantization;

determining whether the gradation value of the target pixel is equal to a specific concentration;

performing addition of a predetermined data according to a position of the target pixel to the gradation value of the target pixel when the gradation value of the target pixel is equal to the specific concentration; and

performing the quantization through the error diffusion process by using the threshold value and the gradation value of the target pixel to which the predetermined data is added.

16. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values; and

a control unit dividing the M-level gradation into (N−

1) portions and changing a corresponding one of the threshold values for each portion according to an input gradation value of a target pixel.

19. The image forming device according to claim 16 wherein the control unit is provided such that two adjacent portions among the (N−

1) portions are set to a first portion and a second portion, a p-th threshold value of the (N−

1) threshold values for the first portion is set to p1, one of the (N−

1) threshold values for the second portion corresponding to the threshold value p1 is set to p2, a one-level high threshold value of the (N−

1) threshold values from the threshold value p2 is set to q2, and the threshold value p1 is equal to or less than the threshold value q2 in an adjoining part of the first portion and the second portion.

20. The image forming device according to claim 16 wherein the control unit is provided such that the threshold values for each portion are changed in accordance with a line that connects a first point where the changed threshold value at a starting point of the portion is smaller than a corresponding one of the:

threshold values for the portion by a predetermined value s, and a second point where the changed threshold value at an end point of the portion is larger than the corresponding one of the threshold values for the portion by a predetermined value e.

21. The image forming device according to claim 16 wherein the control unit is provided such that the threshold values for at least one of the portions are changed in accordance with a line that connects a first point where the changed threshold value at a starting point of the portion is smaller than a corresponding one of the threshold values for the portion by a predetermined value s, and a second point where the changed threshold value at an end point of the portion is larger than the corresponding one of the threshold values for the portion by a predetermined value e.

22. The image forming device according to claim 20 wherein the line which connects the first point and the second point is constituted by a straight line.

23. The image forming device according to claim 20 wherein the line which connects the first point and the second point is constituted by a curve.

24. The image forming device according to claim 20 wherein the line which connects the first point and the second point is constituted by a tangent function curve.

25. The image forming device according to claim 20 wherein the line which connects the first point and the second point is point-symmetrical within the portion.

26. The image forming device according to claim 20 wherein the predetermined value s and the predetermined value e are set to satisfy the conditions s≦

M/2N, e≦

M/2N and s=e.

27. The image forming device according to claim 20 wherein the predetermined value s and the predetermined value e are set to satisfy the conditions s≦

M/2N, e≦

M/2N and s≠

e.

28. The image forming device according to claim 20 wherein the predetermined value s is equal for each of the (N−

1) portions.

29. The image forming device according to claim 20 wherein the predetermined value s is equal for two or more portions of the (N−

1) portions.

30. The image forming device according to claim 20 wherein the predetermined value e is equal for each of the (N−

1) portions.

31. The image forming device according to claim 20 wherein the predetermined value e is equal for two or more portions of the (N−

1) portions.

17. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values; and

a control unit dividing the M-level gradation into (N−

1) portions and changing the (N−

1) threshold values for at least one of the (N−

1) portions according to an input gradation value of a target pixel.

18. The image forming device according to claim 17 wherein the control unit is provided to change a corresponding one of the threshold values for said at least one of the (N−

1) portions according to the input gradation value of the target pixel.

32. An image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values, the method comprising the steps of;

dividing the M-level gradation into (N−

1) portions; and

changing a corresponding one of the threshold values for each portion according to an input gradation value of a target pixel.

33. A computer program embodied therein for causing a computer to execute an image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values, the method comprising the steps of;

dividing the M-level gradation into (N−

1) portions; and

changing a corresponding one of the threshold values for each portion according to an input gradation value of a target pixel.

34. A computer-readable storage medium storing a computer program embodied therein for causing a computer to execute an image forming method in which each pixel of an M-level gradation input image is quantized into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values, the method comprising the steps of;

dividing the M-level gradation into (N−

1) portions; and

changing a corresponding one of the threshold values for each portion according to an input gradation value of a target pixel.

35. An image forming device comprising:

a quantization unit quantizing each pixel of an M-level gradation input image into one of N levels (M>

N) through an error diffusion process by using (N−

1) threshold values; and

an operation unit performing addition or subtraction of a predetermined data according to a position of a target pixel to one of a gradation value of the target pixel or a corresponding one of the threshold values when the gradation value of the target pixel is equal to a specific concentration,

wherein the operation unit comprises a memory that stores a mask in a matrix formation, the memory being accessed when the predetermined data is added or subtracted, and pixel positions where the predetermined data is added or subtracted are determined based on the position of the target pixel, and the number of pixel positions where the predetermined data is added is the same as the number of pixel positions where the predetermined data is subtracted.

36. The image forming device according to claim 35 wherein the pixel positions where the predetermined data is added and the pixel positions where the predetermined data is subtracted are equally arranged on the input image.

37. The image forming device according to claim 35 wherein the pixel positions where the predetermined data is added and the pixel positions where the predetermined data is subtracted are arranged in a 45-degree direction centered on an arbitrary pixel position where the addition or the subtraction is performed.

38. The image forming device according to claim 35 wherein the matrix mask stored in the memory of the operation unit has a two'"'"'s exponential size.

39. The image forming device according to claim 35 wherein the quantization unit is provided to output one of four quantization output values including 0 for a blank dot, 85 for a small dot, 170 for a middle dot, and 255 for a large dot, and the mask stored in the memory of the operation unit has a 16×

16 size, and the operation unit is provided to assign two blank dots and two middle dots for the quantization output value 75 and assign two small dots and two large dots for the quantization output value 170.