Image processing method, program, storage medium, and apparatus

• US 20050025374A1
• Filed: 07/23/2004
• Published: 02/03/2005
• Est. Priority Date: 07/30/2003
• Status: Active Grant

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Inventors: Ishikawa, Hisashi
• Granted Patent: US 7,672,525 B2
• Time in Patent Office: Days
• US Class Current: 382/252
• CPC Class Codes: G06T 2200/28 involving image processing hardware
• CPC Class Codes: H04N 1/4053 with threshold modulated relative to input image data or vice versa

Abstract

An image processing method for executing a quantization process of image data by error diffusion, executes a step of generating first image data by adding a quantization error to input image data, a step of generating second image data by quantizing the first image data by comparing the first image data with a predetermined threshold value, a step of generating third image data by dequantizing the second image data, a calculation step of calculating a quantization error for each pixel on the basis of the difference between the first and third image data, a storage step of storing information indicating a positive/negative sign of the calculated quantization error for a predetermined number of pixels in a storage unit, and a correction value generation step of generating a correction value used to correct at least one of the first image data and the threshold value on the basis of the information which is stored in the storage unit and indicates the positive/negative sign.

Claims

1. An image processing method for executing a quantization process of image data by error diffusion, comprising:

a step of generating first image data by adding a quantization error to input image data;

a step of generating second image data by quantizing the first image data by comparing the first image data with a predetermined threshold value;

a step of generating third image data by dequantizing the second image data;

a calculation step of calculating a quantization error for each pixel on the basis of a difference between the first and third image data;

a storage step of storing information indicating a positive/negative sign of the calculated quantization error for a predetermined number of pixels in a storage unit; and

a correction value generation step of generating a correction value used to correct at least one of the first image data and the threshold value on the basis of the information which is stored in the storage unit and indicates the positive/negative sign.

2. The method according to claim 1, wherein the correction value generation step includes a step of counting the total number of at least one of positive and negative signs on the basis of the information which is stored in the storage unit and indicates the positive/negative sign, and generating the correction value in accordance with the total number.

3. The method according to claim 2, wherein the correction value generated in the correction value generation step decreases with increasing total number of counted positive or negative signs.

4. The method according to claim 2, wherein the correction value generation step includes a step of holding respective values that the total number obtained by counting can assume, and correction values corresponding to the respective values as a correction table in association with each other, and selecting the correction value from the correction table in accordance with the total number value counted on the basis of the information which is stored in the storage unit and indicates the positive/negative sign.

5. The method according to claim 4, wherein the correction table registers absolute values of the correction values, and the correction value generation step includes a step of assigning a negative sign to the absolute value in accordance with the total number of positive or negative signs.

6. The method according to claim 1, wherein the correction value generation step further comprises a step of generating a random number, and the correction value generation step includes a step of generating the correction value on the basis of the information indicating the positive/negative sign and the generated random number.

7. The method according to claim 6, wherein the correction value generation step includes a step of holding a random number shift table which defines in advance shift amounts used to shift a bit sequence indicating the random number in correspondence with values of the total number of positive or negative signs, referring to the shift amount defined in the random number shift table corresponding to the total number of counted positive or negative signs, and generating the correction value on the basis of a value obtained by shifting the random number by the shift amount.

8. The method according to claim 7, wherein the correction value generation step further comprises a limit step of limiting an upper limit value of an absolute value of the value obtained by shifting the random number by the shift amount to be not more than a predetermined value.

9. The method according to claim 6, wherein the correction value generation step includes a step of holding a random number limit table that defines limit values used to limit the range of values that the random number can assume in correspondence with values of the total number of positive or negative signs, referring to the limit value defined in the random number limit table corresponding to the total number of counted positive or negative signs, and generating the correction value on the basis of a value obtained by limiting the random number by the limit value.

10. An image processing program for making a computer execute an image processing method of claim 1.

11. A computer readable storage medium storing an image processing program of claim 10.

12. An image processing apparatus for executing a quantization process of image data by error diffusion, comprising:

a unit for generating first image data by adding a quantization error to input image data;

a unit for generating second image data by quantizing the first image data by comparing the first image data with a predetermined threshold value;

a unit for generating third image data by dequantizing the second image data;

a calculation unit for calculating a quantization error for each pixel on the basis of a difference between the first and third image data;

a storage unit for storing information indicating a positive/negative sign of the calculated quantization error for a predetermined number of pixels in a storage unit; and

a correction value generation unit for generating a correction value used to correct at least one of the first image data and the threshold value on the basis of the information which is stored in the storage unit and indicates the positive/negative sign.

13. The apparatus according to claim 12, wherein said correction value generation unit counts the total number of at least one of positive and negative signs on the basis of the information which is stored in the storage unit and indicates the positive/negative sign, and generates the correction value in accordance with the total number.

14. The apparatus according to claim 13, wherein the correction value generated by said correction value generation unit decreases with increasing total number of counted positive or negative signs.

15. The apparatus according to claim 13, wherein said correction value generation unit holds respective values that the total number obtained by counting can assume, and correction values corresponding to the respective values as a correction table in association with each other, and controls to select the correction value from the correction table in accordance with the total number value counted on the basis of the information which is stored in the storage unit and indicates the positive/negative sign.

16. The apparatus according to claim 15, wherein the correction table registers absolute values of the correction values, and said correction value generation unit assigns a negative sign to the absolute value in accordance with the total number of positive or negative signs.

17. The apparatus according to claim 12, wherein said correction value generation unit further comprises a unit for generating a random number, and generates the correction value on the basis of the information indicating the positive/negative sign and the generated random number.

18. The apparatus according to claim 17, wherein said correction value generation unit holds a random number shift table which defines in advance shift amounts used to shift a bit sequence indicating the random number in correspondence with values of the total number of positive or negative signs, refers to the shift amount defined in the random number shift table corresponding to the total number of counted positive or negative signs, and generates the correction value on the basis of a value obtained by shifting the random number by the shift amount.

19. The apparatus according to claim 18, wherein said correction value generation unit further comprises a limit unit for limiting an upper limit value of an absolute value of the value obtained by shifting the random number by the shift amount to be not more than a predetermined value.

20. The apparatus according to claim 17, wherein said correction value generation unit holds a random number limit table that defines limit values used to limit the range of values that the random number can assume in correspondence with values of the total number of positive or negative signs, refers to the limit value defined in the random number limit table corresponding to the total number of counted positive or negative signs, and generates the correction value on the basis of a value obtained by limiting the random number by the limit value.