Image processing apparatus, image processing method, and program

• US 20100129004A1
• Filed: 11/10/2009
• Published: 05/27/2010
• Est. Priority Date: 11/27/2008
• Status: Active Grant

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Ogawa, Takeshi
• Granted Patent: US 8,208,751 B2
• Time in Patent Office: Days
• US Class Current: 382/274
• CPC Class Codes: H04N 1/4052 by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions
• CPC Class Codes: H04N 1/4057 the pattern being a mixture of differently sized sub-patterns, e.g. spots having only a few different diameters multi-toning H04N1/40087
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

An image processing apparatus, which quantizes M-level image data into N levels (M>N>2) using a multi-level error diffusion method or a multi-level average-error minimization method, includes a γ-conversion unit that performs γ-conversion on an integer part in a real number value after ideal gradation conversion that aims to obtain desired density or brightness, an area-gradation expressing unit that expresses a decimal part in the real number value by area gradation, an integerization unit that adds two values obtained through the γ-conversion and the area gradation to obtain an integerized value corresponding to the ideal gradation conversion, a correction unit that adds a peripheral error to the integerized value, and a threshold setting unit that sets an N−1-type threshold value for a value corresponding to the ideal gradation conversion, wherein the conversion to N levels is performed by using the correction value and the N−1-type threshold value.

Claims

1. An image processing apparatus that quantizes multi-level (M-level) image data into N levels (M>

N>

2) using a multi-level error diffusion method or a multi-level average-error minimization method, the image processing apparatus comprising;

a γ

-conversion unit that performs γ

-conversion on an integer part in a real number value after ideal gradation conversion that aims to obtain desired density or brightness;

an area-gradation expressing unit that expresses a decimal part by area gradation, the decimal part being in the real number value after the ideal gradation conversion;

an integerization unit that adds a value obtained through the γ

-conversion and a value expressed by the area gradation to obtain an integerized value corresponding to a value of the ideal gradation conversion;

a correction unit that adds a peripheral error to the integerized value to obtain a correction value; and

a threshold setting unit that sets an N−

1-type threshold value for a value corresponding to the value after the ideal gradation conversion, wherein

the conversion to N levels is performed by using the correction value and the N−

1-type threshold value, to thereby express the real number value after the ideal gradation conversion so that input multi-level (M-level) image data obtains the desired density or brightness.

2. The image processing apparatus according to claim 1, wherein expression of the area gradation is performed by using a dither process.

3. The image processing apparatus according to claim 1 that is provided with

a LUT (look up table) that

stores therein a value obtained in such a manner that the decimal part is multiplied by a matrix size for the dither process and then multiplied decimal part is rounded, or

stores therein a value obtained by rounding down a value after a decimal point,

for each gradation.

4. The image processing apparatus according to claim 1, wherein a dispersion type dither is used as the dither process.

5. The image processing apparatus according to claim 1, wherein Bayer type dither is used as the dither process.

6. The image processing apparatus according to claim 3, wherein the area gradation includes a value 0 or 1 obtained by binarizing a value stored in the LUT (look up table) using the dither.

7. The image processing apparatus according to claim 1, wherein as a value corresponding to the ideal gradation converted value, the integer part in the real number value after the ideal gradation conversion is a value obtained by adding 1 to a value after the ordinary γ

-conversion, when the decimal part, in the real number value after the ideal gradation conversion, is not 0.

8. An image processing method for an image processing apparatus that quantizes multi-level (M-level) image data into N levels (M>

N>

2) using a multi-level error diffusion method or a multi-level average-error minimization method, the image processing method comprising;

a step of g-converging by a g-conversion unit that performs γ

-conversion on an integer part in a real number value after ideal gradation conversion that aims to obtain desired density or brightness;

a step of area-gradation expressing by an area-gradation expressing unit that expresses a decimal part by area gradation, the decimal part being in the real number value after the ideal gradation conversion;

an step of integerizing by an integerizing unit that adds a value obtained through the γ

-conversion and a value expressed by the area gradation to obtain an integerized value corresponding to a value of the ideal gradation conversion;

a step of correcting by a correcting unit that adds a peripheral error to the integerized value to obtain a correction value; and

a step of threshold setting by a threshold setting unit that sets an N−

1-type threshold value for a value corresponding to the value after the ideal gradation conversion, wherein

the conversion to N levels is performed by using the correction value and the N−

1-type threshold value, to thereby express the real number value after the ideal gradation conversion so that input multi-level (M-level) image data obtains the desired density or brightness.

9. The image processing method according to claim 8, wherein the area-gradation expressing is performed by using a dither process.

10. The image processing method according to claim 8 wherein the image processing apparatus has

a LUT (look up table) that

stores therein a value obtained in such a manner that the decimal part is multiplied by a matrix size for the dither process and then multiplied decimal part is rounded, or

stores therein a value obtained by rounding down a value after a decimal point,

for each gradation.

11. The image processing method according to claim 8, wherein a dispersion type dither is used as the dither process.

12. The image processing method according to claim 8, wherein Bayer dispersion type dither is used as the dither process.

13. The image processing apparatus according to claim 10, wherein the area gradation includes a value 0 or 1 obtained by binarizing a value stored in the LUT (look up table) using the dither.

14. The image processing method according to claim 8, wherein as a value corresponding to the ideal gradation converted value, the integer part in the real number value after the ideal gradation conversion is a value obtained by adding 1 to a value after the ordinary γ

-conversion, when the decimal part, in the real number value after the ideal gradation conversion, is not 0.

15. A program for causing a computer to execute a control process of an image processing apparatus that quantizes multi-level (M-level) image data into N levels (M>

N>

2) using a multi-level error diffusion method or a multi-level average-error minimization method, in order to express a real number value after ideal gradation conversion so that input multi-level (M-level) image data obtains desired density or brightness, the program comprising;

a code for a step of performing γ

-conversion on an integer part in the real number value after the ideal gradation conversion;

a code for a step of expressing a decimal part by area gradation, the decimal part being in the real number value after the ideal gradation conversion;

a code for a step of adding a value obtained through the γ

-conversion and a value expressed by the area gradation to obtain an integerized value, corresponding to an ideal gradation converted value;

a code for a step of adding a peripheral error to the integerized value to obtain a correction value; and

a code for a step of setting an N−

1-type threshold value for conversion to N levels according to a value corresponding to the ideal gradation converted value, wherein

the conversion to N levels is performed by using the correction value and the N−

1-type threshold value.