Method and device for compressing image data

A method performed in a computer for compressing image data having a gradation property, the method comprising the following steps:

• a first step for dividing the image data into predetermined areas, so as to determine a representative value of densities of pixels in each area;
• a second step for determining a density of each pixel included in the area using the representative value of the area and an interpolation rule;
• a third step for testing reproducibility of an image obtained on the basis of the density of each pixel determined in the second step, by using the image data and a dither pattern that is used for a pseudo gradation step performed on the image data;
• and
• a fourth step for performing an encoding process on the basis of the representative value determined in the first step for areas where the reproducibility was good in the test of the third step,
• wherein, in the third step, an error range in a case where the image data are character image data is set smaller than an error range in a case where the image data are picture image data when the reproducibility is tested,
• Wherein each step is performed by said computer.

A method performed in a computer for compressing image data having a gradation property, the method comprising the following steps:

• a first step for dividing the image data into predetermined areas, so as to determine a representative value of densities of pixels in each area;
• a second step for determining, for each area, a density of each pixel included in the area using the representative value of the area and an interpolation rule;
• a third step for comparing a first dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on the image data using a dither pattern with a second dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on a density of each pixel determined in the second step using the dither pattern so as to determine a mismatch value indicating a difference between the first dot pattern and the second dot pattern, and for determining whether or not to use the representative value obtained in the first step for a process for generating compressed data on the basis of the mismatch value and a predetermined criterion value;
• and
• a fourth step for performing the process for generating the compressed data by using the representative value if it was determined that the representative value should be used for generating the compressed data,
• Wherein each step is performed by said computer.

A method performed in a computer for compressing image data having a gradation property, the method comprising the following steps:

• a first step for dividing the image data into predetermined areas, so as to determine a representative value of densities of pixels in each area;
• a second step for determining, for each area, a density of each pixel included in the area using the representative value of the area and an interpolation rule;
• a third step for comparing a first dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on the image data using a dither pattern with a second dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on a density of each pixel determined in the second step using the dither pattern so as to determine a mismatch value indicating a difference between the first dot pattern and the second dot pattern, and for determining for each area whether or not to use the representative value obtained in the first step for a process for generating compressed data on the basis of the mismatch value and a predetermined criterion value;
• and
• a fourth step for performing the process for generating the compressed data by using the representative value of an area where it was determined to use the representative value for generating the compressed data,
• Wherein each step is performed by said computer.

A method performed in a computer for compressing image data having a gradation property, the method comprising the following steps:

• a first step for dividing the image data into predetermined areas, so as to determine a representative value of densities of pixels in each area;
• a second step for determining, for each area, a density of each pixel included in the area using the representative value of the area and an interpolation rule;
• a third step for comparing a first dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on the image data using a dither pattern with a second dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on a density of each pixel determined in the second step using the dither pattern so as to determine a mismatch value indicating a difference between the first dot pattern and the second dot pattern, and for determining a criterion value on the basis of the mismatch value;
• a fourth step for determining, on the basis of the mismatch value and the criterion value, whether or not to use the representative value obtained in the first step for a process for generating compressed data;
• and
• a fifth step for performing the process for generating the compressed data by using the representative value if it was determined that the representative value should be used for generating the compressed data,
• Wherein each step is performed by said computer.

A method performed in a computer for compressing image data having a gradation property, the method comprising the following steps:

• a first step for dividing the image data into predetermined areas, so as to determine a representative value of densities of pixels in each area;
• a second step for determining, for each area, a density of each pixel included in the area using the representative value of the area and an interpolation rule;
• a third step for comparing a first dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on the image data using a dither pattern with a second dot pattern that is an arrangement pattern of dots obtained by performing a pseudo gradation process on a density of each pixel determined in the second step using the dither pattern so as to determine a mismatch value indicating a difference between the first dot pattern and the second dot pattern, and for determining a criterion value on the basis of the mismatch value;
• a fourth step for determining, on the basis of the mismatch value and the criterion value, for each area whether or not to use the representative value obtained in the first step for a process for generating compressed data;
• and
• a fifth step for performing the process for generating the compressed data by using the representative value of an area where it was determined to use the representative value for generating the compressed data,
• Wherein each step is performed by said computer.