Method and device for compressing image data

A method for compressing image data that are processed by a pseudo gradation process using a dither pattern, the method comprising:

• a first step for dividing the image data into predetermined areas;
• a second step for determining whether or not a dot arrangement pattern of the image data in each area matches the dither pattern that was used in the pseudo gradation process;
• a third step that is performed if the patterns match each other in the second step, for determining a representative Value concerning density of the image data in the area supposing dots are made in the pattern area that was prepared in advance when the density value of each pixel of the image data in the area is compared with the density value of each pixel of a corresponding part of the dither pattern and the density value of each pixel of the image data is smaller than the other, the representative value being a value “a” that satisfies the condition
• J≧a≧K
• between a minimum value J of density values of pixels of the dither pattern corresponding to dots that were made in a pattern area and a maximum value K of density values of pixels of the dither pattern corresponding to dots that were not made in the pattern area;
• a fourth step that is performed if the patterns do not match each other in the second step, for dividing the area further into plural small areas and for determining whether or not a dot arrangement pattern of the image data in each small area matches the dither pattern;
• a fifth step that is performed if the patterns match each other in the fourth step, for determining the number of dots that were made or dots that were not made in the area as the representative value in the small area;
• a sixth step that is performed if the patterns do not match each other in the fourth step, for encoding the dot arrangement in the small area;
• and
• a seventh step that is performed if the patterns match each other in the second step or the fourth step, for encoding in accordance with the determined representative value.

A method for compressing image data that are processed by a pseudo gradation process using a dither pattern, the method comprising:

• a first step for dividing the image data into predetermined areas;
• a second step for determining whether or not a dot arrangement pattern of the image data in each area matches the dither pattern that was used in the pseudo gradation process;
• a third step that is performed if the patterns match each other in the second step, for determining a representative value concerning density of the image data in the area supposing dots are made in a pattern area that was prepared in advance when the density value of each pixel of the image data in the area is compared with the density value of each pixel of a corresponding part of the dither pattern and the density value of each pixel of the image data is larger than the other, the representative value being a value “a” that satisfies the condition
• J≧a≧K
• between a maximum value K of density values of pixels of the dither pattern corresponding to dots that were made in a pattern area and a minimum value J of density values of pixels of the dither pattern corresponding to dots that were not made in the pattern area;
• a fourth step that is performed if the patterns do not match each other in the second step, for dividing the area further into plural small areas and for determining whether or not a dot arrangement pattern of the image data in each small area matches the dither pattern;
• a fifth step that is performed if the patterns match each other in the fourth step, for determining the number of dots that were made or dots that were not made in the area as the representative value in the small area;
• a sixth step that is performed if the patterns do not match each other in the fourth step, for encoding the dot arrangement in the small area;
• and
• a seventh step that is performed if the patterns match each other in the second step or the fourth step, for encoding in accordance with the determined representative value.

A method for compressing image data that are processed by a pseudo gradation process using a dither pattern, the method comprising:

• a first step for dividing the image data into predetermined areas;
• a second step for determining whether or not a dot arrangement pattern of the image data in each area matches the dither pattern that was used in the pseudo gradation process;
• a third step that is performed if the patterns match each other in the second step, for determining a representative value concerning density of the image data in the area supposing dots are made in a pattern area that was prepared in advance when the density value of each pixel of the image data in the area is compared with the density value of each pixel of a corresponding part of the dither pattern and the density value of each pixel of the image data is larger than the other, the representative value being a value “a” that satisfies the condition
• J≧a≧K
• between a maximum value K of density values of pixels of the dither pattern corresponding to dots that were made in the pattern area and a minimum value J of density values of pixels of the dither pattern corresponding to dots that were not made in the pattern area;
• a fourth step that is performed if the patterns do not match each other in the second step, for dividing the area further into plural small areas and for determining whether or not a dot arrangement pattern of the image data in each small area matches the dither pattern;
• a fifth step that is performed if the patterns match each other in the fourth step, for determining the representative value in the small area by the same process as the third step;
• a sixth step that is performed if the patterns do not match each other in the fourth step, for encoding the dot arrangement in the small area;
• and
• a seventh step that is performed if the patterns match each other in the second step or the fourth step, for encoding in accordance with the determined representative value.

A device for compressing image data that are processed by a pseudo gradation process using a dither pattern, the device comprising:

• a portion for dividing the image data into predetermined areas;
• a portion for determining whether or not a dot arrangement pattern of the image data in each area matches the dither pattern that was used in the pseudo gradation process;
• a portion for determining a representative value concerning density of the image data in the area if the dot arrangement pattern matches the dither pattern;
• and
• a portion for generating compressed data by using the determined representative value.