Image processor, image formation device, image processing method, and computer-readable medium

1. An image processor comprising:

a gradation value acquisition unit that acquires a gradation value of a pixel of interest which is a pixel sequentially selected as a target of a binarization process from input image data represented by pixels of M gradations, wherein M≧3; and

a pattern determination unit that determines a filling pattern of a group of pixels of output image data corresponding to the pixel of interest according to a corrected gradation value acquired by adding, to the gradation value of the pixel of interest, an error value diffused from a pixel at a periphery of the pixel of interest, wherein the filling pattern includes at least a first pattern in which a predetermined plurality of pixels are filled and which forms a core of a dot and a third pattern in which substantially no pixel is filled and the pattern determination unit determines the filling pattern to be one of the first pattern and the third pattern according to a size relationship between the corrected gradation value and a predetermined threshold value which spatially varies in a periodic manner.

2. The image processor according to claim 1 , wherein

the filling pattern includes a second pattern in which a number of pixels are filled, the number corresponding to the corrected gradation value, and which forms a dot with the first pattern, and

the pattern determination unit determines the filling pattern to be the second pattern when a pixel determined to be the first pattern is present among predetermined pixels adjacent to the pixel of interest, and determines the filling pattern to be one of the first pattern and the third pattern according to the size relationship between the corrected gradation value and the predetermined threshold value when no pixel determined to be the first pattern is present among the predetermined pixels.

3. The image processor according to claim 2 , wherein

at least one of the number of pixels to be filled in the first pattern and the number of pixels to be filled in the second pattern varies according to the gradation value of the pixel of interest so that the number becomes smaller as a density becomes smaller.

4. The image processor according to claim 3 , wherein

when at least the number of pixels to be filled in the first pattern varies according to the gradation value of the pixel of interest, the number of pixels to be filled in the first pattern is a fixed value in a low-density region in which the gradation value of the pixel of interest is less than a predetermined value.

5. An image formation device comprising:

an image data acquisition unit that acquires input image data represented by pixels of M gradations, wherein M≧3;

an image processor that converts the input image data into output image data represented by pixels of two gradations; and

an image formation unit that forms an image based on the output image data on a recording medium, wherein

the image processor comprises:

a gradation value acquisition unit that acquires a gradation value of a pixel of interest which is a pixel sequentially selected as a target of a binarization process from the input image data; and

a pattern determination unit that determines a filling pattern of a group of pixels of output image data corresponding to the pixel of interest according to a corrected gradation value acquired by adding, to the gradation value of the pixel of interest, an error value diffused from a pixel at a periphery of the pixel of interest, wherein the filling pattern includes at least a first pattern in which a predetermined plurality of pixels are filled and which forms a core of a dot and a third pattern in which substantially no pixel is filled and the pattern determination unit determines the filling pattern to be one of the first pattern and the third pattern according to a size relationship between the corrected gradation value and a predetermined threshold value which spatially varies in a periodic manner.

6. A method of processing an image, the method comprising:

acquiring a gradation value of a pixel of interest which is a pixel sequentially selected as a target of a binarization process from input image data represented by pixels of M gradations, wherein M≧3; and

determining a filling pattern of a group of pixels of output image data corresponding to the pixel of interest according to a corrected gradation value acquired by adding, to the gradation value of the pixel of interest, an error value diffused from a pixel at a periphery of the pixel of interest, wherein

the filling pattern includes at least a first pattern in which a predetermined plurality of pixels are filled and which forms a core of a dot and a third pattern in which substantially no pixel is filled, and

the determining the pattern includes determining the filling pattern to be one of the first pattern and the third pattern according to a size relationship between the corrected gradation value and a predetermined threshold value which spatially varies in a periodic manner.

7. A non-transitory computer-readable medium storing a program causing a computer to execute a process for processing an image, the process comprising:

acquiring a gradation value of a pixel of interest which is a pixel sequentially selected as a target of a binarization process from input image data represented by pixels of M gradations, wherein M≧3; and

determining a filling pattern of a group of pixels of output image data corresponding to the pixel of interest according to a corrected gradation value acquired by adding, to the gradation value of the pixel of interest, an error value diffused from a pixel at a periphery of the pixel of interest, wherein

the filling pattern includes at least a first pattern in which a predetermined plurality of pixels are filled and which forms a core of a dot and a third pattern in which substantially no pixel is filled, and

the determining the pattern includes determining the filling pattern to be one of the first pattern and the third pattern according to a size relationship between the corrected gradation value and a predetermined threshold value which spatially varies in a periodic manner.

8. The non-transitory computer-readable medium according to claim 7 , wherein

the filling pattern includes a second pattern in which a number of pixels are filled, the number corresponding to the corrected gradation value, and which forms the dot with the first pattern, and

the determining the pattern includes determining the filling pattern to be the second pattern when a pixel determined to be the first pattern is present among predetermined pixels adjacent to the pixel of interest, and determining the filling pattern to be one of the first pattern and the third pattern according to the size relationship between the corrected gradation value and the predetermined threshold value when no pixel determined to be the first pattern is present among the predetermined pixels.

9. The non-transitory computer-readable medium according to claim 8 , wherein

at least one of the number of pixels to be filled in the first pattern and the number of pixels to be filled in the second pattern varies according to the gradation value of the pixel of interest so that the number becomes smaller as a density becomes smaller.

10. The non-transitory computer-readable medium according to claim 9 , wherein

when at least the number of pixels to be filled in the first pattern varies according to the gradation value of the pixel of interest, the number of pixels to be filled in the first pattern is a fixed value in a low-density region in which the gradation value of the pixel of interest is less than a predetermined value.