Image processing apparatus and image forming apparatus

1. An image processing apparatus, comprising:

a halftone processing unit configured to perform a halftone process for a target image and generate a binary image corresponding to the target image; and

a multi-level gradation processing unit configured to (a) set a target pixel as a dot pixel in the binary image, (b) determine whether a dot exists on plural first periphery pixels with a predetermined first distance from the target pixel and plural second periphery pixels with a predetermined second distance from the target pixel or not, (c) derive a gradation level of the target pixel among predetermined three or more gradation levels on the basis of a number of dots on the plural first periphery pixels and a number of dots on the plural second periphery pixels and thereby generate a multi-level gradation image.

2. The image processing apparatus according to claim 1 , wherein the multi-level gradation processing unit (a) determines whether a predetermined exception condition is satisfied on the target pixel or not, and (b) performs an offset process, if the exception condition is satisfied on the target pixel;

the offset process adjusts the derived gradation level; and

the exception condition is that the target pixel and dot pixels among peripheral pixels in a predetermined range from the target pixel appear in a straight line.

3. The image processing apparatus according to claim 2 , wherein if the exception condition is satisfied on the target pixel, the multi-level gradation processing unit determines that the target pixel is a character pixel that is a part of a character, and performs the offset process corresponding to the character pixel.

4. The image processing apparatus according to claim 1 , wherein the multi-level gradation processing unit (a) determines whether a predetermined exception condition is satisfied on the target pixel or not, and (b) performs an offset process, if the exception condition is satisfied on the target pixel;

the offset process adjusts the derived gradation level; and

the exception condition is that in a peripheral pixel area of a predetermined range from the target pixel, a number of parts in which a dot pixel and a non-dot pixel appear adjacent to each other excesses a predetermined threshold value.

5. The image processing apparatus according to claim 4 , wherein if the exception condition satisfies on the target pixel, the multi-level gradation processing unit determines that the target pixel corresponds to an intermediate gradation level pixel that is a part of a gradation image, and performs the offset process corresponding to the intermediate gradation level pixel.

6. The image processing apparatus according to claim 1 , wherein the first periphery pixels are outermost peripheral pixels in a pixel area of vertical 3 pixels by horizontal 3 pixels with a center set on the target pixel; and

the second periphery pixels are outermost peripheral pixels in a pixel area of vertical 5 pixels by horizontal 5 pixels with a center set on the target pixel.

7. An image forming apparatus, comprising:

an image processing apparatus; and

a printing device configured to print a multi-level gradation image;

wherein the image processing apparatus comprises:

a halftone processing unit configured to perform a halftone process for a target image and generate a binary image corresponding to the target image; and

a multi-level gradation processing unit configured to (a) set a target pixel as a dot pixel in the binary image, (b) determine whether a dot exists on plural first periphery pixels with a predetermined first distance from the target pixel and plural second periphery pixels with a predetermined second distance from the target pixel or not, (c) derive a gradation level of the target pixel among predetermined three or more gradation levels on the basis of a number of dots on the plural first periphery pixels and a number of dots on the plural second periphery pixels and thereby generate the multi-level gradation image.