Image processing apparatus including low-linear-density dot region detection unit, and image forming apparatus including the same

A low-linear-density dot region detecting device which detects a low-linear-density dot region of image data, comprising:

• a maximum/minimum density detection unit detecting whether a center pixel of a matrix of N×N pixels in the image data has a maximum or minimum density among the pixels of the matrix;
• a determination unit determining whether the absolute value of a difference between an average of respective densities of all pairs of diagonal pixels of the matrix at positions symmetrical with respect to the center pixel and a density of the center pixel is larger than a predetermined threshold value;
• a peak pixel detection unit detecting peak pixels of the matrix, each peak pixel being the center pixel detected as having the maximum or minimum density, and the absolute value of the difference with respect to the center pixel determined as being larger than the threshold value;
• a peak pixel erasing unit making the detected peak pixels inactive when a periodicity between the detected peak pixels is below a fixed value;
• and
• a low-linear-density dot region detecting unit detecting one of neighboring pixels of the matrix, including a target pixel, as being a low-linear-density dot region based on a relationship between the number of the peak pixels in a two-dimensional target region and the number of the peak pixels in each of two-dimensional neighboring regions of the target region.

An image processing apparatus including a low-linear-density dot region detection device and an image processing device, the image processing device processing image data and outputting the processed image data, the low-linear-density dot region detection device comprising:

• a maximum/minimum density detection unit detecting whether a center pixel of a matrix of N×N pixels in the image data has a maximum or minimum density among the pixels of the matrix;
• a determination unit determining whether the absolute value of a difference between an average of respective densities of all pairs of diagonal pixels of the matrix at positions symmetrical with respect to the center pixel and a density of the center pixel is larger than a predetermined threshold value;
• a peak pixel detection unit detecting peak pixels of the matrix, each peak pixel being the center pixel detected as having the maximum or minimum density, and the absolute value of the difference with respect to the center pixel determined as being larger than the threshold value;
• a peak pixel erasing unit making the detected peak pixels inactive when a periodicity between the detected peak pixels is below a fixed value;
• and
• a low-linear-density dot region detecting unit detecting one of neighboring pixels of the matrix, including a target center pixel, as being a low-linear-density dot region based on a relationship between the number of the peak pixels in a two-dimensional target region and the number of the peak pixels in each of two-dimensional neighboring regions of the target region,
• wherein the image processing device performs selectively one of a first image processing for pixels of the low-linear-density dot region detected by the low-linear-density dot region detection device and a second image processing for pixels of another dot region of the image data.

An image processing apparatus which processes image data and outputs the processed image data, comprising:

• a determination unit determining whether the absolute value of a difference between an average of respective densities of all pairs of diagonal pixels of a matrix of N×N pixels at positions symmetrical with respect to a center pixel of the matrix and a density of the center pixel is larger than a predetermined threshold value;
• a peak pixel detection unit detecting peak pixels of the matrix, each peak pixel being the center pixel detected as having a maximum or minimum density among the pixels of the matrix, and the absolute value of the difference with respect to the center pixel determined as being larger than the threshold value;
• a dot region detecting unit detecting one of neighboring pixels of the matrix, including the peak pixels, as being a dot region based on a relationship between the number of the peak pixels in a two-dimensional target region and the number of the peak pixels in each of two-dimensional neighboring regions of the target region;
• a low-linear-density dot region detection unit detecting one of the neighboring pixels of the matrix as being a low-linear-density dot region based on the relationship of the number of the peak pixels between the target region and each neighboring region;
• and
• an image processing switching unit switching one of a first image processing and a second image processing to the other, when processing the pixels of the image data, based on a result of the detection of the dot region detection unit and a result of the detection of the low-linear-density dot region detection unit.

An image forming apparatus including an image processing apparatus and an image output device, the image processing apparatus processing image data and outputting the processed image data, and the image output device printing an image based on the processed image data from the image processing apparatus, the image processing apparatus comprising:

• a maximum/minimum density detecting unit detecting a center pixel of a matrix of N×N pixels in the image data that has a maximum or minimum density among the pixels of the matrix;
• a center pixel selecting unit selecting one of neighboring pixels of the matrix adjacent to the center pixel as being an active center pixel;
• a diagonal pixel selecting unit selecting one of pairs of diagonal pixels of the matrix at positions symmetrical with respect to the center pixel as being active diagonal pixels;
• a determination unit determining whether the absolute value of a difference between an average of densities of all the pairs of diagonal pixels selected by the diagonal pixel selecting unit and a density of the center pixel selected by the center pixel selecting unit is larger than a predetermined threshold value;
• a peak pixel detecting unit detecting peak pixels of the matrix, each peak pixel being the center pixel detected as having the maximum or minimum density by the maximum/minimum density detecting unit, and the absolute value of the difference with respect to the center pixel is determined as being larger than the threshold value by the determination unit;
• and
• a dot region detecting unit detecting one of the neighboring pixels including the peak pixels as being a dot region based on a relationship between the number of the peak pixels in a two-dimensional target region and the number of the peak pixels in each of two-dimensional neighboring regions of the target region.

A color copier including an image processing apparatus, an image reading device and an image printing device, the image reading device reading an image data from a document by color separation and supplying the image data to the image processing apparatus, and the image printing device forming a reconstructed image based on a processed image data output by the image processing apparatus and printing the reconstructed image, the image processing apparatus comprising:

• a maximum/minimum density detecting unit detecting a center pixel of a matrix of N×N pixels in the image data that has a maximum or minimum density among the pixels of the matrix;
• a center pixel selecting unit selecting one of neighboring pixels of the matrix adjacent to the center pixel as being an active center pixel;
• a diagonal pixel selecting unit selecting one of pairs of diagonal pixels of the matrix at positions symmetrical with respect to the center pixel as being active diagonal pixels;
• a determination unit determining whether the absolute value of a difference between an average of densities of all the pairs of diagonal pixels selected by the diagonal pixel selecting unit and a density of the center pixel selected by the center pixel selecting unit is larger than a predetermined threshold value;
• a peak pixel detecting unit detecting peak pixels of the matrix, each peak pixel being the center pixel detected as having the maximum or minimum density by the maximum/minimum density detecting unit, and the absolute value of the difference with respect to the center pixel is determined as being larger than the threshold value by the determination unit;
• and
• a dot region detecting unit detecting one of the neighboring pixels including the peak pixels as being a dot region based on a relationship between the number of the peak pixels in a two-dimensional target region and the number of the peak pixels in each of two-dimensional neighboring regions of the target region.