Image processing apparatus, image processing method and storage medium

1. An image processing apparatus comprising:

a first halftone processing unit configured to perform halftone processing on multilevel input image data to generate first halftone image data;

a first filter unit configured to perform smoothing filter processing on the first halftone image data to generate first filter-processed data, which processing is performed for each pixel of interest referring to a neighboring first region including the pixel of interest;

a second filter unit configured to perform smoothing filter processing on the multilevel input image data to generate second filter-processed data, which processing is performed for each pixel of interest referring to the first region;

a first difference evaluation unit configured to convert the first filter-processed data, from the first filter unit, and the second filter-processed data, from the second filter unit, to equalize the number of tones per pixel, and calculate a difference between the converted first and second filter-processed data, respectively from the first and second filter units, for each pixel as a first moiré intensity;

a third filter unit configured to perform smoothing filter processing on the first halftone image data to generate third filter-processed data, which processing is performed for each pixel of interest referring to a neighboring second region including the pixel of interest, and wherein the second region is smaller than the first region;

a fourth filter unit configured to perform smoothing filter processing on the multilevel input image data to generate fourth filter-processed data, which processing is performed for each pixel of interest referring to the second region;

a second difference evaluation unit configured to convert the third filter-processed data, from the third filter unit, and the fourth filter-processed data, from the fourth filter unit, to equalize the number of tones per pixel, and calculate a difference between the converted third and fourth filter-processed data, respectively from the third and fourth filter units, for each pixel as a second moiré intensity; and

a moiré intensity selection unit configured to select one of the first moiré intensity and the second moiré intensity based on a feature amount in the multilevel input image data in a predetermined region including the pixel of interest, and output the selected moiré intensity.

2. The apparatus according to claim 1 , further comprising:

a second halftone processing unit configured to perform halftone processing on the multilevel input image data to generate second halftone image data, wherein characteristics of the halftone processing by said second halftone processing unit is different from those of the halftone processing by said first halftone processing unit; and

a selection unit configured to select the first halftone image data when the moiré intensity is not more than a predetermined threshold, and select the second halftone image data when the moiré intensity is more than the predetermined threshold.

3. The apparatus according to claim 1 , wherein

said first halftone processing unit is further configured to perform halftone processing based on a predetermined halftone processing cycle, and

the first region has a shape corresponding to the halftone processing cycle.

4. The apparatus according to claim 3 , wherein said first halftone processing unit is further configured to perform dither processing on the multilevel input image data using a threshold matrix having the halftone processing cycle as a threshold cycle.

5. The apparatus according to claim 4 , wherein the first region corresponds to a region where thresholds included in the threshold matrix appear the same number of times on a plane filled with the threshold matrix.

6. The apparatus according to claim 4 , wherein said first filter unit and said second filter unit are further configured to perform, on a plane filled with the threshold matrix, smoothing filter processing while setting, for each pixel of the first region, a weight coefficient corresponding to an appearance frequency of each threshold or a total weight corresponding to each threshold inside the first region.

7. The apparatus according to claim 4 , wherein said third filter unit and said fourth filter unit are further configured to perform filter processing using a filter whose reference pixel region includes pixels in equal number in vertical and horizontal directions.

8. The apparatus according to claim 1 , wherein the feature amount is an edge amount.

9. The apparatus according to claim 1 , wherein the feature amount is a dynamic range.

10. An image processing method comprising the steps of:

performing halftone processing on multilevel input image data to generate first halftone image data;

performing smoothing filter processing on the first halftone image data to generate first filter-processed data, which processing is performed for each pixel of interest referring to a neighboring first region including the pixel of interest;

performing smoothing filter processing on the multilevel input image data to generate second filter-processed data, which processing is performed for each pixel of interest referring to the first region;

converting the first filter-processed data and the second filter-processed data to equalize the number of tones per pixel;

calculating a difference between the converted first and second filter-processed data for each pixel as a first moiré intensity;

performing smoothing filter processing on the first halftone image data to generate third filter-processed data, which processing is performed for each pixel of interest referring to a neighboring second region including the pixel of interest; and

wherein the second region is smaller than the first region;

performing smoothing filter processing on the multilevel input image data to generate fourth filter-processed data, which processing is performed for each pixel of interest referring to the second region;

converting the third filter-processed data and the fourth filter-processed data to equalize the number of tones per pixel;

calculating a difference between the converted third and fourth filter-processed data for each pixel as a second moiré intensity;

selecting one of the first moiré intensity and the second moiré intensity based on a feature amount in the multilevel input image data in a predetermined region including the pixel of interest; and

outputting the selected moiré intensity;

wherein at least one of said steps is performed by a computer.

11. A non-transitory computer-readable storage medium storing an executable program for causing a computer to function as the image processing apparatus of claim 1 .

12. The apparatus according to claim 1 , wherein said first filter unit and said second filter unit perform the smoothing filter processing using the same filter.

13. The apparatus according to claim 1 , wherein said third filter unit and said fourth filter unit perform the smoothing filter processing using the same filter.

14. The apparatus according to claim 1 , wherein the second region includes equal number of pixels in vertical and horizontal directions.

15. The apparatus according to claim 13 , wherein said third filter unit and said fourth filter unit are further configured to perform the smoothing filter processing using an isotopic filter.

16. An image processing apparatus comprising:

an acquisition unit configured to acquire multilevel input image data;

a halftone processing unit configured to perform halftone processing on the multilevel input image data to generate halftone image data; and

a moiré intensity output unit configured to:

output a first region value based on a difference between the halftone image data and the multilevel input image data, at a first region including neighboring pixels around a pixel of interest, as a moiré intensity for the pixel of interest in case that the number of edge pixels in a predetermined region around a pixel of interest in the multilevel input image data is less than a predetermined number; and

output a second region value based on a difference between the halftone image data and the multilevel input image data, at a second region including neighboring pixels around a pixel of interest, as a moiré intensity for the pixel of interest in case that the number of edge pixels in the predetermined region around the pixel of interest in the multilevel input image data is equal to or more than the predetermined number;

wherein the second region is smaller than the first region.

17. The apparatus according to claim 16 , wherein the difference between the halftone image data and the multilevel input image data is obtained from the halftone image data and the multilevel input image data which are normalized so that the numbers of tones per pixel are equalized.

18. An image processing method comprising the steps of:

acquiring multilevel input image data;

performing halftone processing on the multilevel input image data to generate halftone image data;

outputting a first region value based on a difference between the halftone image data and the multilevel input image data, at a first region including neighboring pixels around a pixel of interest, as a moiré intensity for the pixel of interest in case that the number of edge pixels in a predetermined region around a pixel of interest in the multilevel input image data is less than a predetermined number; and

outputting a second region value based on a difference between the halftone image data and the multilevel input image data, at a second region including neighboring pixels around a pixel of interest, as a moiré intensity for the pixel of interest in case that the number of edge pixels in the predetermined region around the pixel of interest in the multilevel input image data is equal to or more than the predetermined number;

wherein the second region is smaller than the first region;

wherein at least one of said steps is performed by a computer.