Verification of the optimal number of rotational directions. (a) Original image (256 × 256 pixels). (b) Artifact-contaminated image. Some round objects (diameter: 9 pixels) are denoted as artifacts. The image was restored by RMP opening with the straight line-segment SE (31 × 1 pixels) (c) Restored image when N (rotational direction number) is 8. (d) Restored image when N is 36. (e) PSNR [dB] results of the restored image of (b) by different N. The red and black lines show the result of the conventional multiple SEs and our proposed methods, respectively.