Descreening in Photoshop

I found a Photoshop recipe to remove the moire patterns that result from scanning magazines and books. See here for why I need this.

Here is an article on About.com on the subject.

  1. Scan the image at a resolution approximately 150-200% higher than what you need for final output.
  2. Go to Filter > Noise > Median.
  3. Use a radius between 1-3. Typically the higher the quality of the source, the lower the radius can be. Use your own judgement, but you will probably find that 3 works well for newspapers, 2 for magazines, and 1 for books.
  4. Go to Image > Image Size (Image > Resize > Image Size in Elements) and resample to the desired image size and resolution using the bicubic resampling option.
  5. Make sure you are zoomed to 100% magnification.
  6. Go to Filter > Sharpen > Unsharp Mask.
  7. Exact settings will depend on the image resolution, but these settings are a good starting point: Amount 50-100%, Radius 1-3 pixels, Threshold 1-5. Use your eye as the final judge.

Tips

  1. If you still see a pattern after applying the Median filter, try a slight gaussian blur before resampling. Apply just enough blur to reduce the pattern.
  2. If you notice halos or glows in the image after using Unsharp Mask, go to Edit > Fade. Use settings: 50% Opacity, Mode Luminosity. (Not available in Elements.)

I did an experiment to follow these instructions with a picture of a Chinese ‘Bullet Train’ I scanned from Fortune Magazine Asia-Pacific.

  • I did three scans at 150dpi, 300dpi and 600dpi. (I can work out how to do that using my Thai user interface to Scan Gear).
  • I tried using the technique to make a better 300dpi picture from the 600dpi scan. Here is a crop of the file as scanned:
Descreen test 1 - 600 dpi before

Descreen test 1 - 600 dpi before

  • That’s a good example of a moire pattern!
  • Here’s a screenshot after I applied the Median filter, radius 2:
Descreen test 2 - median filter (radius=2)

Descreen test 2 - median filter (radius=2)

  • Much better.
  • I kept the file the same dimensions but reduced it to 300dpi with this Image Resize setting:
Photoshop Image Size Options

Photoshop Image Size Options

  • Which left the picture looking like this:
Descreen test 3 - Resampled

Descreen test 3 - Resampled

  • Finally I applied the Unsharp Mask Filter with the following empirically derived settings:
Photoshop Unsharp Mask Settings

Photoshop Unsharp Mask Settings

  • Here’s a screenshot of the picture after I applied the Unsharp Mask Filter:
Descreen test 4 - Unsharp Mask

Descreen test 4 - Unsharp Mask

  • Finally, here is a before and after comparison using Lightroom’s Compare View:
Descreen test 5 - Before & After

Descreen test 5 - Before & After

Conclusion

The recipe worked well. If I can work out how to use Thai language Scan Gear I will compare it with using the descreen option provided with the driver.

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