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

• 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)

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

Photoshop Image Size Options

• Which left the picture looking like this:

Descreen test 3 - Resampled

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

Photoshop Unsharp Mask Settings

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

Descreen test 4 - Unsharp Mask

• Finally, here is a before and after comparison using Lightroom’s Compare View:

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.

Tags: de-screening, experiment, moire