How can I reduce CMYK halftone patterns when scanning printed artwork?
Asked 1/6/2021
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I’m scanning a printed CD cover on a flatbed scanner and the CMYK halftone dot pattern becomes very visible at high scan resolutions. A simple Gaussian blur helps, but it either leaves grid/moiré artifacts or removes too much detail. Is there a better way to scan or process printed material to reduce halftone patterns and moiré while keeping as much detail as possible? I’m using GIMP, but any general technique or tool is fine.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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Please note - it has been a while since I've worked with this kind of scanning.
I think the repeating pattern (especially visible in the wood section of the image after blurring) can be caused by interference between the the scanner's grid of pixels and the grids used in the CMYK halftone screens. The traditional way to reduce the effect would be to try rotating the CD cover on the flatbed (maybe try 15, 33 and 45 degrees) and seeing if that helps the end result (after gaussian blur). You might also want to downsize the image after the blur to try and restore sharpness and reduce the visibility of any halftone dots.
Also - looking at the R, G, B channels separately the amount of blurring seems different with each one. Using your sample pre-blur image I was able to get pretty good results by using different blur settings for each channel.
Originally by user2228. Source · Licensed CC BY-SA 4.0
user2228
5y ago
0
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Yes. What you’re seeing is likely moiré/interference between the scanner’s pixel grid and the printed CMYK halftone screens. A better approach than just blurring is:
- Rotate the artwork on the flatbed and rescan. Try angles such as about 15°, 33°, or 45°.
- Apply blur only as much as needed after scanning.
- Then downsize the image to the final resolution; this can reduce visible halftone dots and restore a cleaner look.
- If possible, inspect the RGB channels separately, since the halftone pattern may be stronger in some channels than others; treating channels differently can improve results.
So the practical workflow is: high-resolution scan, experiment with slight rotation, apply modest Gaussian blur, then resize down. There isn’t a perfect recovery of continuous-tone detail from halftone print, but changing the scan angle and resampling usually gives a better result than blur alone.
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