A few scanning tips

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Descreen

The array of dots used in the magazine halftone printing process is called a screen, and the software filter to remove moiré is called a Descreen filter. Scanner TWAIN driver software like Microtek ScanWizard and Umax VistaScan have very good Descreen filters that will greatly minimize moiré patterns. The Descreen filters have selectable parameters to match the screen frequency of the original image halftone (like newspaper or magazine). The additional Descreen processing does lengthen the scan time. It also greatly increases memory requirements for the processing the images. It is about the best solution, try it first. Moiré will always be a serious factor when scanning images in printed matter.

Using the Descreen filter slows the scan considerably, and the result is not quite perfect, but it is still pretty close to magic. Scanning at 2X the desired resolution, and then resampling back to half size before sharpening can help polish the results (2X not done below on this page, but was done on the previous page).

There are many types of screens used in printing, but they will all produce Moiré patterns. The appearance can vary considerably, the pattern can be large or small, spots or checkerboard or crosshatching, very dominant or fairly mild. Higher resolutions do not show such a noticeable pattern, but the image can still be degraded and murky overall.

Such screened prints are simply poor substitute for the original real photograph for use as a master copy of an image. These screened dots are a formidable obstacle.

Be certain to judge your own scanned images when viewed on the monitor at Full Actual 1:1 image size, because the monitor screen dots affect it too. The monitor screen can be a principal part of the interference. Often it looks worse when shown reduced in size on the video.

Descreen has a blurring effect, that's the entire point of it.

Do NOT use the Descreen filter when scanning real photographs.
Descreen is used only for images in printed matter, and not for real photo images. Also it is NOT used for scanning regular printed TEXT (line art mode would probably be for text documents).

Cloth and Needlework

The thread texture in woven cloth fabric is grid-like, and scanning real cloth can give similar moiré. Descreen may be helpful then too, but cloth has a relatively coarse texture (as compared to 2400 dpi prepress screen dots, most cloth is perhaps 20 to 100 threads per inch). So first just try scanning at fairly high resolution, 300 or even 600 dpi, and then resample smaller to desired image size if necessary.

Scanning the fine grid in printed needlework patterns can also cause similar problems. Colored patterns require color of course. Probably best as indexed color mode, adaptive palette, perhaps 16 or 32 colors, in a TIF, GIF, or PNG file. JPG files are inappropriate for this kind of detail. I'd try 300 dpi for printing the color detail. This is graphics, not photo, so pixel or graphics mode resampling wil likely be better than bicubic resampling.

But these patterns are also often printed as black line art too, representing color with black symbols, in which case color mode or grayscale mode is not necessary, nor desirable. So when appropriate, the best procedure to eliminate moiré in printed copies of such fine black symbol detail would be to scan at 600 dpi in line art mode. It should then print great, as is. See the line art threshold section.

The JPG compression is rather high for all images on this page, because the page is big and slow. Perhaps the images have finished loading by the time you have read this far down the page.

Some samples

No descreen   With descreen

The two images above were scanned at 50 dpi from the magazine. The first one suffers from the expected moiré pattern. The second one used the ScanWizard Descreen filter set to Magazine resolution.

This image was scanned at 75 dpi, without the Descreen filter. The pattern looks different at different resolutions. Descreen will fix this one too, trust me, but this web page is already too large and slow to show it again.


You should always critically examine any image by viewing it only at "Actual" size on the video monitor, even if it is for printing and is way too huge for the screen. The same image can look VERY different on the screen when displayed smaller, like 2/3 or 1/8 size, and such smaller examinations are not very meaningful. If you are not viewing at Actual size, you are not seeing the image pixels that will print, you are only seeing a resampled second copy, which will never be seen again at any other time.

150 Before   150 After

The images above are scanned at 150 dpi, with and without the Descreen filter. Moiré patterns have many different appearances, this one is almost polka-dotted.

300 dpi   600 dpi

The first image above was scanned at 300 dpi. The larger second image was scanned at 600 dpi. The Descreen filter was NOT used on either image. These images, especially the 600 dpi image, look very much the same as the real magazine page when examined under a 8 power magnifying glass. You can see the screen dots used to print the image in the magazine. The circular rosette is itself an expected moiré pattern in the original caused by the 4 color CMYK screens. I would not say there was no moiré in the scanned image, but the 600 dpi image really does look just like the magnified original.

Do you see how higher scan resolution helps prevent moire?

Continued


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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