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Old 08-01-2017, 07:36 AM   #1
sammler
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Default Advanced scanning and post processing - prints with registration problems - descreening

Sometimes there is that one picture we treasure but the print we're trying to scan has some printing issues which make the scan always look worse than the original. No matter what scan settings or descreening options we try, the result always looks fuzzy or blurry.

A lot of times the problem is a minor or major registration issue. What is it? You may know that commercial printing, while it may use the same color separation as your home inkjet printer, does not print pixel by pixel, but rather prints whole pages or images at once. To do that, the image to be printed is separated into the 4 printing colors cyan, magenta, yellow and black (CMYK) before the actual printing happens. So four different screens are laid over the paper one at a time before each color is applied. The trick with these commercial printing machines is to make sure the paper doesn't move and the printing masks are put into the *exact* positions before the appropriate color is applied. The printers call this alignment "registration". With cheap publications more often than with quality ones, this registration can be off. In most cases it is one color mask or screen that is misaligned and colored dots appearing slightly shifted in one direction. You can easily see this at high contrast edges as a colored shadow. A cyan line along the left edge of a picture, a yellow line at the bottom, etc.

When scanning magazines, most people turn on descreening. Some scanning software does it automatically when detecting a color separated source. In the case of sources with registration errors, the descreening just makes the situation worse blurring even more detail out of the image. The good news is that you can correct these registration problems with a combination of scan settings and post processing. The bad news is that the process requires a bit of work and an advanced photo editor like photoshop or the gimp.

The basic steps are:
* scan at twice the desired resolution with descreening and color correction turned off
* realigning the misaligned color plane
* manually descreening

As a teenager I took the following clipping from a magazine. I think it was from an ad for subscription rewards. I chose it for this example since it has the worst registration problems I've seen. Here is a photo I took and a 300 dpi scan with descreening and a bit of post processing:



Doesn't look horrible, but clearly out of focus. When you look at the guy's collar with a magnifying glass or zoom in at higher resolution you can clearly see the problem(s):



There's not just one color plane off, they all look off: cyan, magenta and yellow borders are everywhere. To fix this, you need a decently calibrated scanner that will scan cyan as cyan and not map to a slightly different color that has red or yellow components. It's also important to turn off white balancing and color correction for that reason. If the image has a color cast that is part of the image, you can fix that after the alignment correction. If there is a color cast because of the print aging, you should fix that before. Then you need to scan at twice the desired resolution to make realignment easier and to leave room for manual descreening later. I would also recommend saving in a lossless format like TIFF since you will be doing some editing later. Here is the image scanned at 600dpi:



And here are a couple of crops of the girl's face illustrating the problem and the final outcome:



Scanners operate in RGB as that is what the light source often is (RGB LEDs in most cases) and that is also what the color sensors (CCD or CIS) use. So your scan will be an RGB image. But to fix the registration issue we need the image back in CMYK space. In photoshop there is a menu option "Image -> Mode -> CMYK Color". After converting your image to CMYK, select "Window -> Channels". Now you have five options: CMYK is the combined view of all channels and the four individual base colors. Click through all of them. Each one will show a black and white picture showing just this base color. Choose one that has the best contrasty edges as your reference layer. Usually black is my first choice, but in this case magenta looks like an equally good choice. Now figure out which layers are "off". Click the first color, in this case cyan. Then switch on the color you chose as a base, in our case black, by clicking the "eye" icon next to the base color. That will show you a cyan and black image and you can see where the cyan and black colors don't align.

Now for the realigning work: Select the "move" tool from your tool palette. But instead of using the mouse to move your layer, use the keyboard to move in pixel increments. E.g. If you see cyan shadows on the right and bottom edges in your cyan and black image, use the up and left keys to reduce the shadows. If you see shadows appearing on the top or left, you moved too far. A lot of times you can't completely eliminate these shadows. Just try to best you can to minimize them.

When you think you have a layer aligned well, there is an easy way to verify success: Turn the view of the reference layer on and off in rapid succession by clicking the "eye" icon next to your reference layer repeatedly. If the object you are looking at looks stationary, you did well. If it looks to be jumping around, you're not done yet.

Do this for all three colors that are not your reference color. Then switch back to CMYK view and your image should look much better. Convert your image back to RGB since that is probably going to be your target color space. Almost all JPEG images are in RGB space. Some software / browsers / servers will even reject CMYK encoded JPEG images.

Now would be a good time to fix blemishes, etc. with the healing brush, patch tool or the clone stamp. This would also be a good time to fix color problems like oversaturation and color casts.

As the final step, you can now get rid of the colored dots and do a manual descreening: Choose a blur filter or a median filter with a radius just big enough to smooth out those separate dots. Then resize the images by a factor of two to reach your desired resolution. This should give you a good image already. It will probably have edges that are a bit too soft, so apply some unsharp mask to taste. As always, moderation is the king here.

And here is my final version of this image:


Last edited by sammler; 08-10-2017 at 09:19 PM.. Reason: clarification on color correction
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