Barefoot Liam

How to scan colour images from old books, with Gimp, in 18 easy steps.

1. Some people report that a piece of opaque green paper placed behind the page will help reduce show-through; either that or black card seems to help.
2. Look at the image, using a magnifying glass if necessary, and see if it is made of lots of small dots, or is printed with solid areas of dots.  If the book is from before 1850 or so, colours are likely either to have been added by hand, perhaps with a water-colour brush, or in separate print runs of solid colour. If the book is after 1860 or so, there will probably be dots.  You’ll need this information later.  You might also want to note down the page number.
3. Make sure the page is flat on the scanner’s glass!  If necessary, weigh it down a little with another book. If you bend the book back somewhat you can loosen the binding a little, which helps.
4. If you are using Gimp, you can scan the image by going to the File menu and choosing Create, and under that xsane.  If you are using Photoshop it’s most likely under the File/Acquire menu.   For an image made of dots (screened) you will need much higher resolution than you might expect, because capturing the exact size and shape of each of those dots determines the amount of detail you will get in your final image.  You can experiment, but I find there is often a noticeable difference between 600 and 1200dpi, and always a noticeable difference between 300 and 600.
5. When you choose the area to scan (usually by dragging a rectangle), make the area a little larger than the image all round.  There’s no need to go wild here, as scanning too much will just eat up memory and disk space, but you want enough extra to give you room to rotate the image to make it perfectly level afterwards, and to make sure you don’t accidentally miss a bit if the preview isn’t accurate.
6. Your scanning software (e.g. xsane) will probably have a button to auto-adjust levels, and you should press it.  You want the darkest part of the picture to be black and the lightest part to be white (or nearly white) in the output.  I find that I have to increase the gamma to about 1.2 with xsane, though, to keep the detail.
7. Once you have scanned the image, you will want to save a copy.  I always include the page number, but since the book is still sitting on the scanner at this point, you can see why I suggested remembering the number!  I tend to use filenames like 0314-Taj-Mahal-raw.png for the original and 0314-Taj-mahal-cleaned.png for the version after I have processed it (which I am about to describe).
8. Now you have saved the image, you need to make it look like something you want to keep.  The first step is to check the colour range; in Gimp this is colours/levels, and you can use the auto button on that dialogue box to get a good first attempt.  It’s probably good enough for now, but if the colours go weird, choose reset and instead move the little triangles so they point to the upper and lower ends of the histagram. Now you have an image with colour values spanning the whole range; that’s important because as we process the image it means we won’t lose as much detail.
9. Next we remove the dots.  You can do that with a Gaussian blur.  (if there were no dots, you can skip this step).  Choose Filters/Blur/Gaussian Blur.  When the dialogue appears, make sure the preview is ticked, and move the slider to about 11.  For 1200dpi or higher you may need it to be 17 or higher.  Your goal is to find the smallest number at which you can’t see any dots in the preview.  Use the scrollbars (and resize the filter window too) until you find a part of the image that’s fairly light in colour, as the dots are most visible there.  Usually the dots are in clusters, called rosettes, and you need to blue enough that the rosettes are gone too.  Once you find that number, go one or two higher, so, if the rosettes (or at least the dots if you can’t see rosettes) are gone with a bur radius of 12 in the preview, you’ll increase the radius to 13 or 14.   Then press OK to blur the image.  It may take a while.
   Note: If your image has black line art and screened colour, you can experiment using selective gaussian blur to keep the black sharp.
10. Once the image is blurred, you can rotate it, if needed, so it’s upright.  This is easiest in gimp using the Rotate tool with the tool options set to reverse/corrective and the preview set to grid; then you can move the grid so it lines up with something horizontal or vertical in the image.   You can change the spacing of the grid lines in tool options to help the grid line up.  Choose Cubic for the rotation algorithm, because you don’t want to preserve detail here that is just a remnant of the dots.
    Note: if you rotate before you descreen, you will introduce new artefacts that are difficult to remove. So you do the blur first and then the rotate.
11. After rotating, flatten the image (rotate introduces transparency, which we don’t want here).
12. We are nearly done, don’t worry.  At this point, I’d save a new copy of the image with the -cleaned- in its filename, because we’ve done enough work that we’d be cross if there was a power failure and we had to start over.
13. Make sure the image fits in your window (c0ntrol-shift-E in Gimp) and decide how bad the colours look.  Use Colours/Curves and experiment a little.  Or use Colours/levels and use the “white point” eye dropper to click on part of the image outside the picture, where there is plain paper.  Your goal now is to get something that looks roughly like the printed original.
14. Optional step, but very wise: go over the image slowly, at 50% magnification, looking for things like dead insects or hairs or other things you want to fix, and use the clone tool as needed (with a soft brush) to get rid of them.
15. When you’re happy, save the file again.  Use PNG, because it’s lossless.
16. Now you have a nice clean descreened image, but it’s blurry and it’s too big.  No problem: scale it down!  Use image/Scale Image, and go down to, say, 25%, and the image will look much better.  If you want a really sharp image, scale down further, but remember to keep that cleaned PNG file at fill size.  Now, we’re nearly done!
17. After you downsize an image you often need to sharpen it, using Filters/Enhance/Sharpen.  Some people prefer unsharp mask, but I find that’s best for photographs rather than scans of screened prints of photographs. I also prefer to be subtle with sharpening, but again it’s a matter of preference.
18. You can save the smaller scaled-down image as JPEG; I find if there are isolated blobs of red that I need to use the 1×1 subsampling to keep the colour right.

With practice you can do all this fairly quickly. I find the slowest part is usually fixing any problems like hairs or stuff that falls out of the binding of some older books.  You know, $10,000 bills and ancient Roman golden rings. Or more likely tiny pieces of paper, string or leather.  Wipe the scanner glass between scans with a clean piece of kitchen paper, wiping gently always in the same direction, so that pieces of grit don’t make scratches.

Oh. If your image did not have tiny dots, but was printed solid, or if there was detail in the black