The process is more complicated than simply pushing a button, depending on the condition of the original cover being scanned (as well as the scan itself) it can take a considerable amount of work to make a scan presentable. Basically, these are the things that have to be done:
- If necessary, sharpen the image
- If necessary (and possible), remove or clean up the moiré effect
- Rotate, scale and crop the scan to the final dimensions
- Remove or clean up imperfections
- Colour adjustment
Let me explain a bit more in-depth. I use Photoshop myself, so I'll refer to tools and techniques in that program, but I'm sure the basic process works much the same regardless of what software you use.
- If the scan is poor, most often due to insufficient pressure on the scanner lid while scanning, parts of the image may become blurry. Usually there's not a whole lot you can do about that, but in some cases the blurriness can be salvaged by manually sharpening the affected areas using the Sharpen tool (unless of course you can simply copy whatever should go there from a similar, unscathed part of the scan, or redraw it by hand, or whatever). Any text on a cover should of course be legible, so if a scan is too blurry or low-res for the text to show up well, it's probably a good idea to re-type the text manually, making it look as close to the original as possible.
- The moiré effect is another "hallmark" of a poor scan. Avoiding it is mostly in the hands of whoever scans the material, but most of the time it won't be 100% eliminated. If the original scan is 600dpi or higher, and scanned with the "descreen" setting, in most cases it will look perfectly fine when scaled down to 300dpi. Otherwise, clever applications of the Dust & Scratches and Gaussian Blur filters, among others, can sometimes make a poor scan look a little better. Perhaps for this reason more than any other, it is very desirable to be working with high-resolution scans (600dpi or higher).
- The final image should be 300dpi, and the image should stretch to all four corners without any white edges around the border. There are basically two ways to do this: In most cases, at least when a cover involves any kind of custom work (IE anything from fitting into a template to a completely custom cover), I use the Ruler tool to draw a line along the edge of the scan (or any other straight horizontal or vertical line) and Rotate-Arbitrary to make the scan completely perpendicular. After that it's just a matter of using the art in whatever fashion is appropriate. The second way is to create a new document with the final dimensions, paste the scan in a new layer, and then use the Free Transform command and scale, rotate and skew the image by hand until the corners of the scan line up with the corners of the canvas.
- Removing imperfections is generally the most time consuming part of cleaning up a scan. Imperfections can be anything from scratches, creases and tears to sticker residue, stains or torn off parts that are missing completely. Any and all of these should be remedied as well as possible. The Healing Brush and Clone Stamp are your friends! Any imperfection on an area that isn't intricately detailed can easily be taken care of with the Healing Brush, and wherever that's not enough, the Clone Stamp is usually very helpful. Ideally, you should fine comb the entire image for any imperfections; oftentimes small scratches on the cover, dust on the scanner bed, imperfections in the original print and other small things can go by unnoticed, so you'll want to go over the whole thing quite carefully (this is where the time consuming part comes in). Even the most pristine original WILL have some imperfections when scanned, so just because it's not a 20 year old banged up cardboard box, it's worth making sure no ugly artefacts rear their ugly heads.
- Colour adjustment is a vital, and perhaps sometimes overlooked aspect of cleaning up a scan. Many older covers have colours that are sunfaded, but again, even a brand new cover can have its issues due to colours being picked up a certain way in the scan. Proper colour and contrast can make the difference between a half-decent and great looking final cover. The most basic (but also the best) way to adjust overall contrast is by using the Levels command. Adjust the sliders so the black parts are completely black, and the white parts completely white, without the rest of the image being too dark, too bright, or otherwise suffering. Past that there are a plethora of other commands you can use to tweak the colours, such as Hue/Saturation, Colour Balance, Brightness/Contrast, Replace Colour, and others. Just use your own good judgement and play around with the different options and sliders until everything looks good. Most of the time any of these commands can (and should) be applied to the whole scan, but in the case of a badly sunfaded spine for example, you'll want to isolate the spine and tweak it, so that the colours match those of the front and back of the case.
I don't know if any of this is of much help to you (I have no idea how well you know your way around Photoshop, for example), but that's my basic method, anyway. If there's interest perhaps I could show some specific examples later on.
