User:Grythumn
Illustrations, and the scanning and processing thereof
So you vanna be a Content Provider, but are afraid of big scary images? Or you just want to be sure you don't mess it up for the person who ultimately processes the images? You've come to the right place.
First, you need to understand what types of illustrations you are likely to encounter. First, there is the simple breakdown of color vs black and white or gray; grayscale images are much more common. Then you break them into line art (almost always black and white), dithered or screened illustrations, and continuous tone illustrations. Scanning each has different criteria for turning out the best output.
Major Types of Illustrations
Line Art
First, line art. Line art is very sensitive to resolution, but undemanding in terms of color depth (color depth is how many colors the computer breaks the image into when digitizing it.) Line art should be scanned at 600 DPI or better, 8-bit grayscale. This can be safely reduced to 4-bit or 2-bit (16 color or 4 color, respectively) grayscale (assuming a clean scan, no gutter, foxing, or other issues) at this stage, but conversion to black and white should be left for the PP. Indeed, they are generally downscaled and left in grayscale for final presentation. Line art is very sensitive to shear-based deskewing; this is one reason it should not be scanned into Abbyy Finereader and then exported.
Screened Images
Secondly, dithered or screened illustrations. These are very common in 19th and 20th century books. They simulate various colors by depositing patterns of dots or colors that are blended by the eye into continuous colors. The distance between the dots is called the screen rule or screen frequency, and is generally measured in lines per inch. The most important thing to remember is that the books MUST be scanned at at least twice the screen rule, and preferably three times. The screen rule varies by application; 75 LPI is common in newspapers and other low-quality material; a few of the finest art books approach 200 LPI. If you don't want to worry about any of this, 600 DPI is a 'safe' resolution.
Continuous Tone Images
Third, and perhaps the most forgiving, are the continuous tone illustrations. Some are actual photographic prints, others are lithographs. Lithographs tend to have better color reproduction and detail, but are relatively rare. The level of detail varies widely from picture to picture, but generally 300 DPI is sufficient for photographic images, and 600 DPI for higher quality lithographs. These do not require descreening, but may require cleanup or deskewing.
General Preparation
Not all of these steps are required for all images.
Deskewing
One of the most basic corrections for an image is deskewing; the rotating of an image so that the image is oriented correctly. It is very common for a scan to be off up to five degrees from the ideal position. There are two common techniques for performing this correction: Shearing, and rotating. Shearing is fast, and does not produce any additional colors. Rotating is slower, and interpolates what would be in the corrected pixel position based upon the neighboring values. For many applications, shearing is fine; Abbyy Finereader uses it on all pages, and while it may create slight distortions in the letters, then end result is acceptable. However, when applied to a straight line, it results in a jagged line where the edge is sheared.
For illustrations, however,