Tip: Some scanner control programs set their scan resolutions in "dots-per-inch" rather than "pixels-per-inch". The values used are the same whichever term is employed. However, the term "pixels-per-inch" will be used in these notes for scanner settings to avoid confusion with printer resolution settings, which are always referred to in "dots-per-inch".

Line-art images are made up of a simple black and white dot pattern which closely resembles the way that printing devices recreate images on the page, so this kind of image is most likely to reproduce with maximum fidelity, within the limits of the printer's inherent resolution. By contrast, images that contain shades of grey have to be processed to simulate the shades (a procedure called halftoning, described in a later section).

Image Size

For line images, the best results will be achieved with an original that is the same size or larger than the desired printed size within the finished document. If you scan a small original and then enlarge it, either in the scanner software or the DTP package, the individual dots that make up the image will be correspondingly enlarged, degrading the image quality. If a small original is all you have available, you will need to scan it at a higher resolution (see below) so as to minimise this effect.

Image Resolution

Almost all scanners have a variable setting for the scan resolution (expressed as a number of pixels per inch, or 'ppi', sometimes written as dots per inch - 'dpi'), either via a switch on the scanner itself or as an option in the controlling software. For line-art images, you should normally scan at a resolution that matches the print resolution of the final output device. Scanning at a lower resolution will result in a coarser image, while scanning at a higher level will make the image file on disk unnecessarily large. The various resolution levels for different output devices are summarised overleaf. The law of diminishing returns sets in when you scan at very high resolutions; for commercially printed output ask your printing firm what scan resolution they recommend.

The scanned line-art image of the typewriter (left) is shown reproduced at its original scan size. It was scanned at 300 ppi to match a typical 300 dpi (dots per inch) office laser printer. The detail of the same image below shows how the dot pattern becomes apparent when the image is scaled up within a DTP package.

Exceptions

If you require an image simply for positioning purposes, for example if the final document will have its images scanned professionally by a repro house and added when printing plates are made, you need only scan at a fairly low resolution, just sufficient to be able to recognise the image. This will keep the image file size acceptably low. You may occasionally wish to use a low resolution scan as a special effect, to produce a deliberately coarse or 'bitty' image.

Alternatively, if you have to work from a small original, and know that this will have to be significantly enlarged when placed in the DTP document, you can use a higher resolution to compensate for the dot-enlargement when the image is scaled up. The file size will increase but this is unavoidable in these circumstances.

Tip: The key to obtaining good results with line-art is to start with a clean, clear original. You can save yourself time later by removing any spots or blemishes from the original before you scan it. It should also be flat and un-creased (fix it on a piece of card with non-permanent adhesive, such as photographic spray-mount, if you like), otherwise unwanted shadows may appear. If it is very small you may want to enlarge it on a photocopier before beginning the scan.

Brightness and Contrast

If your scanner has brightness and contrast controls these should normally be left at the middle or default setting; check before you scan and adjust them only if the image is unsatisfactory. If you scan a coloured original as line-art you may find that certain colours drop out of the scan; some older hand-held greyscale scanners use a coloured scanning light which will not register the light's complementary colour correctly. Thus a scanner with a red light will tend to under-react to areas of red, making them appear too dark in the scanned image. If this happens, try photocopying the original first to convert it to black & white.