In the enlargement of a typical black-and-white bitmap shown above, the image would be stored numerically as a sequence of ones and zeros. Thus the first row of dots would be represented by the sequence 001111100000 00000111. The file header (the first few bytes of data) provides information on the width and height of the image, and its complexity, so that the data can be reassembled into an image. Many bitmap file formats reduce the image file size by encoding a sequence of pixels where the same colour recurs as a single number.

The data produced by a scanner is stored in the form of a bitmap. A bitmap is essentially a pattern of dots, displayed either on screen or on paper, which reproduces the original pattern of the image. The image is reconstructed line by line from the data stored in the computer file. The level of fidelity with which the image is reproduced depends both on the size and number of dots stored in the image file when it is scanned, and on the ability of the output device (whether printer or computer screen) to recreate that pattern accurately. Often the limitations of the printer will outweigh the abilities of the scanner.

The amount of detail included by the original scan also has a direct relation to the size of the image file when stored on a computer disk. This contrasts with the kind of image produced by a vector drawing package (such as Corel Draw), where the information is stored as a series of mathematical lines, curves and filled areas which usually require less disk storage space and will display and print accurately at any scale to the best abilities of the printer or screen in use. However, a vector format image cannot reproduce the subtle shadings and naturalism of a scanned bitmap; it tends to produce images that are somewhat "technical" in appearance.

(Above left) a bitmap image from Windows Paintbrush; (Above right) a vector image (courtesy ProMedia Software).

Bitmap File Formats

One of the drawbacks of graphics work on computers is the plethora of different image file formats that have been created by various software developers over the years. However, there are is one format, TIFF, which is supported by almost all products and has emerged as the nearest thing to a common standard. There are sub-variations of TIFF, but on the whole a TIFF image produced by a scanner will be readable by most image processing and DTP packages. On the Macintosh, the PICT format is also commonly used. If necessary, file conversion utilities are available at moderate cost (often as shareware) to overcome the occasional compatibility problem. Kodak's Photo-CD (.PCD) format is also emerging as a useful storage medium for colour images derived from transparencies and negatives.

"My photographs do me an injustice. They look just like me" PHYLLIS DILLER

TIFF Images

TIFF stands for Tagged Image File Format, and its files (on PC) have the extension .TIF (for example, MYFILE.TIF). This format extends from simple black-and-white scans of line drawings up to full colour versions at the high end, and is probably the best format to use in most cases. Some scanner software packages can create compressed TIFF files which take up less hard disk space, which is useful if you plan to scan large images, and as long as your DTP software can accept these compressed files. TIFF format is often the best for transfer of files between PC and Apple Macintosh, where many programs support it.

PCX Images

PCX is one of the most commonly supported formats on PC, although it lacks some of the capabilities of TIFF, especially where more than 16 colours or shades of grey are required, and is becoming obsolete. It was commonly used by older "paint" programs such as PC Paintbrush, Windows Paintbrush and Deluxe Paint. PCX can be helpful if you plan to use such a package to retouch or modify a scanned image before transfer to the DTP system, especially when working with colour. PCX files are labelled with the PCX file extension (for example,. MYFILE.PCX). Nowadays though TIFF format is usually a better bet.

BMP Images

Some scanner software that runs under the Windows environment also supports Window's own .BMP bitmap format. This can be used as an alternative to .PCX if you plan to edit your pictures with Windows Paintbrush, or you are using a Windows DTP program or word processor that can import .BMP files. RLE (Run Length Encoded) files are a compressed sub-variant of BMP, used occasionally by programmers to include images (such as logos) in software. Plain BMP format is not a good choice for most DTP work as its files are not compressed and therefore use more disk space than (say) TIFF.

Continued...