Graphic images that have been processed by a computer can usually be divided into two distinct categories. Such images are either bitmap files or vector graphics. If you work in prepress, you need a good comprehension on the advantages and disadvantages of both types of data. These pages tries to explain these differences.

As a general rule scanned images are bitmap files while drawings made in applications like Corel Draw or Illustrator are saved as vector graphics. But you can convert images between these two data types and it is even possible to mix them in a file. This sometimes confuses people.

Bitmap data

Bitmaps images are exactly what their name says they are: a collection of bits that form an image. The image consists of a matrix of individual dots (or pixels) that all have their own color (described using bits, the smallest possible units of information for a computer).

Let’s take a look at a typical bitmap image to demonstrate the principle:

To the left you see an image and to the right a 250 percent enlargement of the top of one of the mountains. As you can see the image consists of hundreds of rows and columns of small elements that all have their own colour. One such element is called a pixel -short for picture element). The human eye is not capable of seeing each individual pixel so we perceive a picture with smooth gradations.

The number of pixels you need to get a realistic looking image depends on the way the image will be used. One of the next pages goes into more detail on this.

Types of bitmap images
Bitmap images can contain any number of colors but we distinguish between four main categories:

1. Line-art. These are images that only contain two colors, usually black and white. Sometimes these images are referred to as bitmaps because a computer has to use only 1 bit (on=black, off=white) to define each pixel.

   

2. Grayscale images, which contain various shades of grey as well as pure black and white.

   

3. Multitones: such images contain shades of two or more colours. The most popular multitone images are duotones, which usually consist of black and a second spot color (often a Pantone color). The example below contains black and Pantone Warm Red.

   

4. Full color images. The colour information can be described using a number of color spaces: RGB, CMYK or Lab for instance.

Characteristics of bitmap data
Bitmap data can take up a lot of room. A CMYK A4-size picture that is optimized for medium quality printing (150 lpi) takes up 40 MB. Compression can reduce the size of the file.

The image with the enlargement showed one of the main disadvantages of bitmap images: once they are enlarged to much, they look unnatural and blocky. But reducing a picture too much also has a bad influence as it looses sharpness.

Bitmaps are fairly simple to output, as long as your RIP or printer has sufficient memory.

Applications that can handle bitmap data
There are hundreds of applications on the market that can be used to create or modify bitmap data. In prepress, one application, Adobe Photoshop, completely dominates the market. This doesn’t mean that cheaper alternatives like Corel Photo-Paint, should be disregarded.

File formats that are used for bitmap data
Bitmap data can be saved in a wide variety of file formats. Among these are:

• BMP: limited file format that is not suitable for use in prepress.
• EPS: flexible file format that can contain both bitmap and vector data.
• GIF: mainly used for internet graphics
• JPEG: or rather the JFIF file format, which is mainly used for internet graphics
• PDF: versatile file format that can contain just about any type of data including complete pages, not yet widely used to exchange just images
• PICT: file format that can contain both bitmap and vector data but that is mainly used on Macintosh computers and is not very suitable for prepress.
• TIFF: the most popular bitmap file format in prepress

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