Bitmap versus vector graphics

Digital images can usually be divided into two distinct categories. They are either bitmap files or vector graphics. If you work in prepress, you need a good understanding on the advantages and disadvantages of both types of data. These pages try to explain the differences.

• As a general rule digital pictures and scanned images are bitmap files. These are sometime also called raster images.
• Drawings made in applications like Adobe Illustrator or Corel Draw are saved as vector graphics.

Technically both data formats are completely different. The end result however can look virtually identical in either format. As a general rule bitmaps are typically used to depict lifelike images whereas vector graphics are more often used for abstract images such as logos. There are however numerous exceptions to this rule. It is often impossible to determine whether an image is a bitmap or a vector file just by looking at it.

• Vexel art, for instance, are bitmap images that have been manipulated to look as if they are vector data.  The technique is used to create attention-grabbing realistic images that have an artificial and sharpened look to them.
• Talented artists like Yukio Miyamoto can draw photorealistic images using vectors.

Photorealistic vector art from Yukio Miyamoto

You can convert a bitmap image into a vector file. A vector image can be transformed into a bitmap. There are even file formats that can combine both types of data into a single file.

Bitmap images

Bitmap 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:

Example of a bitmap image

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 color. 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 there are 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.
   

   Example of a lineart image

2. Grayscale images, which contain various shades of gray as well as pure black and white.Typically 256 shades of gray (8-bit) are used even though the human visual system needs only 100 tints to perceive an image as life-like.
   

   Example of a grayscale image

3. Multitones: such images contain shades of two or more colors. 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.
   

   Example of a duotone image

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

Example of a color image

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 too much, they look unnatural and blocky. Reducing their sizes also has an impact on image quality as images lose a bit of 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: an outdated and limited file format that is not suitable for use in prepress.
• EPS: a flexible file format that can contain both bitmap and vector data. It is gradually being replaced by PDF.
• 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,it is 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.
• PSD: the native file format of Adobe Photoshop (which can also contain vector data such as clipping paths)
• TIFF: a popular and versatile bitmap file format

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