File Preparation: Understanding Image File Formats

Understanding Image File Formats: Raster vs Vector

The software applications commonly used in the graphic arts industry can be grouped into graphics programs (drawing, painting and image editing) and page layout programs (combine graphics and text). In general, there are two classes of digital image data created by graphics programs: vector and bitmap.

Vector Images
Vector, or Object-Oriented images are a collection of geometric shapes – lines, curves, and color information, or fills. Graphics created using most illustration applications are vector, or object-oriented files. The advantage of vector images is that they can be modified easily and resolution is determined by the output device – the image is literally drawn according to instructions. A vector image can be produced at one size and be enlarged in your page layout program without losing any quality. Vector graphics are the best choice for type (especially small type) and bold graphics, such as logos, which require crisp, clear lines that can be scaled to virtually any size.

Vector images have their disadvantages as well. Vector files don't handle photographic detail very well, and they're often difficult to exchange across computer platforms. Also, complex vector files can require a lot of computer processing power to render the image. Thus, a vector image is not the ideal file format for a photograph. Instead, a bitmapped image should be used. Drawing programs such as Macromedia Freehand™ and Adobe Illustrator™ are vector-based programs.

Raster Images
Raster, or Bitmap images are good at reproducing the subtle shading found in continuous-tone images, such as photographs and grayscale images. Images captured by a scanner, manipulated in image editing software or created in paint software are usually raster, or bitmapped, files. Bitmap images are described pixel by pixel and contain color value information, in bits, for individual pixels. Bitmap images are resolution-dependent; that is, they represent a fixed number of pixels. As a result, they can appear jagged and lose detail if they are enlarged in your page layout application, or if they are printed at a higher resolution than they were scanned.

Bitmap or Raster file formats have many advantages. They maintain the fidelity of photographic originals very well, they typically require less processing time, and often they can be easily exchanged or converted to other raster file formats. A vector image can be converted to a bitmap image. Converting a bitmap image to a vector image, however, is nearly impossible. Bitmap images are not easily edited. Adobe Photoshop™ is an example of a bitmap-based program.

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