Vector and raster

There are basically two types of images on a computer - vector and raster

Vector images are composed of mathematical defined lines and curves called vectors. They also include color and position properties. These objects can be moved or resized or rotated as independent objects. The program retains the definition of these objects mathematically. Vector graphics are resolution-independent, meaning they can be displayed on output devices of varying resolutions without loosing any quality. No matter what size they are they retain a crisp definition. These images are associated with drawing programs.

Raster images on the other hand are composed of tiny squares called pixels arranged within a grid. The object is described by the specific location and color value of each pixel in the grid, creating an image much like a mosaic. They are often referred to as bitmap images. When you modify a bitmap graphic, you modify pixels, rather than lines or curves. Bitmap images are resolution-dependent because the data describing the image is fixed to a grid of a particular size. Editing a bitmap graphic can change the quality of its appearence. In particular, resizing a bitmap graphic can make the edges of the image ragged as pixels are distributed within the grid. Displaying a bitmap graphic on an output device that has a lower resolution than the image itself also degrades the quality of its appearence. These images work best with photographic images or images created in a painting program.

What is resolution?

The unit of measurement used to determine the size of an image and the way an image in displayed on your monitor.

Image resolution refers to the size of the file in pixels per inch (ppi).

Monitor resolution determines how your image is displayed on your monitor, called dots per inch (dpi).

Output devise resolution determines the quality of a final printed image measured in both dpi and lines per inch, (lpi).

Image Resolution
Digital images are represented in pixels They are usually measured in ppi. The more pixels per square inch the higher the resolution and the larger the file size. i.e. a one inch square of an image scanned at 72ppi contains 5,184 pixels ( 72 pixels wide by 72 pixels high ), and has a file size of 6K. The same one inch square of scanned image at 144 ppi contains 20,736 pixels ( 144 pixels wide by 144 pixels high ). It has a file size of 21K.

Monitor Resolution
Monitors have a fixed resolution, determined by the manufacturer. Macintosh displays images at 72 dpi and IBM - compatibles display images at 96 dpi.

the monitor resolution determines the size of the image displayed on the screen, and should not be confused with image resolution. Because an image may have a higher resolution than the monitor on which it is displayed. The image image with a higher resolution will appear larger on screen than in print. This is because a pixel is a fixed size on the monitor and the only way it has to display a higher resolution is to make the image appear larger on the monitor. So a picture that is 144 ppi will be twice as large on the monitor than an image at 72 ppi.

Output devise resolution
Output resolution refers to the number of dpi, dots per square inch that the output devise produces.

i.e. laser writers typically have a resolution of 300 or 600 dpi. A high quality image setter can print at a range of 1200 to 4800 dpi.

Look for our next tutorial for artists on Color Models