What is a Raster Image?

A raster image, also called a bitmap, is a way to represent digital images. The raster image takes a wide variety of formats, including the familiar .gif, .jpg, and .bmp. A raster image represents an image in a series of bits of information which translate into pixels on the screen. These pixels form points of color which create an overall finished image.

When a raster image is created, the image on the screen is converted into pixels. Each pixel is assigned a specific value which determines its color. The raster image system uses the red, green, blue (RGB) color system. An RGB value of 0,0,0 would be black, and the values go all the way through to 256 for each color, allowing the expression of a wide range of color values. In photographs with subtle shading, this can be extremely valuable.

When a raster image is viewed, the pixels usually smooth out visually for the user, who sees a photograph or drawing. When blown up, the pixels in a raster image become apparent. While this effect is sometimes a deliberate choice on the part of an artist, it is usually not desired. Depending on resolution, some raster images can be enlarged to very large sizes, while others quickly become difficult to see. The smaller the resolution, the smaller the digital image file. For this reason, people who work with computer graphics must find a balance between resolution and image size.


Resolution refers to the number of pixels per inch (PPI) or dots per inch (DPI) in the image. The higher the resolution, the greater the number of pixels, allowing for a greater gradation of color which will translate better as the image is enlarged. Of course, the more pixels, the more individual points of data to be stored, as well. For high quality photography, a high DPI is preferred because the images will look more appealing to the viewer. For small images which do not need to be blown up, or when quality is not important, a low DPI can be used.

The alternative to a raster image is a vector image, which uses a mathematical formula to draw a picture. A vector image defines points and the paths that connect them to form a digital representation of an image. Because mathematics can be easily scaled, a vector image can be enlarged but still have smooth edges. However, vector images are limited. They are most suitable for typography, line art, and illustrations. A raster image remains the best choice for a photograph or shaded drawing.