A raster image, also called a bitmap, is a way to represent digital images. It can be created in a wide variety of formats, including the familiar .gif, .jpg, and .bmp. The image is represented in a series of bits of information that translate into pixels on the screen. These pixels form points of color that 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 that determines its color. This format 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 values. In photographs with subtle shading, this can be extremely valuable.
When the image is viewed, the pixels usually smooth out visually for the user, who sees a photograph or drawing. When blown up, however, the individual dots of color 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 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, so 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 that 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 that do not need to be blown up, or when quality is not important, a low DPI can be used.
The alternative type is the 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 the image. Because mathematics can be easily scaled, this type can be enlarged but still have smooth edges. Their use is limited, however, and they are most suitable for typography, line art, and illustrations. A raster image usually remains the best choice for a photograph or shaded drawing.