Raster graphics are digital images represented by a matrix or grid of pixels commonly called a bitmap. Each pixel or dot displays a unique color and together all of these colored dots create an image. Every pixel in a bitmap is stored as one or more bits in computer memory. Raster graphics with a greater number of colors and pixels will require more bits and take up more memory. Typical file formats for raster graphics include .jpg, .gif, .tiff, and .bmp.
Since raster graphics are represented in a grid structure, the width and height are usually indicated by the number of rows and columns rather than a particular unit of measurement. The size of an image might be described as 640 x 480 meaning that there are 640 pixels in a row and 480 pixels in a column. The resolution or degree of sharpness present in an image is calculated by determining the number of pixels per inch (PPI) or dots per inch (DPI). A higher resolution raster graphic will have smaller pixels that result in a more detailed image. When raster graphics need to be displayed or printed at larger sizes, it is helpful to use a higher resolution so that the image does not appear grainy.
Black and white raster graphics contain only black and white pixels and each pixel requires just one bit in memory. A colored raster graphic requires additional bits because three values are necessary to represent each of the red, green, and blue components of the pixel. The color depth for an image is represented by the number of bits per pixel and as the color depth increases, more colors are available for display. An image with an 8-bit color depth would have 256 different colors available compared to an image with a 12-bit color depth that would allow for 4,096 colors. Graphics with increased color depth are more adept at displaying shading.
Raster graphics are only one of the two common graphic types used to digitally represent 2-D images. Vector graphics consist of points and paths and the mathematical relationships that connect them to create an image. A vector graphic is infinitely scalable in size, whereas a raster graphic is resolution dependent and changes in size will detrimentally affect the visual quality of the image. Vector graphics are often used for type work and line illustrations while raster graphics are most suitable for photographs and images with continuous tones and shading.