Raster graphics are digital images represented by a matrix or grid of pixels, often called a bitmap. Each pixel or dot displays a unique color, and together all these colored dots form an image. Each pixel in a bitmap is stored as one or more bits in computer memory. Raster graphics with more colors and pixels require more bits and take up more memory. Typical file formats for raster graphics are .jpg, .gif, .tiff and .bmp.
Because raster charts are represented in a grid structure, width and height are often represented by the number of rows and columns rather than a specific unit of measure. The size of an image can be defined as 640 x 480; that means there are 640 pixels in a row and 480 pixels in a column.
The resolution, or degree of sharpness, found in an image is calculated by determining pixels per inch (PPI) or dots per inch (DPI). A higher resolution raster graphic will have smaller pixels resulting in a more detailed image. When raster graphics need to be viewed 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 only one bit of memory. A color raster graphic requires additional bits because three values are required to represent each red, green, and blue component of the pixel.
The color depth of an image is represented by the number of bits per pixel, and the greater the color depth, the more colors are available for viewing. An image with 8-bit color depth has 256 different colors compared to an image with 12-bit color depth, which would allow 4,096 colors. Graphics with increased color depth are more adept at shading.