Digital illustration or computer illustration is the use of digital tools to produce images under the direct manipulation of the artist, usually through a pointing device such as a graphics tablet or, less commonly, a mouse. It is distinguished from computer-generated art, which is produced by a computer using mathematical models created by the artist. It is also distinct from digital manipulation of photographs, in that it is an original construction "from scratch". Photographic elements such as background or texture may be incorporated into such works, but they are not necessarily the primary basis. Pointing devices Mice are not very precise for drawing, so a graphics tablet is often an important tool for most digital illustrators, because it allows the user to make a mark easily in any direction, in a way that reflects the natural or "lively" line made by the human hand. In addition to flexibility of movement, an industry-standard digital drawing tablet has a pressure-sensitive surface, allowing the illustrator to make marks that vary from faint to bold, and from thin to broad, similar to how one would work with a brush. These variations mimic traditional wet and dry media. A hybrid graphics tablet/screen might be helpful, since the artist can see more accurately where to place strokes in the image, but the hardware is currently much more expensive. Illustration software There are two main types of tools used for digital illustration: bitmapped (also known as "raster") and vector applications. Bitmap applications are commonly called "painting" programs, such as Adobe Photoshop, while vector applications, such as Adobe Illustrator, are called "drawing" programs. These terms reflect the difference in look-and-feel between the images created in each type of program. With a bitmap application, the content is stored digitally in fixed rows and columns of pixels, which can be created in separate layers for more easily isolating and manipulating different parts of the image. A bitmap image contains information about each pixel's hue (color), luminance (brightness), and saturation (intensity of color). When the pointing device moves over an area of the image, new colors and values are applied to the underlying pixels. Painting tools allow the easy creation of "fuzzy" imagery, including effects such as glows and soft shadows, and textures such as fur, velvet, stone and skin, and are heavily used in photo-retouching. With vector-based tools, the content is stored digitally as resolution-independent mathematical formulae describing lines (open