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Computer illustration or digital 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 tablet or 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 may be incorporated into such works, but they are not necessarily the primary basis
Mice are not very precise for drawing, so a graphics tablet is an important tool for a digital illustrator, 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. These variations mimic traditional wet and dry media. Drawing on a digital drawing tablet starts to feel natural after about a week of practice. 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.
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 paths), shapes (closed paths), and color fills, strokes or gradients. Vector paths are constructed of anchor points and path segments by using the pointing device to click and drag. Many vector graphics are readily available for download from online databases which can then be edited and incorporated into larger projects. Drawing tools typically create precise lines, shapes and patterns with well-defined edges and are superb for working with complex constructions such as maps and typography. Digital illustrations may include both raster and vector graphics in the same work. A bitmap image file may be saved in a format which embeds a layer of vector information, and vector image file may include imported bitmap images.