Computer Graphics

I

Introduction

Computer Graphics, the generation of graphs, tables, and static or moving images by computer systems, usually for display on a monitor screen. The complexity of the graphic material depends on the software program being used and on the amount of computer memory available. It is now quite common for synthetically produced computer graphic images to alter or to be integrated with visual and spatial information sampled from the real world through techniques such as motion capture.

II

The Development of Computer Graphics

Computer graphics and animation, now familiar through film and video games, are also widely used in science and industry for computer-aided design and the graphical representation of data. In films, highly sophisticated computer graphics are widely used in the creation of special effects. One of the earliest instances of this is the 1982 Disney film Tron, in which computer-generated imagery (CGI) was extensively used. A full 15 minutes of the film consists of moving images generated entirely by computer. In addition to this, there are over 200 scenes in the film that utilize computer-generated backgrounds. Since then, CGI has become such an integral part of the film world that several major movies have been based entirely on computer graphics.

Before the 1960s there was no such thing as computer graphics. At that time, computers operated in batch mode and there was no interaction between the user and the computer, other than the initial submission of a job and the final collection of the results. This situation changed radically in 1962 with the development of a software program called Sketchpad by Ivan Sutherland. This was the first instance of a user interface based on graphics and it paved the way for computers to be used as aids in the production of drawings and images.

III

Vector and Raster Graphics

The physical device used with the Sketchpad program to produce graphic images was the cathode ray tube (CRT), an essential element of the television set at that time. In order to produce images on the CRT screen that served as the computer’s monitor, there had to be some way of generating two-dimensional graphics from the computer’s output. The two basic approaches to this are vector and raster graphics. With the former precise geometric data, layout, and style is stored. This includes the coordinate positions of points, the connections between points (to form lines or paths), the colour, and the thickness of the shapes. Most vector graphic systems also have a library of standard shapes such as circles and rectangles.

With raster graphics, a grid of picture elements, each with its own colour and brightness is projected onto the screen. This grid is composed of a number of horizontal and vertical lines: a standard PC display has 1,024 of the former, 768 of the latter. At the intersection of each horizontal and vertical line is a point known as a pixel (or picture element) and it is the combination of all the pixels on the screen that define the image. It is possible to adjust the image resolution by having more lines (for higher resolution) or fewer (for lower resolution).

IV

Three-Dimensional Images

With the growing sophistication of computer equipment, the focus of computer graphics in the 1980s moved from the rendition of two-dimensional (2D) images to three-dimensional (3D) ones. The demands of a more complex image rendition meant that a lot more information now had to be stored on the computer. As well as rendering simple flat surfaces, three-dimensional objects require information on shading and other more subtle effects.

The basic concept that underpins all modern computer graphics is the three-dimensional polygon (a geometrical plane figure with three or more straight sides). Images are based on collections of polygons and this means that computer graphics software has to store the coordinates that define each of the polygons that comprise the 3D representation. The points, lines, and surfaces that define the polygons may be drawn or derived from measurements (for example, of a moving object or person).

The software that has been developed to render modern computer graphics does much more than simply store geometric information. The realistic images that people now expect to see require a lot more than the assembly of a massive collection of polygons into recognizable shapes. They also require techniques for shading, texturing, and rasterization of the images. The last of these, rasterization, involves the conversion of a vector-defined image into a series of pixels that can be rendered as a 3D image on a video display. Texturing is concerned with how surfaces look after being shaded, depending on the shading method, and how the image is interpreted during shading.