Computer Graphics

Concepts

The key concept in computer graphics is the representation of an “analog” piece of visual information (a picture made up from a potentially infinite number of brush strokes in any number of colors or gray shades) in a digital format (a table of numbers chosen from a set of fixed values). The advantage to the digital format is that, while a picture drawn on paper is essentially a static item that can be changed only with difficulty, a digital image can be repeatedly transformed by applying various algorithms (predefined mathematical processes) to the stored numbers. For example, an image can be “mirrored” simply by reversing the order of the numbers that represent it, or doubled in size by repeating every number in the table.

Computer-graphic systems have much in common with television displays, in which pictures are built up from thousands of individual dots or squares by electron guns scanning systematically across a phosphor-coated screen. Raster graphics, as this process is called, is very different from an earlier (and much more expensive) form of computer imaging known as vector graphics, in which pictures were built up by plotting successive lines between coordinates a little like the trace on an oscilloscope. Imagine drawing a picture of a house. With raster graphics, the roof might be built up from a collection of red dots that happen to make the shape of a triangle and the main building from a collection of blue dots that happen to make up a square. With vector graphics, the roof would actually be plotted out as a red triangle and the main building as a blue square.

In the crudest computer imaging system, each pixel (the smallest element of a picture) is represented by a single bit (binary digit), with a 1 meaning that the pixel should be lit and a 0 that it should be unlit. As the pixels can only be on or off, this system is good enough only for rendering images in black and white; the resolution of an image (how detailed it can be) is determined by the size of the pixels (the number of pixels in a given area) and thus by the number of bits of memory available. The entire image is effectively a map of bits, or a bitmap. Using 8 bits per pixel, it becomes possible to represent 28 or 256 colors (or gray shades) at each point of the image. With 24 bits per pixel, 16 million colors are possible. For a typical PC screen size of 800 x 600 pixels, that gives a total memory requirement of (800 x 600 x 24)/8 or roughly 1.4 megabytes.

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