English_topic_1_kurs / computer graphics.doc

Computer Graphics

1. INTRODUCTION

Computer Graphics, images created by using a computer. Computer graphics are used in a variety of fields, including entertainment, science, education, medicine, business, and industry. Computer graphics also help make computers easier to use. Graphical user interfaces (GUIs) allow computer users to interact with their computers by selecting small images called icons to execute instructions, eliminating the need to use text commands.

2. HOW COMPUTER GRAPHICS ARE MADE

Creating a computer-generated image generally follows three separate stages: modeling, rendering, and display. These stages result in a completed image.

3. Modeling Stage

In the first stage of assembling a computer graphic, known as the modeling stage, a computer graphics artist acquires enough data to represent an image in three dimensions (3D). The data are usually grouped into three distinct classes, according to how the data are acquired. The first class of data is geometric and is usually created with special modeling software or acquired by using a 3D scanner, a device that converts an image or model into digital data. Objects such as buildings, for example, are approximated with a set of simple shapes such as triangles and spheres. The second class of data involves placing acquired images, usually from a camera or a video recorder, in 3D space. The third class relates to volume data, such as that acquired from a CAT scan. This type of data is mostly used in medical applications.

4. Rendering Stage

During the second stage of image creation, known as the rendering stage, a two-dimensional image is produced from the three-dimensional input data. The method with which an image is rendered depends on the type of input data, the quality of the desired image, and the amount of time needed to render the image. For example, high-quality computer graphics images that are indistinguishable from a photograph take about 24 hours to render. Low-quality images, such as the type used in video games, can be rendered in 1/60 of a second. For geometric data, the rendering process may be as simple as projecting all the objects onto the computer's screen, or it may be as complex as a full lighting simulation involving dozens of light sources. The process is similar to placing a large sheet of glass in a room and having an artist paint everything seen through the glass onto the glass. Only in computer graphics that sheet of glass is the computer monitor, the artist is the computer itself, and the brushstrokes are guided by mathematical formulas. The end result of the rendering stage is an image that consists of a large number of colored picture elements known as pixels arranged in a grid like the squares on a sheet of graph paper. Together, these pixels form the image.

5. Display Stage

The final stage in creating a computer graphics image is the display stage. It involves making the image suitable for display. Although the rendering stage produces two-dimensional data, which would theoretically be suitable for display on computer monitors or in computer printouts, further processing is needed for various purposes. For example, the colors of phosphor available for the image on the computer monitor are different from the colors of ink available in the printer. Therefore, during the display stage the computer graphics artist may change the color representation of the image so that the printed image will look the same as it does on the monitor.

6. COMPUTER GRAPHICS AND COMPUTER MEMORY

Before an image can be displayed on a computer screen or monitor, it must be created by a computer program in a special part of the computer's memory, called a frame buffer. The frame buffer is usually located on the computer’s graphics card. One method of producing an image in the frame buffer is to use a block of memory called a bitmap to store small, detailed figures such as a text character or an icon (small image). Frame buffer memory can also store other information, such as the color of each pixel.

7. Using Memory to Represent Color

Computers store and manipulate colors by representing them as a combination of three numbers. For example, in the Red-Green-Blue (RGB) color system, the computer uses one number each to represent the red, green, and blue primary components of the color. Alternate schemes may represent other color properties such as the hue (frequency of light), saturation (amount), and value (brightness).

If one byte of memory is used to store each color component in a three-color system (red, green, and blue), then over 16 million color combinations can be represented. The computer graphics artist, however, restricts the number of color combinations used in creating an image, particularly those combinations where the human eye cannot discern a difference between two colors. These restrictions reduce the amount of memory and processing time needed to create the image.

An alternate method of color representation, known as color mapping, uses only one number per color combination, storing each number in a table of available colors like a painter's palette. The problem with color mapping is that the number of colors in the palette is usually too small to create realistically colored images. Choosing the colors that make the best image given the available palette is called color quantization, and it becomes a very important part of the image-making process. Another method, called dithering, alternates the limited palette colors throughout the image—much like the patterns of dots in a newspaper comic strip—to give the appearance of more colors than are actually in the image.

8. Aliasing and Anti-Aliasing

Since a computer monitor is essentially a grid of colored squares arranged like a sheet of graph paper, diagonal lines tend to be displayed with a jagged “stair step” appearance. This effect, called aliasing, can be lessened by calculating how close each pixel is to the ideal line of the drawn image and then basing the pixel's color on its distance from this line. For example, if the pixel is directly on the line, it may be given the darkest color, and if it is only partially on the line, it may be given a lighter color. This process effectively smoothes the line.

9. USES OF COMPUTER GRAPHICS

As recently as the 1980s, all the things that people built and used were originally designed with the use of rulers and other mechanical tools of the graphic artist. Today things are designed and built electronically with computers. The cars that people drive are designed on a computer screen and tested in computer graphics simulations. Before doctors perform surgery, they use computer graphics in looking at X-ray images of the patient. Photographs used to be stored on film. Now photos can be taken with digital cameras and sent as attachments to e-mail messages. You are most likely reading this text on a computer monitor. Today, almost all the things built and used in the world were originally designed with computer graphics.