Architecture (Несколько текстов для зачёта), страница 16

The first OOP language was Smalltalk, developed by Alan Kay at the Palo Alto Research Center of the Xerox Corporation in the early 1970s. By using objects, Smalltalk allowed programmers to focus on and specify the task to be performed from the top down, rather than laboring on detailed, ground-up procedures, which were embedded in the language structure. Smalltalk, however, has not found widespread use.

The most popular OOP language is C++, developed by Bjarne Stroustrup at Bell Laboratories in the early 1980s. In May 1995 Sun Microsystems, Inc. released Java, a new OOP language, which has drawn worldwide interest. In some ways Java represents a simplified version of C++, but it adds other features and capabilities as well, and is particularly well suited for writing interactive applications to be used on the World Wide Web.

Ada

Ada, in computer science, a procedural programming language designed under the direction of the U.S. Department of Defense (DOD) in the late 1970s and intended to be the primary language for DOD software development. Ada, named after (Augusta) Ada Byron, Countess of Lovelace, who was a pioneer in the field of computers, was derived from Pascal but has major semantic and syntactical extensions, including concurrent execution of tasks, overloading of operators, and modules.

Java (computer)

Java (computer), in computer science, object-oriented programming language introduced in 1995 by Sun Microsystems, Inc. Java facilitates the distribution of both data and small applications programs, called applets, over the Internet. Java applications do not interact directly with a computer’s central processing unit (CPU) or operating system and are therefore platform independent, meaning that they can run on any type of personal computer, workstation, or mainframe computer. This cross-platform capability, referred to as “write once, run everywhere,” has caught the attention of many software developers and users. With Java, software developers can write applications that will run on otherwise incompatible operating systems such as Windows, the Macintosh operating system, OS/2, or UNIX.

To use a Java applet on the World Wide Web (WWW)—the system of software and protocols that allows multimedia documents to be viewed on the Internet—a user must have a Java-compatible browser, such as Navigator from Netscape Communications Corporation, Internet Explorer from Microsoft Corporation, or HotJava from Sun Microsystems. A browser is a software program that allows the user to view text, photographs, graphics, illustrations, and animations on the WWW. Java applets achieve platform independence through the use of a virtual machine, a special program within the browser software that interprets the bytecode—the code that the applet is written in—for the computer’s CPU. The virtual machine is able to translate the platform-independent bytecode into the platform-dependent machine code that a specific computer’s CPU understands.

Applications written in Java are usually embedded in Web pages, or documents, and can be run by clicking on them with a mouse. When an applet is run from a Web page, a copy of the application program is sent to the user’s computer over the Internet and stored in the computer’s main memory. The advantage of this method is that once an applet has been downloaded, it can be interacted with in real time by the user. This is in contrast to other programming languages used to write Web documents and interactive programs, in which the document or program is run from the server computer. The problem with running software from a server is that it generally cannot be run in real time due to limitations in network or modembandwidth—the amount of data that can be transmitted in a certain amount of time.

Java grew out of a research project at Sun Microsystems in the early 1990s that focused on controlling different consumer electronics devices using the same software. The original version of Java, called Oak, needed to be simple enough to function with the modest microprocessors found in such consumer devices. Following the introduction of the National Center for Supercomputing Applications’ (NCSA) Mosaic browser in 1993, Oak was recast by Sun Microsystems developers. In 1994 Sun Microsystems released a Java-compatible Internet browser, called HotJava, that was designed to read and execute Java applets on the WWW. Netscape Communications licensed Java from Sun Microsystems in November 1995, and its Navigator 3.0 browser supports Java applications. Microsoft also licensed Java, in 1996, for its Internet Explorer 3.0 browser. Microsoft developed a programming language, called Visual J++, to integrate Java, through its ActiveX technology, with its browser. Visual J++ is optimized for the Windows operating system. Various other WWW browsers are also capable of supporting Java applications and applets.

JavaSoft, a division of Sun Microsystems with responsibility for Java and its business development, has created JavaOS, a compact operating system for use on its own JavaStation network computers, now in development, as well as, possibly, in cellular telephones and pagers.

Structured Query Language

Structured Query Language (SQL), in computer science, a database sublanguage used in querying, updating, and managing relational databases. Derived from an IBM research project that created Structured English Query Language (SEQUEL) in the 1970s, SQL is an accepted standard in database products. Although it is not a programming language in the same sense as C or Pascal, SQL can either be used in formulating interactive queries or be embedded in an application as instructions for handling data. The SQL standard also contains components for defining, altering, controlling, and securing data. SQL is designed for both technical and nontechnical users.

Hypertext Markup Language (HTML)

Hypertext Markup Language (HTML) in computer science, the standard text-formatting language since 1989 for documents on the interconnected computing network known as the World Wide Web. HTML documents are text files that contain two parts: content that is meant to be rendered on a computer screen; and markup or tags, encoded information that directs the text format on the screen and is generally hidden from the user. HTML is a subset of a broader language called Standard Generalized Markup Language (SGML), which is a system for encoding and formatting documents, whether for output to a computer screen or to paper.

Some tags in an HTML document determine the way certain text, such as titles, will be formatted. Other tags cue the computer to respond to the user's actions on the keyboard or mouse. For instance, the user might click on an icon (a picture that represents a specific command), and that action might call another piece of software to display a graphic, play a recording, or run a short movie. Another important tag is a link, which may contain the Uniform Resource Locator (URL) of another document. The URL can be compared to an address where a particular document resides. The document may be stored on the same computer as the parent document or on any computer connected to the World Wide Web. The user can navigate from document to document simply by clicking on these links. HTML also includes markups for forms, that let the user fill out information and electronically send, or e-mail, the data to the document author, initiate sophisticated searches of information on the Internet, or order goods and services.

The software that permits the user to navigate the World Wide Web and view HTML-encoded documents is called a browser. It interprets the HTML tags in a document and formats the content for screen display. Since HTML is an accepted standard, anyone can build a browser without concerning themselves with what form various documents will assume, unlike documents produced by typical word processors, which must be translated into a different format if another word processing application is used. Most sites on the World Wide Web adhere to HTML standards and, because HTML is easy to use, the World Wide Web has grown rapidly. HTML continues to evolve, however, so browsers must be upgraded regularly to meet the revised standards.

Computer Graphics

Computer Graphics, two- and three-dimensional images created by computers that are used for scientific research, artistic pursuits, and in industries to design, test, and market products. Computer graphics have made computers easier to use. Graphical user interfaces (GUIs) and multimedia systems such as the World Wide Web, the system of interconnected worldwide computer resources, enable computer users to select pictures to execute orders, eliminating the need to memorize complex commands.

Before an image can be displayed on the screen it must be created by a computer program in a special part of the computer's memory, called a frame buffer. 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. A graphical image is created by dividing the computer's display screen into a grid of tiny dots called pixels. Frame buffer memory can also store other information, such as the color of each pixel.

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, then over 16 million color combinations can be represented. But in the creation of a large image, allowing so many combinations can be very costly in terms of memory and processing time. An alternate method, 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 for the palette, called color quantization, 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.

Bézier Curve: Computer Graphics Tool

A Bézier curve is used in computer graphics programs to illustrate numerous shapes. Graphic artists move the handles to create many different shapes, while the anchor points remain stationary.

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 smooths the line.

Image processing is among the most powerful and important tools in computer graphics. Its underlying techniques are used for many applications, such as detecting the edge of objects; enhancing images and removing noise in medical imaging; and blurring, sharpening, and brightening images in feature films and commercials.

Image warping lets the user manipulate and deform an image over time. The most popular use of image warping is morphing, in which one image deforms and dissolves into another. Morphing is different from similar processes, in which one image simply fades into another, because the actual structures of the original image change. To morph an image, the user specifies corresponding points on the original and final objects that the computer then distorts until one image becomes the other. These transformation points are usually either a grid overlaid on each object or a specific set of features, such as the nose, eyes, and ears of two faces to be morphed.

Many uses of computer graphics, such as computer animation, computer-aided design and manufacturing (CAD/CAM), video games, and scientific visualization of data such as magnetic resonance images of internal organs, require drawing three-dimensional (3D) objects on the computer screen. The drawing of 3D scenes, called rendering, is usually accomplished using a pipeline or assembly-line approach, in which several program instructions can, at any given time, be executed in various stages on different data.

This graphics pipeline is implemented either with special-purpose 3D graphics microprocessors (hardware) or with computer programs (software). Hardware rendering can be expensive, but it enables the user to draw up to 60 images per second and to make immediate changes to the image. Software renderers are very slow, requiring from a few hours to a full day to render a single image. However, computer animation almost always uses software renderers because they provide greater control of the images and potentially photo-realistic quality.

The first step in a rendering pipeline is the creation of 3D objects. The surface of an object, such as a sphere, is represented either as a series of curved surfaces or as polygons, usually triangles. The points on the surface of the object, called vertices, are represented in the computer by their spatial coordinates. Other characteristics of the model, such as the color of each vertex and the direction perpendicular to the surface at each vertex, called the normal, also must be specified. Since polygons do not create smooth surfaces, detailed models require an extremely large number of polygons to create an image that looks natural.