Architecture (Несколько текстов для зачёта), страница 17

Another technique used to create smooth surfaces relies on a parametric surface, a two-dimensional (2D) surface existing in three dimensions. For example, a world globe can be considered a 2D surface with latitude and longitude coordinates representing it in three dimensions. More complex surfaces, such as knots, can be specified in a similar manner.

Once these models have been created, they are placed in a computer-generated background. For example, a rendered sphere might be set against a backdrop of clouds. User instructions specify the object's size and orientation. Then the colors, their locations, and the direction of light within the computer-generated scene, as well as the location and direction of the viewing angle of the scene, are selected.

At this point, the computer program generally breaks up complex geometric objects into simple “primitives,” such as triangles. Next, the renderer determines where each primitive will appear on the screen by using the information about the viewing position and the location of each object in the scene.

Once a primitive has been located, it must be shaded. Shading information is calculated for each vertex based on the location and color of the light in the computer-generated scene, the orientation of each surface, the color and other surface properties of the object at that vertex, and possible atmospheric effects that surround the object, such as fog.

Graphics hardware most commonly uses Gouraud shading, which calculates the lighting at the vertices of the primitive, and interpolates, or blends, colors across the surface to make the object appear more realistic. Phong shading represents highlights by blending the lighting and colors in a direction perpendicular to the surface at each vertex, the normal, and calculating the lighting at each pixel. This provides a better approximation of the surface but requires more calculation.

Several techniques permit the artist to add realistic details to models with simple shapes. The most common method is texture mapping, which maps or applies an image to an object's surface like wallpaper. For example, a brick pattern could be applied to a rendered sphere. In this process only the object's shape, not features of the texture, such as the rectangular edges and grout lines of the brick, affect the way the object looks in lighting; the sphere still appears smooth. Another technique, called bump mapping, provides a more realistic view by creating highlights to make the surface appear more complex. In the example of the brick texture, bump mapping might provide shadowing in the grout lines and highlights upon some brick surfaces. Bump mapping does not affect the look of the image's silhouette, which remains the same as the basic shape of the model. Displacement mapping addresses this problem by physically offsetting the actual surface according to a displacement map. For example, the brick texture applied to the sphere would extend to the sphere's silhouette, giving it an uneven texture.

Once the shading process has produced a color for each pixel in a primitive, the final step in rendering is to write that color into the frame buffer. Frequently, a technique called Z buffering is used to determine which primitive is closest to the viewing location and angle of the scene, ensuring that objects hidden behind others will not be drawn. Finally, if the surface being drawn is semitransparent, the front object's color is blended with that of the object behind it.

Because the rendering pipeline has little to do with the way light actually behaves in a scene, it does not work well with shadows and reflections. Another common rendering technique, ray tracing, calculates the path that light rays take through the scene, starting with the viewing angle and location and calculating back to the light source. Ray tracing provides more accurate shadows than other methods and also handles multiple reflections correctly. Although it takes a long time to render a scene using ray tracing, it can create stunning images.

In spite of its generally accurate portrayal of shadows and reflections, ray tracing calculates only the main direction of reflection, while real surfaces scatter light in many directions. This scattered-light phenomenon can be modeled with global illumination, which uses the lighting of the image as a whole rather than calculating illumination on each individual primitive.

Many scientific applications of computer graphics require viewing 3D volumes of data on a 2D computer screen. This is accomplished through techniques that make the volume appear semitransparent and use ray tracing through the volume to illuminate it.

Floppy Disk

Floppy Disk, in computer science, a round, flat piece of Mylar coated with ferric oxide, a rustlike substance containing tiny particles capable of holding a magnetic field, and encased in a protective plastic cover, the disk jacket. Data is stored on a floppy disk by the disk drive's read/write head, which alters the magnetic orientation of the particles. Orientation in one direction represents binary 1; orientation in the other, binary 0. Typically, a floppy disk is 5.25 inches in diameter, with a large hole in the center that fits around the spindle in the disk drive. Depending on its capacity, such a disk can hold from a few hundred thousand to over one million bytes of data. A 3.5-inch disk encased in rigid plastic is usually called a microfloppy disk but can also be called a floppy disk. See also Computer.

Microfloppy Disk

Microfloppy Disk, in computer science, a 3.5-inch floppy disk of the type used with the Apple Macintosh and with the IBM PS/1 and PS/2 and some compatible computers. A microfloppy disk is a round piece of Mylar coated with ferric oxide and encased in a rigid plastic shell. On the Macintosh, a single-sided microfloppy disk can hold 400 kilobytes (KB); a double-sided (standard) disk can hold 800 KB; and a double-sided high-density disk can hold 1.44 megabytes (MB). On IBM and compatible machines with 3.5-inch disk drives, a microfloppy can hold either 720 KB or 1.44 MB of information.

Disk

Disk, in computer science, a round, flat piece of flexible plastic (floppy disk) or inflexible metal (hard disk) coated with a magnetic material that can be electrically influenced to hold information recorded in digital (binary) form. A disk is, in most computers, the primary means of storing data on a permanent or semipermanent basis. Because the magnetic coating of the disk must be protected from damage or contamination, a floppy (5.25-inch) disk or microfloppy (3.5-inch) disk is encased in a protective plastic jacket. A hard disk, which is very finely machined, is enclosed in a rigid case and can be exposed only in a dust-free environment. Standard floppy and microfloppy disk designs are being phased out in favor of compact discs (CD-ROMs), and new microfloppy designs with upward of one hundred times the storage capacity of their older counterparts. CD-ROMs, however, cannot have information recorded upon them, and recordable compact discs (CD-Rs) remain too expensive for general use. A hybrid of the optical storage techniques used in CD-ROMs and the magnetic storage capabilities of floppy disks, called a floptical or magneto-optical disk, provides large storage capacities and the option to read and write information upon them.

Hard Disk

Hard Disk, in computer science, one or more inflexible platters coated with material that allows the magnetic recording of computer data. A typical hard disk rotates at 3600 RPM (revolutions per minute), and the read/write heads ride over the surface of the disk on a cushion of air 10 to 25 millionths of an inch deep. A hard disk is sealed to prevent contaminants from interfering with the close head-to-disk tolerances. Hard disks provide faster access to data than floppy disks and are capable of storing much more information. Because platters are rigid, they can be stacked so that one hard-disk drive can access more than one platter. Most hard disks have from two to eight platters.

Hard Disk

Hard disks are used to record computer data magnetically. A hard disk drive consists of a stack of inflexible magnetic disks mounted on a motor. As the disks spin at high speeds, read/write heads at the end of a metal fork swing in and out to access sectors of the disks.

Write Protect

Write Protect, in computer science, to prevent the writing (recording) of information, usually on a disk. Write protection can be applied (not necessarily infallibly) either to a floppy disk or to an individual file on a floppy or hard disk. Covering the write-protect notch on a 5.25-inch floppy disk enables programs to read, but not record on, the disk. Moving the slide to open the “notch” on a 3.5-inch disk provides the same protection. Individual files can also be made “read-only” through software commands; read-only files, like protected disks, can be read but not written to.

Double-Density Disk

Double-Density Disk, in computer science, a disk created to hold data at twice the density (bits per inch) of a previous generation of disks. Early IBM PC floppy disks held 180 kilobytes (KB) of data. Double-density disks increased that capacity to 360 KB. Double-density disks use modified frequency modulation encoding for storing data.

Disk Drive

Disk Drive, in computer science, a device that reads or writes data, or both, on a disk medium. The disk medium may be either magnetic, as with floppy disks or hard disks; optical, as with CD-ROM (compact disc-read only memory) disks; or a combination of the two, as with magneto-optical disks. Nearly all computers come equipped with drives for these types of disks, and the drives are usually inside the computer, but may also be connected as external, or peripheral, devices.

The main components of a disk drive are the motor, which rotates the disk; the read-write mechanism; and the logic board, which receives commands from the operating system to place or retrieve information on the disk. To read or write information to a disk, drives use various methods. Floppy and hard drives use a small magnetic head to magnetize portions of the disk surface, CD-ROM and WORM (Write-Once-Read-Many) drives use lasers to read information, and magneto-optical drives use a combination of magnetic and optical techniques to store and retrieve information.

Floppy and hard disk drives store information on magnetic disks. The disk itself is a thin, flexible piece of plastic with tiny magnetic particles imbedded in its surface. To write data to the disk, the read-write head creates a small magnetic field that aligns the magnetic poles of the particles on the surface of the disk directly beneath the head. Particles aligned in one direction represent a 0 while particles aligned in the opposite direction represent a 1. To read data from a disk, the drive head scans the surface of the disk. The magnetic fields of the particles in the disk induce an alternating electric current in the read-write head, which is then translated into the series of 1s and 0s that the computer understands.

Unlike hard or floppy disks, most CD-ROM drives are unable to write data to the CD. Data is initially written to CD-ROM discs by burning microscopic pits into the disc's reflective surface with a laser. To read the information contained on the disc, the drive shines a low-power laser beam onto the surface. When the laser light hits flat spots on the reflective surface of the CD, it bounces back to a photo detector, which records the impulse as a 0. When the laser light hits pits in the surface, it does not reflect light back to the photo detector, and this absence of light corresponds to a 1. Most CD-ROM drives are only capable of reading data and cannot write data to the CD. WORM drives, however, are able to both etch blank CDs and to read data from them.

Magneto-optical (MO) drives combine optical and magnetic technology to read from and write to disks that have the appearance of CD-ROMs in plastic, floppy-disk cases. MO drives can rewrite the MO disks without limitation just as magnetic drives rewrite magnetic media. Although more expensive than standard magnetic or optical drives, MO drives combine speed, large capacity, and high durability of data. see Computer Memory.

CD-ROM

CD-ROM, in computer science, acronym for compact disc read-only memory, a rigid plastic disk that stores a large amount of data through the use of laser optics technology. Because they store data optically, CD-ROMs have a much higher memory capacity than computer disks that store data magnetically. However, CD-ROM drives, the devices used to access information on CD-ROMs, can only read information from the disc, not write to it.

The underside of the plastic CD-ROM disk is coated with a very thin layer of aluminum that reflects light. Data is written to the CD-ROM by burning microscopic pits into the reflective surface of the disk with a powerful laser. The data is in digital form, with pits representing a value of 1 and flat spots, called land, representing a value of 0. Once data is written to a CD-ROM, it cannot be erased or changed, and this is the reason it is termed read-only memory. Data is read from a CD-ROM with a low power laser contained in the drive that bounces light—usually infrared—off of the reflective surface of the disk and back to a photodetector. The pits in the reflective layer of the disk scatter light, while the land portions of the disk reflect the laser light efficiently to the photodetector. The photodetector then converts these light and dark spots to electrical impulses corresponding to 1s and 0s. Electronics and software interpret this data and accurately access the information contained on the CD-ROM.

CD-ROMs can store large amounts of data and so are popular for storing databases and multimedia material. The most common format of CD-ROM holds approximately 630 megabytes. By comparison, a regular floppy disk holds approximately 1.44 megabytes.

CD-ROMs and Audio CDs are almost exactly alike in structure and data format. The difference between the two lies in the device used to read the data—either a CD-ROM player or a compact disc (CD) player. CD-ROM players are used almost exclusively as computer components or peripherals. They may be either internal (indicating they fit into a computer’s housing) or external (indicating they have their own housing and are connected to the computer via an external port).