1.5 A graphics system

Graphics system is any collection of hardware and software designed to make it easier to use graphics input and output in computer programs.

A computer graphics system is a computer system; as such it must have all the components of a general-purpose computer system which includes five major elements namely:

	Processor.
	Memory.
	Frame buffer.
	Output devices.
	Input devices.

Raster displays are always connected to some region of memory sufficiently large to hold all of the pixel values for the display called Frame Buffer.

1.5 A graphics system

Graphics Software

There are two broad classifications for computer-graphics software:

	Special-purpose packages
	General programming packages.

Special-purpose packages are designed for nonprogrammers who want to generate pictures, graphs, or charts in some application area without worrying about the graphics procedures that might be needed to produce such displays.

Examples of such applications include artist's painting programs and various architectural, business, medical, and engineering CAD systems.

By contrast, a general programming package C, C++, or Java. Basic functions in a typical graphics library include those for specifying picture components (lines, polygons, spheres, and other objects), setting color values, selecting views of a scene, and applying rotations or other transformations.

1.5 A graphics system

Some examples of general graphics programming packages are GL (Graphics Library), OpenGL, VRML (Virtual-Reality Modeling language), Java 2D and Java 3D. A set of graphics functions is often called a computer-graphics application programming interface (CG API) because the library provides a software interface between a programming language (such as C++) and the hardware.

Graphics Hardware (Graphics display devices)

The most common hardware devices that are used to display computer graphics include:

	Video monitor.
	Plotters.
	Film recorder.
	Printers.

1.5 A graphics system

Video Monitor (Raster displays)

Most displays used for computer graphics nowadays are raster displays. Raster devices have a display surface on which the image is presented. This surface has a certain number of pixels that it can show, such as 480 rows, each containing 640 pixels.

The most famous raster displays are the video monitor. Typically, the primary output device in a graphics system is a video monitor. The operation of most video monitor is based on the standard cathode-ray tube (CRT) design.

CRT is the most common means for making CG visible. You may be used to calling this device a monitor. CRT is the "screen" part, where you see the picture. It is one of the few types of vacuum tubes still in common use today. It works on the principle that some materials, called phosphors, emit light if you crash enough electrons into them. A spot on the phosphor lights up more brightly as more electrons hit it.

CRT scan rate or refresh rate refers to how fast the electron beams are swept across the screen. CRT consists of (see figure 1.16):

Electron gun which produce electron beam.

1.5 A graphics system

The color CRT monitors displays color pictures by using a combination of phosphors that emit different colored. Three electron guns are exists in this case, one for each of the three main colors: red, green, and blue (RGB) as shown in figure 1.17. All raster displays have a built-in coordinate system that associates a given pixel in an image with a given physical position on the display surface.

Figure 1.17: Color CRT Diagram

1.5 A graphics system

The horizontal coordinate (sx) increases from left to right, and the vertical coordinate (sy) increases from top to bottom. This "upside-down" coordinate system is typical of raster devices (see figure 1.18). The scanning process for sending each pixel value in the frame buffer to the right place on the display surface is shown in figure 1.19(a)-(b) respectively.

Figure 1.18: The built-in coordinate system for the surface of a raster display

1.5 A graphics system

Hard-Copy Raster Devices

One frequently wants a permanent version of an image, usually on paper or film. A number of raster devices produce hard copy of a raster image.

Each draws pictures by transferring information from the frame buffer, dot by dot, to the display medium.

	Printers are used to create hard-copy output on paper or overhead transparencies. Products are available in a wide quality/price range. Examples are Inkjet, laser printers, Wax Transfer, and others.
	Film recorder: in a film recorder, the screen is a strip of photographic film, and the electron beam exposes the film as it sweeps over it in a raster pattern. Film recorders are frequently used to make movies or high-quality 35-mm slides.
	Inkjet plotters produce hard-copy raster images in color.

Graphics Input Primitives and Devices

Many input devices let the user control the computer. Most graphics systems provide a keyboard and at least one other input device. You can look at an input device in two ways: what it is, and what it does. Physically, each device is some piece of machinery, such as a mouse, or keyboard.

1.5 A graphics system

The device fits in the hand in a certain way and is natural for the user to manipulate. The device measures the user's manipulations and sends corresponding numerical information back to the graphics program. We first look at what input devices do, by examining the kinds of data each sends to the program. We then look at a number of input devices in common use today. Types of input graphics primitives

	Each device transmits particular kind of data (e.g., a number, a string of characters, or a position) to the program.
	The different types of data are called input primitives.
	The important input primitives are as follows:

		String: it is the most familiar input primitive, producing a string of characters and thus modeling the action of a keyboard. When an application requests a string, the program pauses while the user types in the string, followed by a terminator character.
		Choice: a choice reports a selection from a fixed number of items. The programmer's model is a bank of buttons, or a set of buttons on a mouse.
		Valuator: a valuator produces a real value between 0.0 and 0.1, which can be used to fix the length of a line, the speed of an action, or the size of a picture.

1.5 A graphics system

		Locator: a basic requirement in interactive graphics is to allow the user to point to a position on the display. The locator input primitiveperforms this function, because it produces a coordinate pair (x,y).
		Pick: the pick input primitive is used to identify a portion of a picture for further processing.

Types of physical input devices

The most common input devices are the mouse, the joystick, and the data tablet. Each provides positional information to the system, and each usually is equipped with one or more buttons to provide signals to the processor.

	Keyboard: An alphanumeric keyboard on a graphics system is used primarily as a device for entering text string, issuing certain commands, and selecting menu options. The keyboard is an efficient device for inputting such nongraphic data as picture labels associated with a graphics display.
	Mouse: Mouse is the most familiar input device of all, because it is easy and comfortable to operate. Douglas Engelbart invented the mouse in 1968.

1.5 A graphics system

Joystick and Trackball: Figure 1.20 shows two similar input devices that control the position of a cursor on nthedisplay. The joystick in part (a) consists of a small, vertical lever (called the stick) mounted on a base. We use the joystick to steer the screen cursor around. The trackball in part (b) is a ball device that can be rotated with the fingers or palm of the hand to produce screen-cursor movement.

Figure 1.20: Joystick and Trackball

1.5 A graphics system

Digitizer: A common device for drawing, painting, or interactively selecting positions is a digitizer. These devices can be designed to input coordinate valuesin either a two-dimensional or a three-dimensional space. One type of digitizer is the graphic tablet (also referred to as a data tablet), which is used to input two-dimensional coordinates by activating a hand cursor or stylus at selected positions on a flat surface (see figure 1.22).

Figure 1.22: A graphics tablet

1.5 A graphics system

	Image scanner: Drawings, graphs, photographs, or text can be stored for computer processing with an image scanner by passing an optical scanning mechanism over the information to be stored.
	Light pens: Such pencil-shaped devices are used to select screen positions by detecting the light coming from points on the CRT. (See figure 1.23).

Figure 1.23: A Light pen with a button activator

1.5 A graphics system

Voice Systems: The voice system input can be used to initiate graphics operations or to enter data. These systems operate by matching an input against a predefined dictionary of words and phrases (See figure 1.24).

Figure 1.24: A graphics tablet

1.5 A graphics system

Touch Panels: As the name implies, touch panels allow displayed objects or screen positions to be selected with the touch of a finger. A typical application of touch panels is for the selection of processing options that are represented as a menu of graphical icons.

	Deflection system (pairs of horizontal and vertical).
	Focusing system: direct the beam toward specified point on the screen.
	Phosphor-coated screen.