Various Input Devices

Various devices are available for data input on graphics workstations. Most systems have a keyboard and one or more additional devices specially designed for interactive input.

An alphanumeric keyboard on a graphics system is used primarily as a device for entering text strings. The keyboard is an efficient device for inputting such non-graphic data as picture labels associated with a graphics display. Keyboards can also be provided with features to facilitate entry of screen co-ordinates, menu selections or graphics functions.

Cursor-control keys and functions keys are common features of general purpose keyboards. Function keys allow users to enter frequently used operations in a single keystroke, and cursor control keys can be used to select display objects or co-ordinate positioning devices. Additionally, a numeric keyboard is often included on the keyboard, for fast entry of numeric data.

A mouse is a small hand-held device used to position the screen cursor. Wheels or rollers on the bottom of the mouse can be used to record the amount and direction of movement. Another method of detecting mouse motion is with an optical sensor. One, two or three buttons are usually included on top of the mouse for signaling the execution of some operation.

The Z-mouse includes three buttons, a thumbwheel on the side, a trackball on the top, and a standard mouse ball underneath. This design provides six degrees of freedom to select the object and to perform the actions.

As the name implies, a trackball is a ball that can be rotated with the finger or palm of the hand, to produce screen-cursor movement. Potentiometers attached to the ball, measure the amount and direction of rotation. Track balls are often mounted on keyboards. While a trackball is a two-dimensional positioning device, a Spaceball provides six degrees of freedom. The Spaceball does not move. Strain gauges measure the amount of pressure applied to the Spaceball to provide input for spatial positioning and orientation as the ball is pushed or pulled in various directions. Spaceballs are used for 3D positioning and selection generations in virtual reality systems.

A joystick consists of a small, vertical lever (called the stick) mounted on a based that is used to steer the screen cursor around. The distance that the stick is moved in any direction from its center position corresponds to screen-cursor movement in that direction. Potentiometers mounted at the base of the joystick measure the amount of the movement, and springs return the stick to the centre position when it is released. One or more buttons can be programmed to act as input switches.

Data glove can be used to grasp a ???virtual??? object. The glove is constructed with a series of sensors that detect hand and finger motions. Electromagnetic coupling between transmitting antennas and receiving antennas is used to provide information about the position and orientation of the hand. Input from a glove can be used to position or manipulate objects in a virtual scene.

A common device for drawing, partitioning or interactively selecting co-ordinates positions on an object is digitizer. These devices can be used to input co-ordinate values in either a 2D or 3D space. One type of digitizer is the graphics tablet which is used to input 2D co-ordinates by activating a hand cursor at selected positions on a flat surface. A hand cursor contains cross hairs for sighting positions, while a stylus is a pencil shaped device that is pointed at positions on the tablet.

Many graphics tablets are constructed with a rectangular grid of wires embedded in the tablet surface. Electromagnetic pulses are generated in sequence along the wires, and an electric signal is induced in a wire coil in an activated stylus or hand cursor to record a tablet position.

Acoustic (or Sonic) tablets use sound waves to detect the stylus position. 3D digitizers use sonic or electromagnetic transmissions to record positions.

Drawings, graph, photos or text can be stored for computer processing with an image scanner by passing an optical scanning mechanism over the information to be stored. The objects are stored in and array, these representations of a picture, can be transformed like rotation, scaling and enhancements.

It allows displayed objects or screen positions to be selected with the touch of a finger. Touch input can be recorded using optical, electrical or acoustical methods. Optical touch panels employ a line of infrared Light-Emitting Diodes (LED) along one vertical edge and along one horizontal edge of the frame. The opposite vertical and horizontal edges contain light detectors. These detectors are used to record which beams are interpreted when the panel is touched. The two crossing beams that are interpreted identify the horizontal and vertical co-ordinates of the screen position selected.

Light pens are pencil shaped devices are used to select screen positions by detecting the light coming from points on the CRT screen. They are sensitive to short bust of light emitted from the phosphor coating; at the instant the electron beam strikes a particular point. Other light sources are not usually detected by a light pen.

An activated light pen, pointed at a spot on the screen as the electron beam lights up the spot, generates on electrical pulse that causes the co-ordinate position of the electron beam to be recorded. As with cursor-positioning devices, recorded light pen co-ordinates can be used to position an object or to select a procession option.

Speech recognizers are used in some graphics workstations as input devices to accept voice commands. The voice-system input can be used to initialize graphics operations to enter data. These systems operate by matching an input against a predefined dictionary of words and phrases. A dictionary is setup for the operator; each word is spoken several times and stored. When a voice command is given, the system searches the dictionary for a match.

The graphics can be obtained as hard-copy output in several formats. The images can be converted as 35-mm slides, photographs or it can be taken as output by a printer or plotter. The quality of the pictures obtained from a device depends on dot size and the number of dots per inch (dpi), or lines per inch (lpi), that can be displayed.

Printers produce output by either impact or non-impact methods. Impact printers??? press formed character faces against an inked ribbon onto the paper. A line printer is an example of an impact device, with the typefaces mounted on bands, chains, drums, or wheels. Non-impact printers and plotters use laser techniques to get images onto paper.

Character impact printers often have a dot-matrix print head containing a rectangular array of protruding wire pins, with the number of pins depending on the quality of the printer. Individual characters or graphics patterns are obtained by retracting certain pins so that the remaining pins from the pattern to be printed.

In a laser device, a laser beam creates a charge distribution on a rotating drum coated with photoelectrical methods such as selenium. Toner is applied to the drum and then transformed to paper. Ink-jet methods produce output by squirting ink in horizontal rows across a roll of paper wrapped on a drum. The electrically charged ink stream is deflected by an electric field to produce dot-matrix patterns.

We can get limited color output on an impact printer by using different colored ribbons. Non-impact printers use various techniques to combine three color pigments (cyan, magenta and yellow) to produce a range of color patterns. Laser and xerographic devices deposit the three pigments on separate passes; ink-jet methods shoot the three colors simultaneously on a single pass along each print line on the paper.

Drafting layouts and other drawings are typically generated with ink-jet or pen plotters. A pen plotter has one or more pens mounted on a carriage, or crossbar, that spans a sheet of paper. Pens with varying colors and widths are used to produce a variety of shadings and line style. Plotter paper can lie flat or be rolled onto a drum or belt. Either, clamps, a vacuum, or an electrostatic charge hold the paper in position.

• Overview Of Graphics System

  1. Applications of computer graphics

  2. Video Display Devices

  3. Architecture and components of Raster-Scan Systems & Random-Scan Systems

  4. Various Input Devices

• Basic Drawing Techniques

  1. DDA Line Algorithm 

  2. Midpoint Circle Algorithm 

  3. Clipping & Point Clipping