Display Types( 2.2)

1.                                            LED (Light Emitting Diode)
2. It is a emissive device where a matrix of diodes is arranged to form the pixel positions in the display and picture definition is stored in refresh buffer. 
3. As in scan line refreshing of a CRT, information is read from refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

LCD (Liquid Crystal Display)

1. It is non-emissive devices which produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid crystal materials that are aligned to either block or transmit the light fully or partially.
2. It has a crystalline arrangement of molecules, which flow like a liquid. 
3. A flat panel display commonly use nematic liquid crystal compound that tend to keep the long axes of the rod shaped molecules aligned. 
4. Two glass plates each containing a light polarizer at right angles to the other plate, sandwich the liquid crystal material.
5. Rows of horizontal transparent conductors are built into one glass plate and columns of vertical conductors are put into the other plate.
6. The intersection of two conductors defines a pixel position.

Passive matrix LCD:

1. Polarized light passing through the material is twisted so that it will pass through the opposite polarizer. The light is then reflected back to the viewer. To turn off the pixel, we apply a voltage to the two intersecting conductors to align the molecules so that the light is not twisted. This type of flat-panel device is referred to as a passive-matrix LCD.

2.  Picture definitions are stored in a refresh buffer, and the screen is refreshed at the rate of 60 frames per second, as in the emissive devices. Back lighting is also commonly applied using solid-state electronic devices, so that the system is not completely dependent on outside light sources. Colors can be displayed by using different materials or dyes and by placing triad of color pixels at each screen location.

3. Active matrix LCD:Transistor is palced at each pixel location using thin flim transistor technology. The transistors are used to control the voltage at pixel locations and to prevent charge from gradually leaking out of the liquid crystal cells. These devices are called active matrix displays.

Software Standards

1. With standard graphics functions software can be moved easily from one hardware system to another and used in different implementations and applications.
2. If we donot have standard programs for graphics, designed for one hardware system often cannot be transferred to another system without extensive rewriting of the programs.Types of software standards for graphics:GKS (Graphical Kernel System)PHIGS (Programmer’s Hierarchical 

• GKS (Graphical Kernel System):

1. This was the first graphics software standard developed by ISO, ANSI and various national standards organizations. 

2. GKS was originally designed for 2D graphics package and later on 3D GKS extension was subsequently developed.

(Programmer’s Hierarchical Interactive Graphics Standard): 

1. It is an extension of GKS with increased capabilities for object modeling, color specifications, surface rendering and picture manipulation.
2. The extension of PHIGS called PHIGS+ was developed to provide 3D surface shading capabilities which was not available in PHIGS.
3. Standard graphics functions are defined as a set of specifications that is independent of any programming language. 
4. PHIGS (Programmer’s Hierarchical Interactive Graphics Standard): 
5. A language binding is then defined for a particular high level programming language which allow for accessing various standard graphics functions from this language. 
6. For example: the general form of the PHIGS and GKS function for specifying a sequence of n-1 connected 2D straight line segment is : polyline(n,x,y). In FORTRAN this procedure is implemented as a subroutine with the name GPL. 
7. In C the procedure would be invoked with ppolyline(n,pts) where pts is the list of coordinate endpoint positions.

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8. PHIGS only provide a specification for basic graphics functions, it doesnot provide a standard methology for a graphics interface to output devices.
9. Computer Graphics Interface (CGI) is a standardization for device interface methods.
10. Computer Graphics Metafile (CGM) system specifies standards for archiving and transporting pictures.

PHIGS Workstations: 

1. The term workstation refers to a computer system with a combination of input and output devices that is designated for a single user. 
2. PHIGS and GKS workstation is used to identify various combinations of graphics hardware and software. 
3. A PHIGS workstation can be a single output device, a single input device, a combination of input and output devices, a file or even a window displayed on a video moniter

       Raster Scan Systems
Video Controller

1. A special purpose processor called video controller or display controller is used to control the operation of the display device. 
2. In addition to the video controller most sophisticated raster system employ other processors as coprocessors and accelerators to implement various graphics operations.
3. In a raster system, a fixed area of the system memory is reserved for the frame buffer and the video controller is given direct access to the frame buffer memory. Frame buffer locations and screen positions are referenced in Cartesian coordinates. 
4. In most of the graphics monitors the coordinates origin from lower left screen corner. The screen surface is then represented at the first quadrant of 2D system with +ve x values increasing to the right and positive y values increasing from bottom to top. 
5. Scan line are labelled from ymax at the top to 0 at the bottom, each scan line screen pixel positions are labelled from 0 to xmax.
6. In video controller, two registers (x register and y register) are used to store the coordinates of the screen pixels where initially, x register is set to 0 and y register is set to ymax.
7. The value stored in the frame buffer for this pixel position is then retrieved and used to set  the intensity of the CRT beam. Then the x-register is incremented by 1 and the process repeated for the next pixel position on the top of the scan line. 
8. After the last pixel on the top of the scan line has been processed, the x-register is reset to 0 and the register y is decremented by 1. After cycling through all pixels along last scan line (y=0) the video controller resets the registers to the first pixel position on the top scan line and the refresh process start over.
9. To speed up the pixel processing, video controller can retrieve multiple pixel values from the refresh buffer on each pass and multiple intensities values are stored in separate register and used to control the CRT beam intensity for a group of adjacent pixels. When that group of pixels has been processed the next block of pixel values is retrieved from the frame buffer.

Raster Scan Display Processor:

1. It is also referred to as graphics controller or display coprocessor. It’s main task is to free the CPU from the graphics routine rasks.
2. It’s major task is to digitizing a picture definition given in an application program into a set of pixel intensity values for storage in the frame buffer. The digitization process is called scan conversion.
3. It can also be used for generating various lines sytles (dashed, dotted, solid), displaying color areas and performing certain transformations and manipulations on displayed objects. As well as to interface with interactive input devices such as a mouse.
4. There are three common methods for storing raster data:

1. Cell encoding

2. Run-length encoding

3. Quad tree

5. To reduce the memory requirements in raster system, frame buffer has been organized in linked list and encoding the intensity information. For this, one way is to store each scan line as a set of integer pairs, where one number repesents the intensity value and the second number represents the number of adjacent pixels on the scan line that are to have that intensity. This method is called run length encoding. This method help to save the storage space if a picture is to be constructed with long runs of a single color each.
6. Another method is cell encoding which encode the raster as a set of rectangular areas i.e. raster is stored as a matrix and its cell values are written into a file by row and column.
7. The disadvantages of encoding runs are :
1. Intensity changes are difficult to make and storage requirements actually increase as the length of the runs decreases. 
2. It is difficult for display controller to process the raster when many short runs are involved.

Random Scan Systems

1. with In this system, an application program is input and stored in the system memory along a graphics package. Graphics commands in the application program are translated by the graphics package into a display file stored in the system memory. 
2. This display file is then accessed by the display processor to refresh the screen. 
3. The display processor cycles through each command in the display file program once during every refresh cycle.
4. Graphics patterns drawn on this system by directing electron bema along the component lines of the pictures.
5. Lines are defined by the values of their coordinate endpoints and these input coordinate values are converted to x and y deflection voltage. 
6. A scene is then drawn one line at a time by positioning the beam to fill in the line between specified endpoints.