COMPUTER GRAPHICS (XCS 354) UNIT I

1. Explain refresh cathode ray tube? 

• A beam of electrons, emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor coated area. 
•  The phosphor that emits a small spot of light at each position contacted by the electron beam 
• One way to keep phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points 
• This type of display is called is called refresh CRT 

Primary components of electron gun are: 

1. Heated metal cathode Heat is supplied to the cathode by directing the beam through a coil of wire called the filament inside the cylindrical cathode structure. 

2. Control grid Intensity of the electron beam is controlled by setting the voltage levels on the control grid, which is a metal cylinder that fits to the cathode. 

• Different kinds of phosphorus are available 
• Besides color the major difference between phosphors is their persistence 
• The maximum number of points that can be displayed without overlap on a CRT is referred to as resolution. 
• Another property is aspect ratio 

2. Explain color CRT monitors? 

A CRT monitor displays color pictures by using a combination of phosphors that emit different colored light. 

By combining the emitted light from the different phosphors, a range of colors can be generated 

Two techniques 

Beam penetration method 

Shadow mask method 

Beam penetration method 

• Display color pictures 
• Two layers of phosphors, usually red and green 
• A beam of slow electrons excites the outer red layer 
• Fast electron penetrates through the red layer and excites the inner green layer 
• An intermediate beam speeds, combinations of red and green light 

Shadow mask method 

• They produce a wider range of colors 
• Has three phosphor color dots at each pixel position 
• One emits red light, another emits green light, and the third emits a blue light 
• The three beams are deflected and focused as a group on to the shadow mask, which contains a series of holes aligned with a phosphor dot patterns. 
• When the three beams passes through a hole in the shadow mask, they activate a dot triangle, which 
• appears as a small color spot on the screen. 

3. Explain direct view storage tubes and liquid crystal displays? 

Liquid crystal displays 

• Refers to the compounds having crystalline arrangement of molecules flow of liquid 

• Two plates each glass plate contains a light polarized that are right angles to each other 

Two types 

Passive matrix LCD 

Active matrix LCD 

Direct view storage tubes 

• Alternative method for maintaining a screen image 

• Stores picture information as a charge distribution 

• Very complex pictures can be displayed at very high resolutions 

• To eliminate the picture section the entire screen must be erased 

• The erasing and redrawing process can take several seconds for a picture 

4.Explain Raster scan systems? 

Several processing units 

Contains a special purpose processor, called video controller or display controller 

Video controller 

• A fixed area of the system memory is reserved for the frame buffer, and the video controller is given access to the frame-buffer memory 

• Tow registers are used to store the coordinates of the screen pixels 

• The value is stored in the frame buffer for this pixel position is then retrieved and used to set the intensity of the CRT beam 

• Initially the x register is set to 0 and the y register is set to ymax 

• Then the x register is incremented by 1, and the process repeated for the next pixel on the top scan line 

• This process is repeated for each pixel along the scan line 

• After the last pixels on the top scan line has been processed, the x register is reset to 0 and the yregister is decremented by 1. 

• The procedure is repeated fro each successive line 

. Frame buffer locations and the corresponding screen positions are referenced as Cartesian coordinates 

CPU 

System Frame 

Memory Buffer 

I/O device 

System Bus 

Video 

controller 

5. Explain the following? 

1. Z-mouse 

2. Joysticks 

3. Touch panels 

4. Image scanners 

5. Data glove 

Z-Mouse: 

• Include three buttons 
•  A thumb wheel on the side, a track ball on the top, and a standard mouse ball underneath. 
•  This design provides six degrees of freedom to select an object from the spatial position. 
• Wit this we can pickup an object, rotate it and we can move it in any direction 
• Used in virtual reality and CAD systems 

Joysticks 

• Consists of small vertical liver mounted on a base 

• Used to move the cursor around the screen 

• The screen cursor is moved according to the distance 

• One or two buttons is usually intended for signaling certain actions 

Touch panels 

• Three types 

• Optical touch panel

• Electrical touch panel 

• Acoustical touch panel 

• Allow selecting the screen position with the touch of finger. Touch input can be recorded using optical, electrical methods

 Image scanners 

•  Drawings, color and black and white photos or text can be given as an input to the computer with an optical scanning mechanism. 
• According to reflected light intensity the gradations of gray scale or color can be stored in an array 

Data glove 

• Constructed with a series of sensors that can detect hand and finger motions . The transmitting and receiving antennas can be structured as a set of three mutually perpendicular cols, forming a three dimensional Cartesian coordinates system. 
• Electromagnetic coupling between the three pairs of coil is used to provide information about the position and orientation of hand.