Computer Graphics Unit1

Computer Graphics

                                               UNIT – I

2 Marks

1. Define Computer graphics.
Computer graphics remains one of the most existing and rapidly growing computer fields. Computer graphics may be defined as a pictorial representation or graphical representation of objects in a computer.

2. What is meant by scan code?
When a key is pressed on the keyboard, the keyboard controller places a code carry to the key pressed into a part of the memory called as the keyboard buffer. This code is called as the scan code.

3. What is meant by refreshing of the screen?
Some method is needed for maintaining the picture on the screen. Refreshing of screen is done by keeping the phosphorus glowing to redraw the picture repeatedly. (i.e.)By quickly directing the electronic beam back to the same points.

4. Define Random scan/Raster scan displays?
Random scan is a method in which the display is made by the electronic beam which is directed only to the points or part of the screen where the picture is to be drawn. The Raster scan system is a scanning technique in which the electrons sweep from top to bottom and from left to right. The intensity is turned on or off to light and unlight the pixel.

5. List out the merits and demerits of Penetration techniques?
The merits and demerits of the Penetration techniques are as follows
• It is an inexpensive technique
• It has only four colors
• The quality of the picture is not good when it is compared to other
techniques
• It can display color scans in monitors
• Poor limitation etc.

6. List out the merits and demerits of DVST?
The merits and demerits of direct view storage tubes [DVST] are as follows
• It has a flat screen
• Refreshing of screen is not required
• Selective or part erasing of screen is not possible
• It has poor contrast
• Performance is inferior to the refresh CRT.
7. What do you mean by emissive and non-emissive displays?
The emissive display converts electrical energy into light energy. The plasma panels, thin film electro-luminescent displays are the examples. The Non emissive are optical effects to convert the sunlight or light from any other source to graphic form. Liquid crystal display is an example.

8. List out the merits and demerits of Plasma panel display?
Merits
• Refreshing is not required
• Produce a very steady image free of Flicker
• Less bulky than a CRT.
Demerits
• Poor resolution of up to 60 d.p.i
• It requires complex addressing and wiring
• It is costlier than CRT.

9. What is persistence?
The time it takes the emitted light from the screen to decay one tenth of its original intensity is called as persistence.

10. What is resolution?
The maximum number of points that can be displayed without an overlap on a CRT is called as resolution.

11. What is Aspect ratio?
The ratio of vertical points to the horizontal points necessary to produce length of lines in both directions of the screen is called the Aspect ratio. Usually the aspect ratio is ¾.

12. What is meant by Addressability?
The Addressability is the number of individual dots per inch (d.p.i) that can be created. If the address of the current dot is (x, y) then the next dot will be (x+y), (x+y+1) etc.
13. What is a dot size?
Dot size may be defined as the diameter of a single dot on the devices output. Dot size is also called as the Spot size. 3
14. What is interdot distance?
Interdot distance is the reciprocal of addressability. If the addressability is large, the interdot distance will be less. The interdot distance should be less to get smooth shapes.
15. What is the difference between impact and non-impact printers?
Impact printer press formed character faces against an inked ribbon on to the paper. A line printer and dot-matrix printer are examples. Non-impact printer and plotters use Laser techniques, inkjet sprays, Xerographic process, electrostatic methods and electro thermal methods to get images onto the papers.
Examples are: Inkjet/Laser printers.
16. What are the features of Inkjet printers?
• They can print 2 to 4 pages/minutes.
• Resolution is about 360d.p.i. Therefore better print quality is achieved.
• The operating cost is very low. The only part that requires replacement is ink cartridge.
• 4 colors cyane, yellow, majenta, black are available.
17. What are the advantages of laser printer
• High speed, precision and economy.
• Cheap to maintain.
• Quality printers.
• Lasts for longer time.
• Toner power is very cheap.
18. What are the advantages of electrostatic plotters?
• They are faster than pen plotters and very high quality printers.
• Recent electrostatic plotters include a scan-conversion capability.
• Color electrostatic plotters are available. They make multiple passes over the paper to plot color pictures.
19. Define pixel?
Pixel is shortened forms of picture element. Each screen point is referred to as pixel or pel.
20. What is frame buffer?
Picture definition is stored in a memory area called frame buffer or refresh buffer.
21. What is bitmap and what is pixmap?
The frame buffer used in the black and white system is known as bitmap which take one bit per pixel. For systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.

22. What is a Vector display or stroke writing or calligraphic display?
Random scan monitors draw a picture one line at a time and for this reason are also referred as vector displays.

23. Where the video controller is used?
A special purpose processor, which is used to control the operation of the display device, is known as video controller or display controller.

24. What do you mean by scan conversion?
A major task of the display processor is digitizing a picture definition given in an application program into a set of pixel-intensity values for storage in the frame buffer. This digitization process is called scan conversion.

25. What is run length encoding?
Run length encoding is a compression technique used to store the intensity values in the frame buffer, which stores each scan line as a set of integer pairs. One number each pair indicates an intensity value, and second number specifies the number of adjacent pixels on the scan line that are to have that intensity value.

16 Marks

1. Explain ref resh cat hode 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. 16
♦ 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 17
♦ 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 framebuffer 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 line18
• 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 I/O device
4. 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
System Bus
CPU
System Frame
Memory Buffer
Video
controller 19
• 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