Graphics display devices

notes

1. Computer Graphics Display Devices

2. Display Devices A display device is a device for presentation of information, such as image or a text, for visual reception, acquired, stored, or transmitted in various forms When the input information is supplied as an electrical signal, the display is called electronic display. Electronic displays are available for presentation of visual information.

3. Cathode Ray Tubes - CRT

4. A CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. Most desktop computers make use of CRTs. The CRT in a computer display is similar to the picture tube in a television receiver. A CRT consists of several basic components as illustrated in fig. Heater element and cathode: Heat is supplied to the cathode by passing current through heater element. The Cathode is cylindrical metallic structure which is rich in electrons. On heating the electrons are released from cathode surface. Control Grid: The control grid is the next element which follows cathode. It almost covers cathode leaving small opening for electrons to come out. Intensity of the electron beam is controlled by setting voltage levels on the control grid. A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of control grid structure. A smaller negative voltage on the control grid will simply decrease the number of electrons passing through the cathode. Thus, we can control the brightness of a display by varying the voltage on the control grid.

5. Accelerating Anode: Positively charged anodes, in the sequence of accelerating anodes, accelerate the electrons towards phosphor screen. Focusing And Deflection Coils: Focusing and Deflection Coils are together needed to force the electron beam to converge into a small spot, as it strikes the screen, otherwise the electrons would repel each other and the beam would spread out as it approaches the screen. Deflection coils produce an extremely low frequency electromagnetic field that allows for constant adjustment of the direction of the electron beam. There are two sets of deflecting coils: horizontal and vertical. Electrostatic focusing is commonly used in television and computer graphics monitor. Phosphor Coating: Finally, when the accelerating electron beam collides on the phosphor coating, a part of Kinetic Energy is converted into light and heat. When the electrons in the beam collide with the phosphor coating they are stopped and their kinetic energy is absorbed by the phosphor, resulting in the screen display.

6. Drawbacks CRT S produce crisp , vibrant images. But they do have serious drawbacks: They are bulky. In order to increase the screen width in a CRT set, you also have to increase the length of the tube (to give the scanning electron gun room to reach all parts of the screen). It has been claimed that the electromagnetic fields emitted by CRT monitors constitute a health hazard and can affect the functioning of living cells, and people are moving towards other forms of digital displays replacing them slowly.

7. Raster Scan Display A raster-scan display is the most common method of drawing images on a CRT screen. In this method, horizontal and vertical deflection signals are generated to move a beam all over the screen in a pattern for displaying any image. The electron beam is swept across the screen one row at a time from top to bottom. The electron beam sweeps back and forth from left to right across the screen. The beam is on, while it moves from left to right. The beam is off, when it moves back from right to left. This phenomenon is called the horizontal retrace, as shown by red lines in the figure. As soon as the beam reaches the bottom of the screen, it is turned off and is rapidly retraced back to the top to start again. This is called the vertical retrace. Raster-scan displays maintain the steady image on the screen by repeating scanning of the same image. This process is known as refreshing of screen.

8. Typically, a graphics display consists of three components: frame buffer, display controller, and a TV screen or monitor.

9. Frame Buffer: The frame buffer stores an image as a matrix of intensity values. It is an interface between what are usually relatively slow graphics computation and the high data rate video image display. In a typical personal computer, the frame buffer is located on the graphics card that manages the video subsystem of the computer. It basically used to be not much more than some extra memory. Stored intensity values are then retrieved from the refresh buffer and displayed on the screen one row at a time. Each intensity value is represented by bit zero (0) or one (1) in the frame buffer.

10. Video or Display controller: The video or display controller has direct access to memory locations in the frame buffer. It is responsible for retrieving data from the frame buffer and passing it to the display device. It reads each successive bytes of data from frame buffer and converts this 0's and 1's in one line into a corresponding video signals, and this line is called a scan line. If the intensity is one (1) then the controller sends a signal to display a dot in the corresponding position on the screen. If the intensity is zero (0) then no dot is displayed. This is the simplest way in which a black and white image is displayed. The display-controller repeats the read, convert, and fill operations at least 60 times per seconds that maintains a study picture on the screen (refresh rate). The frame buffer maps the screen into Cartesian coordinates. Generally, the screen coordinates are taken as a positive xy -plane. Hence the screen is continuously refreshed by scanning from maximum value of ycoordinate down to y = 0.

11. Intensity range for pixel positions depends on the capability of the raster system. In a simple black-and-white system, each screen point is either on or off, so only one bit per pixel is needed to control the intensity of screen positions. For a bit level system, A bit value of 1 indicates that the electron beam is to be turn ON at that position. A bit value of 0 indicates that the beam intensity is to be turn OFF. Additional bits are needed when color and intensity variations can be displayed. Up to 24 bits per pixel are included in high-quality systems, which can require several megabytes of storage for the frame buffer, depending on the resolution of the system. A system with 24 bits per pixel and a screen resolution of 1024 bv 1024 requires 3 Mega Bytes of storage for the frame buffer. Bitmap: On a black-and-white system with one bit per pixel, the frame buffer is commonly called a Bitmap. Pixmap : Systems with multiple bits per pixel, the frame buffer are often referred to as a Pixmap .

12. Random Scan Display A CRT, as a random-scan display unit, has an electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitors draw a picture one line at a time. These are also referred to as vec tor displays (or stroke-writing or calligraphic displays). The component of a picture (lines and curves) can be drawn and refreshed by a random-scan system in any specified order. A pen plotter operates in a similar way and is an example of a random scan, hardcopy device. The refresh rate, on a random-scan system, depends on the number of primitives like lines to be displayed. A picture definition is stored as a set of line-drawing commands in an area of memory called a refresh display file (or a refresh buffer). To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line one by one. After all line-drawing commands have been processed, the system cycles back to the first line command in the list and repeats the procedure of scan, display, and retrace.

13. Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. High -quality vector systems are capable of handling approximately 100,000 short lines at this refresh rate. It is important to note that the faster refreshing of the set of lines could burn out the phosphor. Therefore, when a small set of lines are to be displayed, each refresh cycle is delayed to avoid greater refresh rates, typically 60 frames per second. Random-scan systems are designed for line-drawing applications; hence cannot display realistic shaded scenes. Vector displays generally have a higher resolution than raster systems, as picture definition is stored as a set of line-drawing instructions instead of a set of intensity values for all screen points. These vector displays produce smooth line drawings, because the CRT beam directly follows the line path. A raster system, in contrast, produces jagged lines that are plotted as discrete point sets.

14. 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. The two basic techniques for producing color displays with a CRT are 1. The Beam-Penetration method. 2. The Shadow-Mask method.

15. BEAM-PENETRATION METHOD The beam-penetration method for displaying color pictures has been used with random-scan monitors. Two layers of phosphor, usually RED and GREEN, are coated onto the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphor layers. A beam of slow electrons excites only the outer RED layer. A beam of very fast electrons penetrates through the RED layer and excites the inner GREEN layer. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors, ORANGE and YELLOW. The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. Advantage : Beam penetration has been an inexpensive way to produce color in random-scan monitors, Disadvantage : only four colors are possible, and the quality of pictures is not as good as with other methods.

16. SHADOW-MASK METHODS Shadow-mask methods are commonly used in raster scan systems (including color TV) because they produce a much wider range of colors than the beam penetration method. A shadow-mask CRT has three phosphor color dots at each pixel position. One phosphor dot emits a RED Light, another emits a GREEN light, and the third emits a BLUE light. This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen. 1. Delta-Delta Shadow-Mask method, commonly used in color CRT systems. The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. When the three beams pass through a hole in the shadow mask, they activate a Dot Triangle, which appears as a small color spot on the screen. The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. 2.In-Line arrangement in which the three electron guns, and the corresponding red-green-blue color dots on the screen, are aligned along one scan line instead of in a triangular pattern. This in-line arrangement of electron guns is easier to keep in alignment and is commonly used I n high-resolution color CRTs.

17. The color variations in a shadow-mask CRT by varying the intensity levels of the three electron beams. By turning OFF the RED and GREEN guns, gets the color coming from the BLUE phosphor. Other combinations of beam intensities produce a small light spot for each pixel position, since our eyes tend to merge the three colors into one composite. The color depends on the amount of excitation of the red, green, and blue phosphors. White (or gray) area is the result of activating all three dots with equal intensity. Yellow is produced with the green and red dots only. Magenta is produced with the blue and red dots. Cyan shows up when blue and green are activated equally. In some low-cost systems, the electron beam can only be set to on or off, limiting displays to Eight Colors.

18. RED GREEN BLUE COLOR 0 0 0 Black 0 0 1 Blue 0 1 0 Green 0 1 1 Cyan 1 0 0 Red 1 0 1 Magenta 1 1 0 Yellow 1 1 1 White

19. Flat panel display Although most graphics monitors are still constructed with CRTs, other technologies are emerging that may soon replace CRT monitors. The term flat-panel display refers to a class of video devices that have 1. Reduced volume 2. Weight 3. Power requirements compared to a CRT. The flat panel display (FPD) technology is becoming increasingly common in a wide variety of consumer devices that include cellular phones, digital cameras, liquid crystal display (LCD) televisions, computer displays, and personal digital assistants (PDAs). These FPDs are lighter and much thinner than traditional television and video displays that use a CRT. Current uses for flat-panel displays include small TV monitors, calculators, pocket video games, laptop computers, armrest viewing of movies on airlines, as advertisement boards in elevators, and as graphics displays in applications requiring rugged, portable monitors and pocket notepads.

20. Reading Assignment: LCD