Display devices
The display devices used in graphics system is video monitor. The most common video monitor is based on CRT technology.
Cathode Ray Tube (CRT)
− CRT are the most common display devices on computer today. A CRT is an evacuated glass tube, with a heating element on one end and a phosphor-coated screen on the other end.
− When a current flows through this heating element (filament) the conductivity of metal is reduced due to high temperature. These cause electrons to pile up on the filament.
− These electrons are attracted to a strong positive charge from the outer surface of the focusing anode cylinder.
− Due to the weaker negative charge inside the cylinder, the electrons head towards the anode forced into a beam and accelerated by the inner cylinder walls in just the way that water is speeds up when its flow though a small diameter pipe.
− The forwarding fast electron beam is called Cathode Ray. A cathode ray tube is shown in figure below.
− There are two sets of weakly charged deflection plates with oppositely charged, one positive and another negative. The first set displaces the beam up and down and the second displaces the beam left and right.
− The electrons are sent flying out of the neck of bottle (tube) until the smash into the phosphor coating on the other end.
− When electrons strike on phosphor coating, the phosphor then emits a small spot of light at each position contacted by electron beam. The glowing positions are used to represent the picture in the screen.
− The amount of light emitted by the phosphor coating depends on the no of electrons striking the screen. The brightness of the display is controlled by varying the voltage on the control grid.
Persistence:
− How long a phosphor continues to emit light after the electron beam is removed
− Persistence of phosphor is defined as the time it takes for emitted light to decay to 1/10 (10%) of its original intensity. Range of persistence of different phosphors can react many seconds.
− Phosphors for graphical display have persistence of 10 to 60 microseconds. Phosphors with low persistence are useful for animation whereas high persistence phosphor is useful for highly complex, static pictures.
Refresh Rate:
− Light emitted by phosphor fades very rapidly, so to keep the drawn picture glowing constantly, it is required to redraw the picture repeatedly and quickly directing the electron beam back over the some point. The no of times/sec the image is redrawn to give a feeling of non-flickering pictures is called refresh-rate.
− If Refresh rate decreases, flicker develops.
− For refresh displays, it depends on picture complexity
− Refresh rate above which flickering stops and steady it may be called as critical fusion frequency(CFF).
Resolution:
Maximum number of points displayed horizontally and vertically without overlap on a display screen is called resolution. In other ways , resolution is referred as the no of points per inch(dpi/pixel per inch).
Raster-Scan Display
− Raster Scan Display is based on television technology. In raster-scan the electron beam is swept across the screen, one row at a time from top to bottom. No of scan line per second is called horizontal scan rate.
− As electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory called frame buffer or refresh buffer. Frame buffer holds all the intensity value for screen points.
Figure: A raster-scan system displays an object as a set of points across each screen scan line
Figure: Raster Scan display system
− The stored intensity value is retrieved from frame buffer and painted on the scan line at a time. Home television are common examples using raster display
− Intensity range for pixel position depends on capability of raster system. For B/W system each point on screen are either on or off, so only one bit per pixel is needed to control the pixel intensity. To display color with varying intensity level, additional bits are needed. Up to 24 to 32 bit per pixel are included in high quality systems, which require more space of storage for the frame buffer, depending upon the resolution of the system.
− A system with 24 bit pixel and screen resolution 1024 1024 require 3 megabyte of storage in frame buffer.
1024*1024 pixel = 1024*1024*24 bits = 3 MB
− The frame butter in B/W system stores a pixel with one bit per pixel so it is termed as bitmap. The frame buffer in multi bit per pixel storage, is called pixmap.
− Refreshing on Raster-Scan display is carried out at the rate of 60 or higher frames per second. 60 frames per second is also termed as 60 cycle per second usually used unit Hertz (HZ)
− Returning of electron beam from right end to deft end after refreshing each scan line is horizontal retrace . At the end of each frame, the electron beam returns to the top left corner to begin next frame called vertical retrace.
Interlaced: Display in two pass with interlacing.
Figure: Horizontal retrace and Vertical retrace
Question: Consider a RGB raster system is to be designed using 8 inch by 10 inch screen with a resolution of 100 pixels per inch in each direction. If we want to store 6 bits per pixel in the frame buffer, How much storage(in bytes) do we need for the frame buffer?
Solution: Size of screen = 8 inch ´ 10 inch.
Pixel per inch(Resolution) = 100.
Then, Total no of pixels = 8´100 ´ 10 ´ 100 pixels
Bit per pixel storage = 8
Therefore Total storage required in frame buffer = (800 ´1000´8) bits
= (800 ´1000´8)/8 Bytes
= 800000 Bytes.
Frame Buffer Architecture of Raster Display
1. Indexed-color frame buffer.
In indexed –color frame buffer,
− Each pixel uses one byte in frame buffer.
− Each byte is an index into a color map.
− Each pixel may be one of 324 colors, but only 256 color can be displayed at a time.
− There is a look-up table which has as many entries as there are pixel values.
− The table entry value is used to control the intensity or color of the CRT.
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Figure: Indexed color frame buffer
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Figure : True Color Frame Buffer
2. True-color frame buffer: ( 24 bit or above): In true color frame buffer,
− Each pixel requires at least 3-bytes, one for each primary color (R,G,B)
− Sometimes combined with a look-up table per primary.
− Each pixel can be one of 224 colors.
3 High-color frame buffer
· Popular PC/SVGA standard
· Pixels are packed in a short i.e. each primary color use 5 bit.
· Each pixel can be one of 215 colors
Red Green Blue
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Random scan display: (Vector display)
In random scan system, the CRT has the electron beam that is directed only to the parts of the screen where the picture is to be drawn. It draws a picture one line at a time, so it is also called vector display (or stroke writing or calligraphic display). The component lines of a picture are drawn and refreshed by random scan system in any specified order.
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Figure: Random Scan Display
− The refresh rate of vector display depends upon the no of lines to be displayed for any image. Picture definition is stored as a set of line drawing instructions in an area of memory called the refresh display file (Display list or display file)
− To display a picture, the system cycles through the set of commands (line drawing) in the display file. After all commands have been processed, the system cycles back to the first line command in the list.
− Random scan systems are designed for drawing all component lines 30 to60 times per second. Such systems are designed for line-drawing applications and can not display realistic shaded scenes. Since CRT beam directly follows the line path, the vector display system produce smooth line.
Color CRT
In color CRT, the phosphor on the face of CRT screen are laid in to different fashion. Depending on the technology of CRT there are two methods for displaying the color pictures into the screen.
1. Beam penetration method 2. Shadow mask method
Beam Penetration method:
This method is commonly used for random scan display or vector display. In random scan display CRT, the two layers of phosphor usually red and green are coated on CRT screen. Display color depends upon how far electrons beam penetrate the phosphor layers.
Slow electron excite only red layer so that we can see red color displayed on the screen pixel where the beam strikes. Fast electron beam excite green layer penetrating the red layer and we can see the green color displayed at the corresponding position. Intermediate is combination of red and green so two additional colors are possible – orange and yellow.
So only four colors are possible so no good quality picture in this type of display method.
Shadow Mask Method:
Shadow mask method is used for raster scan system so they can produce wide range of colors. In shadow mask color CRT, the phosphor on the face of the screen are laid out in a precise geometric pattern. There are two primary variations.
1. The stripe pattern of inline tube
2. The delta pattern of delta tube
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Stripe pattern
Delta Pattern
− In color CRT, the neck of tube, there are three electron guns, one for each red, green and blue colors. In phosphor coating there may be either strips one for each primary color, for a single pixel or there may be three dots one for each pixel in delta fashion.
− Special metal plate called a shadow mask is placed just behind the phosphor coating to cover front face.
− The mask is aligned so that it simultaneously allow each electron beam to see only the phosphor of its assigned color and block the phosphor of other two color.
Depending on the pattern of coating of phosphor, two types of raster scan color CRT are commonly used using shadow mask method.
1. Delta-Delta CRT:
− In delta-delta CRT, three electron beams one for each R,G,B colors are deflected and focused as a group onto shadow mask, which contains a series of holes aligned with the phosphor dots.
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Shadow Mask
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Screen
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Electron Gun
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Figure: Shadow mask in Delta-Delta CRT
− Inner side of viewing has several groups of closely spaced red ,green and blue phosphor dot called triad in delta fashion.
− Thin metal plate adjusted with many holes near to inner surface called shadow mask which is mounted in such a way that each hole aligned with respective triad.
− Triad are so small that is perceived as a mixture of colors. When three beams pass through a hole in shadow mask, they activate the dot triangle to illuminate an small spot colored on the screen.
− The color variation in shadow mask CRT can be obtained by varying the intensity level of the three electron guns.
The main draw back of this CRT is due to difficulty for the alignment of shadow mask hole and respective triads.
A precision inline CRT:
This CRT uses strips pattern instead of delta pattern. Three strips one for each R, G, B color are used for a single pixel along a scan line so called inline. This eliminates the drawbacks of delta-delta CRT at the cost of slight reduction of image sharpness at the edge of the tube.
− Normally 1000 scan lines are displayed in this method. Three beams simultaneously expose three inline phosphor dots along scan line.
Architecture of Raster Scan System:
The raster graphics systems typically consists of several processing units. CPU is the main processing unit of computer systems. Besides CPU, graphics system consists of a special purpose processor called video controller or display processor. The display processor controls the operation of the display device.
The organization of raster system is as shown below
System Frame
Memory buffer
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Peripheral device
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Video controller
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Monitor
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System Bus
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CPU
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Figure: A simple Raster System.
· A fixed area of system memory is reserved for the frame buffer. The video controller has the direct access to the frame buffer for refreshing the screen.
· The video controller cycles through the frame buffer, one scan line at a time, typically at 60 times per second or higher. The contents of frame buffer are used to control the CRT beam's intensity or color.
The video controller:
The video controller is organized as in figure below. The raster-scan generator produces deflection signals that generate the raster scan and also controls the X and Y address registers, which in turn defines memory location to be accessed next. Assume that the frame buffer is addressed in X from 0 to Xmax and in Y from 0 to Ymax then, at the start of each refresh cycle, X address register is set to 0 and Y register is set to 0 (top scan line).
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Liner Address
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Figure: Organization of Video Controller.
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As first scan line is generated, the X address is incremented up to Xmax. Each pixel value is fetched and used to control the intensity of CRT beam. After first scan line X address is reset to 0 and Y address is incremented by 1. The process is continued until the last scan line (Y=Ymax) is generated.
Raster-Scan Display Processor:
The raster scan with a peripheral display processor is a common architecture that avoids the disadvantage of simple raster scan system. It includes a separate graphics processor to perform graphics functions such as scan conversion and raster operation and a separate frame buffer for image refresh.
The display processor has its own separate memory called display processor memory.
· System memory holds data and those programs that execute on the CPU, and the application program, graphics packages and OS.
· The display processor memory holds data plus the program that perform scan conversion and raster operations.
· The frame buffer stores displayable image created by scan conversion and raster operations.
The organization is given below in figure
Display
processor
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Peripheral device
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System memory
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System Bus
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Display frame
processor buffer
memory
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Video controller
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Monitor
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CPU
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Figure: Architecture Raster scan system with display processor
2. Vector Display System.
System Memory
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Peripheral device
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Display processor
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System Bus
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Monitor
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CPU
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Move (10,10)
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Move (512,512)
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Figure : Architecture of Vector Display System
− Vector display system consists of several units along with peripheral devices. The display processor is also called as graphics controller.
− Graphics package creates a display list and stores in systems memory (consists of points and line drawing commands) called display list or display file.
− Refresh time around so cycle per second.
− Vector display technology is used in monochromatic or beam penetration color CRT.
− Graphics are drawn on a vector display system by directing the electron beam along component line.
Advantages:
· Can produce output with high resolutions.
· Better for animation than raster system since only end point information is needed.
Disadvantages:
· Cannot fill area with pattern and manipulate bits.
· Refreshing image depends upon its complexity.
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