Lectures

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Overview of Graphics Systems

Muhammad Sarfraz Sarfraz@kfupm.edu.sa

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Objectives

Introduction to Interactive Systems Interactive Output Devices Cathode Ray Tube (CRT) Based Devices:

Random-scan Raster-scan

Flat Panel: Plasma Panels Liquid Crystal Displays (LCD)

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Introduction to Interactive Systems A picture may take the form of a static image or that of an animated form. Whatever the outcome, a system will need to incorporate mechanisms to promote interaction; between the user and the system itself. For interaction to take place, the system needs to employ tools to facilitate input variation and output devices which work on a transitory basis.

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Interactive Output Devices

Transitory output devices can be categorized as: CRT family flat panel clan.

There are two forms of CRT type:

Storage Refresh.

The refresh type includes: Vector Displays Raster displays.

There are two forms of flat panel type: Emitter Non-emitter.

The emitter type includes light emitting diodes (LEDs) and Plasma Panels; whilst LCDs come within the non-emitter range.

LCDs Vector

Raster

LEDs

Plasma Panels

CRT Flat Panel

Storage Refresh Emitter Non-Emitter

Transitory Output Devices

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Cathode Ray Tube (CRT)

Primary transitory output device within a graphics system is a video monitor. It fulfils the basic interactive goal of producing a non-permanent picture. This permits picture modifications, as well as dynamic movements of the whole or sections of a picture.

Video monitors are mainly realized using the standard CRT design.

The operation of a CRT can be summarised as follows:

A beam of electrons (cathode rays) are emitted by an electron

gun. The beam firstly passes through a grid, which controls its

intensity (electron number). The focusing unit forces the electron beam to converge into a

small spot as it hits the screen. X, Y movement of the beam is controlled by a set of respective

horizontal and vertical deflectors. Having being directed, the beam hits the phosphor-coated screen,

which in turn emits a small spot of visible light.

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Fig 1.: Basic Design of a magnetic-deflection CRT.

Fig 2.: Operation of an electron gun with accelerating Anode.

Fig 3.: Electrostatic deflection of the electron beam in a CRT.

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CRT: Attributes Persistence The light generated by the phosphor decays exponentially with time, so that after a known period the spot of light will disappear. The term persistence is used to indicate the length of light emission after an electron beam has been removed. Its precise definition is: The time it takes the emitted light from the screen to decay to one-tenth of its original intensity. This implies that in order to maintain a desired light element (eg an image) on screen, it needs to be constantly refreshed. If the refresh rate is lower than required for a known phosphor, then the image will be seen to flicker. Different phosphors have different persistence rates, ranging from 10 to 60 microseconds for most graphical monitors.

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CRT: Attributes (Persistence) Low persistence phosphors require a high refresh rate to maintain a picture on screen without flicker. This lends itself nicely to displaying images which require dynamic movement, such as in the area of animation. High persistence phosphors are useful for displaying highly complex, static, pictures. Another factor relating to refresh rates is the human eye. The eye remembers a picture for about 70 ms, and if represented at every 28 ms then a flicker free image will result. This means that if a phosphor had zero persistence, we will need to refresh every 28 ms (36 times/sec) to avoid flicker. Phosphors with milliseconds decay rates will permit refresh rates to be dropped to 30 or even 25 times per second.

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CRT: Attributes Resolution The maximum number of points (light spots) that can be displayed without overlap on a CRT is referred to as the resolution. A more precise definition is the number of points which can be displayed within a square centimetre. The definition, in practice, is often simply stated as the total number of points in the horizontal (X) and vertical (Y) directions. How much separation is required between two adjacent light spots to avoid overlap ?

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CRT: Attributes (Resolution) A measure can be made by analysing the shape of the intensity distribution for a spot. This normally takes the form of a Gaussian distribution:

The intensity is greatest at the centre of the spot, and decreases out towards the edges. Two adjacent spots will appear distinct as long as their separation is greater than the diameter at which each spot has an intensity of about 60% of that at the centre of the spot.

The size of a light spot depends also on the intensity level; higher intensity, greater diameter of electron beam and thus the illuminated spot. In practice, the resolution of a CRT is dependent on the type of phosphor used, the intensity level and also on the focusing and deflection systems.

Cross-sectional view of the electron density distribution of the CRT beam.

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CRT: Attributes Aspect Ratio This conveys the ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen. For a given device, it is expressed as: aspect ratio = (Yp/Ys)/(Xp/Xs) where: Yp Total number of vertical points Ys Maximum vertical height Xp Total number of horizontal points Xs Maximum horizontal width Example: A 12-inch screen has a horizontal length of 10 inches and vertical height of 7 inches. What is the aspect ratio given that the screen resolution is 640 x 350 points ?

aspect ratio = (350/7)/(640/10) = 0.78125 (vertical to horizontal) What does this mean ? Aspect ratios are useful for ensuring that a square or a circle, for example, drawn on a screen maintain their geometric features.

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Interactive Devices: Random-scan Display One of the modes in which the CRT is employed to display an image is referred to as a random-scan. In this mode, the electron beam is directed only towards those parts of a screen where a picture is to be drawn. There is no theoretical limit on the addressable screen points, though in practice a Cartesian coordinate is used to assist users. Random-scan monitors draw a picture one line at a time, as required.

CRT Screen

Figure: In order to draw the triangular shape, as shown, the electron beam follows only the desired path. The labels vector or stroke-writing or calligraphic, all refer to a random-scan display unit. The image, generated by means of lines, can be drawn and refreshed in any specified order. The process of image generation makes use of a refresh display file.

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This is often referred to as simply a display list, a display file, a refresh buffer or a display buffer. It occupies an area of memory where all the information required to draw an image is stored. Example: An example of a display buffer for the triangular shape outlined earlier may take the form:

Step Action 1 Move To

(80,85) 2 Draw To

(80,25) 3 Draw To

(30,40) 4 Draw To

(80,85) 5 Go To

2

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Interactive Devices: Random-scan Display A working system embodies a display controller unit, whose main purpose: * is to cycle through the display file, and * to provide the interface for the CRT. Figure: Block diagram of a graphical display system using the random-

scan mode. Refresh rates of a random-scan system depends on the number of lines to be drawn. It practice, a system is designed to draw all the line components of an image 30 to 60 times per second. Systems are constrained to work within the 60 times per second marker, otherwise possibility of burning-out the screen phosphor.

Host CPU Display Buffer

Display Controller CRT

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Interactive Devices: Raster-scan Displays The most common type of display monitor which employs a CRT is the raster-scan device. In this mode, the electron beam of the CRT is swept across the screen, one row at a time from top to bottom. As the beam moves across the row, the intensity of the beam is toggled on and off to generate a pattern of illuminated spots. The toggling process reflects the status level at each stage of the image to be generated.

Figure: The electron beam moves horizontally and vertically (downwards), at each stage setting the correct intensity level.

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Interactive Devices: Raster-scan Displays The process of image generation makes use of an area of memory referred to as a refresh buffer or frame buffer. The frame buffer stores the intensity values for all the screen points. Each screen point is referred to as a pixel or pel (pixel element). In a basic system, each bit in the frame buffer corresponds directly to a screen pixel.

Frame Buffer

1 D/A

CRT S

Electron Gun

Figure: A monochrome raster-scan graphical system showing the relationship between the frame buffer and the CRT screen.

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Interactive Devices: Raster-scan Displays The diagram depicts a bi-level (monochrome) system: bit value electron gun (pixel) 0 off (black - background) 1 on (white - foreground) The size of the frame buffer bears a direct relationship with the resolution of the screen. Example: If the screen resolution of a raster-scan device is 640 x 480, what would be the display storage requirements for a monochrome system? monochrome: 1 bit/pixel frame buffer = 640 x 480 = 307,200 bits

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Interactive Devices: Raster-scan Displays Multiple frame buffers allow grey (and color) levels per each pixel to be realized.

Frame Buffers

1D/A

CRT Screen

Electron Gun

0

1

1 01

Figure: A graphical system employing three frame buffers to support grey intensity levels.

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Interactive Devices: Raster-scan Displays For grey scale implementation, we are working with intermediary levels between the two extremes (black and white):

Black

White

0

1

000

001

010

011

100

101

110

111

Bi-Level Multi-Level

Figure: The effect of introducing additional (in this case, three) frame buffers.

In general, for N frame buffers, we have 2N grey levels, with: 0 background colour (dark) 2N-1 foreground colour (full intensity) What is the shortcoming of using additional frame buffers ? Memory. Required Memory = (bits/pixel) x screen resolution Example: Number of frame buffers = 4 Screen resolution = 1024 x 512 Required image storage = (4) x 1024 x 512 = 2 Mb

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Interactive Devices: Raster-scan Displays Another way of increasing the intensity (grey) levels is to use a look-up table.

Frame Buffers

1

0

1

1 01D/A

CRT Screen

Electron Gun

Look-up table

1 0 1 1

Figure: Increasing the number of grey values by using a look-up table.

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Interactive Devices: Raster-scan Displays The bit pattern from the frame buffers is used as an index for the look-up table. The look-up table has 2N entries, where N is the number of frame buffers being used. The width of the look-up table (W) can take on any size and usually is greater than N. This facilitates 2W intensities, with only 2N different intensities available at anyone time. This arrangement provides more grey levels with a modest increase in required memory.

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Interactive CRT Devices: Colour Colour images are realised through using a combination of phosphors that emit different-coloured light. By mixing the emitted light from each respective phosphor, a range of colours can be produced. Two commercial methods exist for producing colour CRT displays: beam penetration and shadow-masking. In the case of beam penetration, two layers of phosphor (usually red and green) are used. The display colour depends on how far the electron beam penetrates into the phosphor layers.

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Interactive CRT Devices: Colour The speed of the electrons, and hence the screen colour at any point, is controlled by the beam-acceleration voltage. This method has been used in random-scan devices, but it has limited range of colours (usually four) and yields a poor picture quality.

red

green

screen

B

E

A

M

RED

ORANGE/YELLOW

GREEN

Colour generation by means of beam penetration.

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Interactive CRT Devices: Colour Shadow-mask methods are commonly used in raster-scan display systems, including televisions. They produce a much wider range of colours than the beam penetration method. A shadow-mask CRT has three phosphor colour dots (red, green, and blue - RGB) at each pixel position. Three electron guns, one for each colour dot, together with a single shadow-mask grid is employed to implement a colour CRT.

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Interactive CRT Devices: Colour The shadow-mask contains a series of perforations which are aligned with the phosphor dot patterns. The three electron beams are deflected and focused as a group onto the shadow-mask. Upon passing through a perforation, the electron beams activate a dot triangle, which appears a small colour spot on the screen.

Figure: Phosphor dot (triangular) pattern for a shadow-mask CRT.

R B G R G

B G R B R

R B G R G

B G R B R

R B G R G

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Interactive CRT Devices: Colour What is the purpose of the shadow-mask ? The shadow mask ensures that the appropriate phosphor dot is excited at each stage. Other shadow-mask arrangements exist, typically the in-line configuration. The RGB arrangements allows for a variety of colours to be realised by varying the intensity of each of the three electron guns. A basic colour system may use three frame buffers (one for each electron gun) to hold respective intensity levels. In this case, eight colours are available.

Figure: Screen colour for different RGB intensity combinations. How may the range of colours be increased ?

R G B Screen Colour 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

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Flat Panel Displays Please, see pages 44-47 in the text book.