Download Introduction to Computer Graphics

www.knowledgepath.in
Introduction to Computer Graphics
The computer is an information processing machine. It is a tool for storing, manipulating and
correcting data. There are many ways to communicate the processed information to the user.
The computer graphics is one of the most effective and common way to communicate the
processes information to the users. It displays the information in the form of graphics objects.
Such as pictures, diagrams, and graphs instead of simple text. That means with the help of
computer graphics, we can express the data in pictorial form.
In CG, pictures and graphics objects are presented as a collection of picture elements called
pixel. The pixel is the smallest addressable screen element. It is the smallest piece of the
display screen which we can control. The control is achieved by setting the intensity and
color of the pixel, which compose the screen.
Computer Graphics
Computer Graphics is a process of generation of images of virtual scenes using computer
hardware. It is a human oriented system that uses the capability of a computer to create,
transform and display pictorial and symbolic data.
Application of Computer Graphics
Presentation Graphics
Presentation graphics is used to produce illustrations for reports or to generate 35-mm slides
or transparencies for use with projectors. Presentation graphics is commonly used to
summarize financial, statistical, mathematical, scientific, and economic data for research
reports, managerial reports, consumer information bulletins, and other types of reports.
Workstation devices and service bureaus exist for converting screen displays into 35-mm
slides or overhead transparencies for use in presentations. Typical examples of presentation
graphics are bar charts, line graphs, surface graphs, pie charts, and other displays showing
relationships between multiple parameters.
Entertainment
For many entertainment applications, photo realistic techniques are used to render images of
a product. Animations are used frequently in advertising and TV commercials are produced
frame by frame where each frame of a motion is rendered and saved in an image file.
In each successive frame, the motion is simulated by moving object position slightly from the
positions in the previous frame. When all frames in animation sequence have been rendered,
the frames are transferred to film or stored in a video buffer for playback. Film animation
requires 24 frames for each second and if the animation is to be played back on a video
monitor30 frames per seconds are required. A common graphics method employed in many
commercials is morphing where one object is transformed to another.
Education and Training
Computer generated models of physicals, financials and economic systems are often used as
educational aids. Models of physical system, physiological systems, population trends or
equipment such as the color coded diagram can help the trainers to understand the operation
of the system.
For some training application, some special systems are designed. Some of the specialized
systems are the simulators for practice sessions or training of ship contains air-craft pilots,
heavy equipment’s operators and air-traffic control personnel. Some simulators have no video
screens eg. A flight simulator with only a control panel for instrument flying. But most
simulators provide graphics screens for visual operator.
www.knowledgepath.in
Image Processing
Although methods used in computer graphics and image processing overlap, the two areas are
concerned with fundamentally different operations.
In computer graphics, computer is used to create a picture. Image processing on the other
hand applied techniques to modify or interpret existing pictures, such as photographs and TV
scans.
Two principal applications of image processing are:
1. Improving picture quality
2. Machine perception of visual information as used in robotics.
 To apply image processing methods use first digitize a photographs into an image file.
 Then digital methods can be applied to rearrange picture parts, to enhance color
separation, or to improve the quality of shading.
Graphics Systems
The display devices are known as output devices. The most commonly used output device in
a graphics video monitor. The operations of most video monitors are based on the standard
cathode ray tube (CRT) design.
How the Interactive Graphics display works
The modern graphics display is extremely simple in construction. It consists of three
components:
1- A digital memory, or frame buffer, in which the displayed Image is stored as a matrix of
intensity values.
2- A monitor
3- A display controller, which is a simple interface that passes the contents of the frame
buffer to the monitor. Inside the frame buffer the image is stored as a pattern of binary digital
numbers, which represent a rectangular array of picture elements, or pixel. The pixel is the
smallest addressable screen element. In the simplest case where we wish to store only black
and white images, we can represent black pixels by 0's in the frame buffer and white Pixels
by 1's. The display controller simply reads each successive byte of data from the frame buffer
and converts each 0 and 1 to the corresponding video signal. This signal is then fed to the
monitor. If we wish to change the displayed picture all we need to do is to change of modify
the frame buffer contents to represent the new pattern of pixels.
Refresh Cathode-Ray Tubes
www.knowledgepath.in
A basic CRT architecture consist of a e guns, deflection (Moving Around) plates, phosphorus
coated screen and coil. A beam of electrons (Cathode rays) emitted by an elctrogun, passes
through focusing and deflects system that direct the beam towards specified position on the
phosphor-coated screen.
The phosphor then emits a small spot of light at each position contacted by the electron beam.
Because the light emitted by the phosphor fades very rapidly, some methods is needed for
maintaining the screen picture. One way to keep the phosphor glowing is to redraw the
picture repeatedly by quickly directing the e-beam back over the same point. This type of
displays is called Refresh CRT.
The primary component of electron gun in a CRT are the heated metal cathode and a control
grid. Heat is supplied to the cathode by directing a current via the coil of wire called filament.
This causes electrons to be “boiled off” the hot cathode surface. In the vacuum inside the
CRT envelope, the free, negatively charged electrons are then accelerated toward the
phosphor coating by a high positive voltage.
The focusing system in a CRT is needed to force the electron beam to converge into a small
spot as it strikes the phosphor. Generally the focusing system is accomplished with either
electric or magnetic fields. This system is commonly used in television or computer monitor.
Raster Scan Display
The most common type of graphics monitor CRT is the raster-scan display, based on
television technology. In a raster scan system, the electron beam is swept across the screen,
one row at a time from top to bottom. As the electron beam move across each row, the beam
intensity is turned on and off to create a pattern of illuminated spot. Picture definition is
stored in a memory area called refresh buffer or frame buffer. This memory area holds the set
of intensity values for all the screen points. Stored intensity values are then retrieved from the
refresh buffer and painted on screen one row (Scan Line) at a time. Each screen point is
referred to as a pixel or picture element. The capability of a raster scan system to store
intensity information for each screen point makes it well suited for the realistic display of
scenes containing shading and color pattern. Home television sets and printers are example of
other systems using raster scan methods.
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
www.knowledgepath.in
needed to control the intensity of screen positions. For this system, a bit value 1 indicates that
the electron beam is to be turned on at that position and a value 0 indicates that the beam
intensity is to be off.
Other 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 MB 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 by 1024 requires 3 MB of storage of frame buffer. On a black and
white system with one bit per pixel, the frame buffer is commonly called a bitmap. For
system with multiple bits per pixel, the frame buffer is often called as a pixmap.
Refreshing on raster-scan displayed is carried out at the rate of 60 to 80 frames per second,
although some system designed for higher refresh rates. Refresh rates are described in units
of cycles per second, where cycle corresponds to one frame. At the end of each scan line, the
electron beam returns to the left side of screen to begin displaying next line. The return to the
left of the screen, after refreshing each scan line, is called the horizontal retrace of the
electron beam. At the end of each frame, the electron beam returns to the top left corner of
the screen to begin the next frame is called as vertical retrace.
Random Scan Display
When operated as a random-scan display unit, a CRT has the 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 and for this reason are also referred to as vector displays (or stroke-writing
or calligraphic displays).
www.knowledgepath.in
Refresh rate on a random-scan system depends on the number of lines to be displayed.
Picture definition is now stored as a set of line drawing commands in an area of memory
referred to as the refresh display file. Sometimes the refresh display file is called the display
list, display program, or simply the refresh buffer.
To display a specified picture, the system cycles through the set of commands in the display
file, drawing each component line in turn. After all line drawing command have been
processed, the system cycle back to the first line of the command in the list.
Random-scan systems are designed for line drawing applications and cannot display realistic
shaded scenes.
Since picture definition is stored as a set of line drawing instructions and not as a set of
intensity values for all screen points, vector displays generally have higher resolution than
raster systems. Also, vector displays produce smooth line drawings because the CRT beam
directly follows the line path. A raster system, produced a jagged lines.
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 the beam-penetration method and the shadow-mask 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.
www.knowledgepath.in
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. Beam penetration has been an inexpensive way to produce color in random-scan
monitors, but only four colors are possible, and the quality of pictures is not as good as with
other 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. 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.
We obtain 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, we get only the color coming from the
blue phosphor.
Direct View Storage Tubes
An alternative method for maintaining a screen image is to store the picture information
inside the CRT instead of refreshing the screen.
A direct-view storage tube (DVST) stores the picture information as a charge distribution just
behind the phosphor-coated screen.
Two electron guns are used in a DVST. One, the primary gun, is used to store the picture
pattern. A primary gun produces high speed electrons which strike on the storage grid to draw
the picture pattern. The continuous low speed electrons from flood guns pass through the
control grid.
www.knowledgepath.in
Advantages of DVST
Refreshing of CRT is not required.
Because no refreshing is needed, very complex pictures can be displayed at very high
resolutions without flicker. It has a flat screen
Disadvantages of DVST
Erasing requires removal of entire picture on the storage grid. Thus erasing & redrawing
process take several seconds.
Erasing of selected portion of the screen is not possible. Due to these disadvantages storage
displays have been largely replaced by raster system.
Flat-Panel Display
The flat-panel display refers to a class of video devices that have reduced volume, weight and
power requirement compared to the CRT. You can hang them on walls or wear them on your
wrists. Current uses for flat-panel displays include calculators, video games, monitors, laptop
computer, graphics display.
The flat-panel display are divided into two categories:
 Emissive Displays - The emissive displays are devices that convert electrical energy into
light. Example are plasma panel and LED (Light-Emitting Diodes).
 Non-Emissive Displays - The Non-emissive displays use optical effects to convert sunlight
or light from some other source into graphics patterns. Example is LCD (Liquid-Crystal
Device)
Liquid-Crystal Display
Each pixel of an LCD typically consists of a layer of molecules aligned between two
transparent electrodes, and two polarizing filters (parallel and perpendicular), the axes of
transmission of which are (in most of the cases) perpendicular to each other. Without the
liquid crystal between the polarizing filters, light passing through the first filter would be
blocked by the second (crossed) polarizer.
Before an electric field is applied, the orientation of the liquid-crystal molecules is
determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still
the most common liquid-crystal device), the surface alignment directions at the two
electrodes are perpendicular to each other, and so the molecules arrange themselves in a
helical structure, or twist. This induces the rotation of the polarization of the incident light,
and the device appears gray. If the applied voltage is large enough, the liquid crystal
molecules in the center of the layer are almost completely untwisted and the polarization of
the incident light is not rotated as it passes through the liquid crystal layer. This light will
then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel
will appear black. By controlling the voltage applied across the liquid crystal layer in each
pixel, light can be allowed to pass through in varying amounts thus constituting different
levels of gray.
LCD with top polarizer removed from device and placed on top, such that the top and bottom
polarizers are parallel.
The optical effect of a twisted nematic device in the voltage-on state is far less dependent on
variations in the device thickness than that in the voltage-off state. Because of this, these
devices are usually operated between crossed polarizers such that they appear bright with no
voltage (the eye is much more sensitive to variations in the dark state than the bright state).
These devices can also be operated between parallel polarizers, in which case the bright and
dark states are reversed. The voltage-off dark state in this configuration appears blotchy,
however, because of small variations of thickness across the device.
www.knowledgepath.in
Both the liquid crystal material and the alignment layer material contain ionic compounds. If
an electric field of one particular polarity is applied for a long period of time, this ionic
material is attracted to the surfaces and degrades the device performance. This is avoided
either by applying an alternating current or by reversing the polarity of the electric field as the
device is addressed (the response of the liquid crystal layer is identical, regardless of the
polarity of the applied field).
Displays for a small number of individual digits and/or fixed symbols (as in digital watches
and pocket calculators) can be implemented with independent electrodes for each segment. In
contrast full alphanumeric and/or variable graphics displays are usually implemented with
pixels arranged as a matrix consisting of electrically connected rows on one side of the LC
layer and columns on the other side, which makes it possible to address each pixel at the
intersections. The general method of matrix addressing consists of sequentially addressing
one side of the matrix, for example by selecting the rows one-by-one and applying the picture
information on the other side at the columns row-by-row.
Cathode Ray Tubes
Liquid Crystal Displays (LCDs)
Advantages
 Fast repsonse (high resolution possible)
 Full color (large modulation depth of Ebeam)
 Saturated and natural colors
 Inexpensive, matured technology
 Wide angle, high contrast and brightness
Advantages
 Small footprint (approx 1/6 of CRT)
 Light weight (typ. 1/5 of CRT)
 Low power consumption (typ. 1/4 of
CRT)
 Completely flat screen - no geometrical
errors
 Crisp pictures - digital and uniform colors
 No electromagnetic emission
 Fully digital signal processing possible
 Large screens (>20 inch) on desktops
Disadvantages
 Large and heavy (typ. 70x70 cm, 15 kg)
 High power consumption (typ. 140W)
 Harmful DC and AC electric and
magnetic fields
 Flickering at 50-80 Hz (no memory
effect)
 Geometrical errors at edges
Disadvantages

High price (presently 3x CRT)

Poor viewing angle (typ. +/- 50 degrees)

Low contrast and luminance (typ. 1:100)

Low luminance (typ. 200 cd/m2)
Plasma Display Panel
Plasma displays are bright, have a wide color gamut, and can be produced in fairly large
sizes, up to 262 cm (103 inches) diagonally. They have a very low-luminance "dark-room"
black level, creating a black some find more desirable for watching movies. The display
panel is only about 6 cm (2½ inches) thick, while the total thickness, including electronics, is
less than 10 cm (4 inches).
Plasma displays use as much power per square meter as a CRT or an AMLCD television.
Power consumption will vary greatly depending on what is watched on it. Bright scenes (say
a football game) will draw significantly more power than darker scenes. The xenon and neon
www.knowledgepath.in
gas in a plasma television is contained in hundreds of thousands of tiny cells positioned
between two plates of glass.
Long electrodes are also sandwiched between the glass plates, in front of and behind the
cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent
display electrodes, which are surrounded by an insulating dielectric material and covered by a
magnesium oxide protective layer, are mounted in front of the cell, along the front glass plate.
Control circuitry charges the electrodes that cross paths at a cell, creating a voltage difference
between front and back and causing the gas to ionize and form a plasma; as the gas ions rush
to the electrodes and collide, photons are emitted.
In a monochrome plasma panel, the ionizing state can be maintained by applying a lowlevel
voltage between all the horizontal and vertical electrodes – even after the ionizing voltage is
removed. To erase a cell all voltage is removed from a pair of electrodes. This type of panel
has inherent memory and does not use phosphors. A small amount of nitrogen is added to the
neon to increase hysteresis. In color panels, the back of each cell is coated with a phosphor.
The ultraviolet photons emitted by the plasma excite these phosphors to give off colored
light. The operation of each cell is thus comparable to that of a fluorescent lamp. Every pixel
is made up of three separate sub pixel cells, each with different colored phosphors. One sub
pixel has a red light phosphor, one sub pixel has a green light phosphor and one sub pixel has
a blue light phosphor. These colors blend together to create the overall color of the pixel,
analogous to the "triad" of a shadowmask CRT.
Thin Film Electro-Luminescent Display
Electroluminescence is the result of radiative recombination of electrons and holes in a
material (usually a semiconductor). The excited electrons release their energy as photons light. Prior to recombination, electrons and holes are separated either as a result of doping of
the material to form a p-n junction (in semiconductor electroluminescent devices such as
LEDs), or through excitation by impact of high-energy electrons accelerated by a strong
electric field (as with the phosphors in electroluminescent displays).
Powder phosphor-based electroluminescent panels are frequently used as backlights to liquid
crystal displays. They readily provide a gentle, even illumination to the entire display while
consuming relatively little electric power. They do, however, require relatively high voltage.
Recently, blue, red and green emitting thin film electroluminescent materials have been
developed that offer the potential for long life and full color electroluminescent displays. In
either case, the EL material must be enclosed between two electrodes and at least one
electrode must be transparent to allow the escape of the produced light.
Glass coated with indium oxide or tin oxide is commonly used as the front (transparent)
electrode while the back electrode is or is coated with reflective metal. Additionally, other
transparent conducting materials, such as carbon nanotubes coatings or PEDOT can be used
as the front electrode. Unlike neon and fluorescent lamps, EL lamps are not negative
resistance devices so no extra circuitry is needed to regulate the amount of current flowing
through them. In principle, EL lamps can be made in any color. EL devices have low power
consumption when compared with neon signs, and have a wide range of applications such as
their use on advertising boards and safety signs.
Light-Emitting Diode (LED)
A light-emitting diode (LED) is a semiconductor diode that emits incoherent narrowspectrum light when electrically biased in the forward direction of the p-n junction. This
effect is a form of electroluminescence. An LED is usually a small area source, often with
extra optics added to the chip that shapes its radiation pattern. The color of the emitted light
www.knowledgepath.in
depends on the composition and condition of the semi conducting material used, and can be
infrared, visible, or near-ultraviolet. An LED can be used as a regular household light source.
Like a normal diode, an LED consists of a chip of semi conducting material impregnated, or
doped, with impurities to create a p-njunction. As in other diodes, current flows easily from
the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Chargecarriers—electrons and holes—flow into the junction from electrodes with different voltages.
When an electron meets a hole, it falls into a lower energy level, and releases energy in the
form of a photon. The wavelength of the light emitted, and therefore its color, depends on the
band gap energy of the materials forming the p-n junction.
In silicon or germanium diodes, the electrons and holes recombine by a non-radiative
transition which produces no optical emission, because these are indirect band gap materials.
The materials used for an LED have a direct band gap with energies corresponding to nearinfrared, visible or near-ultraviolet light. LEDs are usually built on an n-type substrate, with
an electrode attached to the ptype layer deposited on its surface.
P-type substrates, while less common, occur as well. Many commercial LEDs, especially
GaN/InGaN, also use sapphire substrate. Substrates that are transparent to the emitted
wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive
index of the package material should match the index of the semiconductor, otherwise the
produced light gets partially reflected back into the semiconductor, where it may be absorbed
and turned into additional heat, thus lowering the efficiency. An anti-reflection coating may
be added as well.