Computer Graphics can use many different output devices, such as monitors, printer, plotter, etc. but the most common display device is the Cathode Ray Tube (CRT) monitor.
In a CRT the focusing system acts like a light lens with a focal length such that the center of focus is the screen. The horizontal and vertical deflectors allow the electron beam to be focused on any spot on the screen. The screen is coated with a special organic compound called a phosphor. For color systems there are groups of three different phosphors, one to produce red shades, one for green shades, and one for blue shades.
Electrons hit the screen phosphor molecules and cause a ground state to singlet excited state transition. Most of the phosphors relax back to the ground state by emitting a photon of light which is called fluorescence. This happens very rapidly so that all of the molecules which fluoresce do so in under a millisecond. Some of the molecules convert from an excited singlet state to an excited triplet state in a process called inter-system crossing. This is quantum mechanically forbidden, which means it still happens but only by a relatively small fraction of the phosphors and is a slower process. These phosphors then emit light, called phosphorescence, that decays slower but still rapidly (in about 15-20 milliseconds) so there is the need to refresh the screen by redrawing the image.
The physical process is depicted below:
Phosphors are characterized by color (usually red, green, or blue) and persistence, which is the time for the emitted light to decay to 10 % of the initial intensity. High persistence phosphors allow for a lower refresh rate to avoid flicker, e.g., the original IBM PC monochrome monitor had a high persistence phosphor. This allowed it to have good resolution for text with inexpensive electronics. However, this is poor for animation since a "trail" is left with moving objects.
Low persistence phosphors are good for animation but require a high refresh rate to prevent flicker. A refresh rate of 50 - 60 Hz is usually sufficient to prevent flicker, but some systems refresh at even higher rates such as 72-76 Hz.
The user can vary the voltage on the control grid to attenuate the electron flow. This is done with the "intensity" knob:
high - voltage --> few e- (are repelled)
low or 0 - voltage --> high number of e-
The maximum number of points that can be displayed without overlap is called the resolution and is usually given as the number of horizontal points versus the number of vertical points These are called pixels (picture elements), e.g. a monitor might have a resolution of 1024 X 768 pixels.
The maximum resolution may be determined by the characteristics of the monitor for a random scan system or by a combination of the monitor and graphics card memory for a raster scan system. For a random scan system the resolution can be up to 4096 X 4096
The aspect ratio equals the ratio of vertical pixels/horizontal pixels for an equal length line. It sometimes refers to the ratio of the horizontal dimension/vertical dimension. Examples:
If have the above monitor and a resolution of 640 X 480 pixels
==> horizontal --> 640/8 = 80 pixels/inch ==> vertical --> 480/6 = 80 pixels/inch Therefore, it has "square" pixels, i.e. the same size in the vertical and horizontal directions. If the resolution is 320 X 200 pixels, then: ==> horizontal --> 320/8 = 40 pixels/inch ==> vertical --> 200/6 = 33 1/3 pixels/inch
So the size of a horizontal pixel does not equal the size of a vertical pixel. We must correct for this in our image display, e.g. line or object drawing. Otherwise all of the drawings will be distorted.
Computer graphics systems can be random scan or raster scan.
In a Random Scan system, also called vector, stroke-writing, or calligraphic the electron beam directly draws the picture.
Advantages of random scan:
A large technology advance was the modified random scan system, called the Direct View Storage Tube (DVST) system from Tektronix in the late 1960's.
The primary electron gun draws the picture by knocking out electrons from the storage grid, producing a positively charged pattern. Low speed electrons from the flood gun are attracted to the storage grid, and pass through the positively charge pattern spots (past the collector grid) to hit the screen. So once a picture is drawn it stays until erased by putting a charge over all of the storage grid so that the electrons hit all of the screen (producing a green flash on a green screen monitor). There is no animation or color capability but a very complex, high density image is possible. The DVST was much cheaper then a regular random scan system since it did not require a built in CPU (the host computer could be used to draw the image once).
Note that a Pen - Plotter and Laser Light Show are also random scan display devices.
A Raster Scan device scans the screen from top to bottom in a regular pattern. This is common TV technology.
The electron beam is turned on/off so the image is a collection of dots which are painted on screen one row (scan line or raster line) at a time. Note: A raster is a matrix of pixels covering the screen (output display) area and consists of raster lines.
Question: How to tell the electron beam when to turn on/off?
Answer: We must store the pattern in a special graphics memory area called a frame buffer (also called a bit map) where each memory location corresponds to a pixel. So have display processor scan memory and then turn electron beam on/off depending if bit is 1 or 0. We may have an interlaced scan, i.e., the first scan does the even lines 0,2,4,... then the second scan does the odd lines 1,3,5,... Must do at least > 45 Hz or else get flicker.
Look at the memory requirements for a monochrome system, where a pixel is either on or off: 640 X 480 resolution --> 307,200 bits/8 bits/byte = 38,400 bytes. In the 1960's and 70's, memory was very expensive (>= $1,000,000/Mbyte) so there were few raster scan systems, mostly random scan. Today with memory inexpensive, most graphics systems (99%) are raster scan.
There may be several bits/pixel for color or gray scale intensity, e.g., 8 bits/pixel (0 --> 255) gives 256 colors. This is sometimes called 8 bit planes. So the memory requirements for 1024 x768 with 8bits/pixel is 768 K bytes.
Look at speed requirements for memory access:
60 Hz for 1024 x 768 X 8 = 768 Kbytes = 786,432 bytes
Read 786 X 10^3 bytes in 1600 X 10^-5 sec.
or 1 byte in 2 X 10-8 sec. = 20 ns
Also, look at rough estimation of scanning rates: frequency X number
of vertical lines:
Example: for an IBM VGA 60 X 480 = 30 KHz; for 1024 x 768 --> 60 X 768 = 46 Khz
PC's all use raster scan and they can easily do color/shading
Look at color CRT - use different phosphors, usually Red, Green, Blue then combine.
Can use 2 methods:
First method (random scan) : beam penetration - have 2 layers (R,G) phosphors. Low speed e- excite R, very high speed e- excite G, intermediate do both so get yellow or orange-- so control e- beam voltage.
Second Method (raster scan) uses a shadow mask with three electron guns: red, green, & blue.
So by combining different intensities from guns can get different colors (more later).
Example: R G B 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
More expensive systems will have multiple levels of each color, not just on/off. They do this with Digital to Analog Converters (DACs). This converts a digital number, e.g., from 0 to 255, to a voltage, e.g., from 0.0 to 5.0 volts. So, for example, a color intensity of 100 would be converted to a voltage of about 100/255 * 5.0 = 1.96 volts. This is the voltage that would be applied to that color's electron gun so about 40% of the maximum possible value of that color's phosphors would be hit by electrons.
Note : Printers are also raster scan devices and can give fairly high quality output (dot matrix or laser).
Look at frame buffer for VGA 640 X 480 monochrome resolution --> 1 bit/pixel (on/off) 640/8 = 80 bytes/row X 480 rows = 28,800 bytes so the central processor performs the scan conversion of the scene, writes 1's, 0's into the frame buffer and then the display processor scans frame buffer and turns electron gun on/off.
Overall Hardware System for interactive graphics.
For random scan the host system generates a display file of graphics
commands which is executed by the display processor. For cheap, e.g. PC's, raster scan
systems the host processor performs can conversion (from mathematical model to a frame
buffer image). The display processor then just reads the frame buffer and turns the
electron beams on/off. More expensive raster scan systems will have a graphics processor
that performs some scan conversion.
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