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Computer Graphics display technologies(Computer graphics tutorials)

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Computer Graphics display technologies(Computer graphics tutorials)

  1. 1. CRT (Cathode Ray Tube) Control grid – determines the rate at which the electron will pass thro. Electron beam- electrons travel without any hindrance from the air/dust as the tube is a vacuum. Phosphor coated screen – It glows when struck by electrons. conductive coating - to soak up the electrons that pile up at the screen-end of the tube. Focusing anode – It attracts scattered electron to a focal point. Accelerating anode – It gives the anode a high velocity so that we can use the velocity/momentum to give the light we want. 1
  2. 2. CRT Monitors A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image. In a cathode ray tube, the "cathode" is a heated filament. The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons generated by an electron gun that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. This screen is coated with phosphor, an organic material that glows when struck by the electron beam. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. There are three ways to filter the electron beam in order to obtain the correct image on the monitor screen: shadow mask, aperture grill and slot mask. These technologies also impact the sharpness of the monitor's display. Let's take a closer look at these now. 1. Shadow-mask A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display. The unwanted beams are blocked or "shadowed." 2. Aperture-grill Monitors based on the Trinitron technology, which was pioneered by Sony, use an aperture-grill instead of a shadow-mask type of tube. The aperture grill consists of tiny vertical wires. Electron beams pass through the aperture grill to illuminate the phosphor on the faceplate. Most aperture-grill monitors have a flat faceplate and tend to represent a less distorted image over the entire surface of the display than the curved faceplate of a shadow-mask CRT. However, aperture-grill displays are normally more expensive. 2
  3. 3. 3. Slot-mask A less-common type of CRT display, a slot-mask tube uses a combination of the shadow-mask and aperture-grill technologies. Rather than the round perforations found in shadow-mask CRT displays, a slot-mask display uses vertically aligned slots. The design creates more brightness through increased electron transmissions combined with the arrangement of the phosphor dots. Advantages of phosphor - electron r easily knocked off to give light - once electrons starts losing energy, phosphor stay glowing for some time – persistence PERSISTENCE Different kind of phosphor are used in a CRT. Each has a diff time durations during which it continues to emit light after the beam is removed. Persistence is defined as the time it takes for the emitted light from the screen to decayto 1/10th of its origin in intensity. Lower persistence phosphor require higher refresh rates to maintain a picture on the screen w/o flicker. The phosphor with low persistence is useful for animation. A high persistence phosphor is useful for displaying high complex static pictures. Resolution. The max No. of points that can b displayed on the screen on a CRT w/o overlap is called resolution. Typical resolution of high definition system is 1280 by 1024. Screen size. The physical size of a graphics monitor is given by the length on the screen diagonally n normally quoted in inches. Aspect Ratio It gives the ratio of vertical points to horizontal points necessary to produce equal length lines in both direction of the screen. 3
  4. 4. raster graphics a raster graphics image or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a display medium.. A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent). In a raster scan display system the electron beam is swept across the screen one row at a time from top to bottom and from left to right. As the elec beam moves across each row, the beam intensity is turned on or off to create a pattern of illuminated spots. The spots to be turned on are dependent on the picture to be drawn. The definition of this picture is stored in a memory area called the refresh buffer or frame buffer. 4
  5. 5. This memory area holds the intensity valuesfor all screen points. The intensity values are read from memory area and used to ‘paint’ each point on the screen one row at a time. This row is called a scan line and each point is called a pixel (picture element). The ordering of pixels by rows is known as raster order, or raster scan order... Rasterization The term rasterization can in general be applied to any process by which vector information can be converted into a raster format. In normal usage, the term refers to the popular rendering algorithm for displaying three-dimensional shapes on a computer. Rasterization is currently the most popular technique for producing real-time 3D computer graphics. Real-time applications need to respond immediately to user input, and generally need to produce frame rates of at least 20 frames per second (and preferably 60 fps or more). Compared to other rendering techniques such as ray tracing, rasterization is extremely fast. However, rasterization is simply the process of computing the mapping from scene geometry to pixels and does not prescribe a particular way to compute the color of those pixels. Interlacing Its a method of encoding a bitmap image such that a person who has partially received it sees a degraded copy of the entire image. When communicating over a slow communications link, this is often preferable to seeing a perfectly clear copy of one part of the image, as it helps the viewer decide more quickly whether to abort or continue the transmission. Interlacing is supported by the following formats: • GIF • PNG • JPEG 5
  6. 6. • PGF Interlacing is also known as "progressive" encoding, because the image becomes progressively clearer as it is received. FLAT PANEL DISPLAY PLASMA PANEL The basic idea of a plasma display is to illuminate tiny, colored fluorescent lights to form an image. Each pixel is made up of three fluorescent lights -- a red light, a green light and a blue light. What is Plasma? The central element in a fluorescent light is a plasma, a gas made up of free-flowing ions (electrically charged atoms) and electrons (negatively charged particles). Under normal conditions, a gas is mainly made up of uncharged particles. That is, the individual gas atoms include equal numbers of protons (positively charged particles in the atom's nucleus) and electrons. The negatively charged electrons perfectly balance the positively charged protons, so the atom has a net charge of zero. If you introduce many free electrons into the gas by establishing an electrical voltage across it, negatively charged particles rush toward the positively charged area of the plasma, and positively charged particles rush toward the negatively charged area. In this mad rush, particles are constantly bumping into each other. These collisions excite the gas atoms in the plasma, causing them to release photons of energy How the system works Its composed of 2 glass plates. The 1st plate is brought to the 2nd plate until the space btn them is small. The edges are sealed off and space is left with air. The air inside is then removed and replaced with the plasma gas (e.g neon). Properties of the gas - Must produce light when ionized 6
  7. 7. - Must be easily ionized - Produce the correct color of gas when ionized. LIQUID CRYATAL DISPLAY. In a LCD display, there are 2 polarizers i.e vertical and horizontal polarizer. A polarizer is a component that filters light. A vertical polarizer filters vertical component of light and allow horizontal component of light thro. As light strikes the first filter, it is polarized. The molecules in each layer of the liquid crystal then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through. For a particular voltage the liquid material at that intersection of electrons changes the orientation of that liquid crystal of that intersection. The horizontal component is converted to a vertical component hence transmit light. If we apply an electric charge to liquid crystal molecules, they untwist. When they straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. Consequently, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas. INTERACTIVE DEVICES They are devices that help in input of data in the system and also help in giving out the processed information. They include: Mouse 7
  8. 8. Space balls – right handed co-ordinate system. The space ball doesn’t move. It has a strain gauge that measure the amount of pressure applied to the space ball to provide input. Trackball – it’s an upside down mouse Touch pad Touch panel LOGICAL INPUT DEVICES The need for interactive comp graphics sys has resulted in the dev of diff kinds of physical interactive devices which function differently from each other. The actions of pointing, selecting, sketching, placing or erasing in a direct manner and the instantaneous sys response to this action make the interaction possible and natural – however it becomes diff for a comp graphics application to deal with all the diff kinds of operating sys. Most sys therefore introduce an interface btn the application program and the sys itself with the peculiarities of diff interactive devices. To make graphical packages independent of the devices used to input, functions can be structured according to the data description to be handled by each function. This approach provides a logical input devices classification in terms of the kind of data to be input by the device. 1. Locator – it’s a device for specifying a co-rdinate position x,y, in world co-ord input to the graphics package by the user selecting a point. 2. stroker – a device for specifying a series of cor-ord positions 3. string – a device for specifying input text 4. valuater – a device for specifying scalar values. 5. choice – a device for selecting menu options 6. pic – a device for selecting picture components. 8

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