Now, the functions and applications of computer graphics have been widely recognized, and a large number of graphics hardware and software systems have been applied to almost all fields. General-purpose computers and even many handheld calculators are already commonly equipped with graphics capabilities for 2D and 3D applications. A variety of interactive input devices and graphics software packages can also be used on a personal computer. For high-performance applications, there are many advanced dedicated graphics hardware systems and technologies to choose from. This chapter explores the basic characteristics of graphics hardware and graphics software packages.
1 video display device
Graphics systems typically use a video display as their primary output device. Historically, the operation of most video monitors has been based on the standard Cathode Ray Tube (CRT) design, but some other technologies have also emerged. become more and more popular.
1.1 Refresh CRT
Figure 2.1 shows the basic working principle of CRT. The electron beam (cathode ray) emitted by the electron gun passes through the focusing system and the deflection system, and shoots to the designated position on the screen coated with the phosphor layer. At each location hit by the electron beam, the phosphor layer produces a small bright spot. Since the light emitted by the phosphor decays quickly, there must be some way to preserve the screen image. One approach is to store the graphics information on the CRT as a charge distribution. This charge distribution is used to keep the phosphor in an activated state. But the way to maintain the brightness of the phosphor is to quickly control the electron beam to redraw the image repeatedly. This type of display is called a refresh CRT (refresh CRT), and the frequency at which pictures are drawn repeatedly on the screen is called the refresh rate.
The main components of a CRT electron gun are a thermally excited metal cathode and a control grid (see Figure 2.2). The cathode is heated by energizing a coil called the filament, causing heated electrons to "boil off" the surface of the cathode. In the vacuum inside the CRT package, negatively charged free electrons accelerate towards the fluorescent screen under the action of a higher positive voltage. This accelerating voltage can be generated by a positively charged metal coating in the CRT package near the phosphor screen, or by using an accelerating anode (see Figure 2.2). Sometimes, the electron gun structure puts the accelerating anode and the focusing system in the same part.
The intensity of the electron beam is controlled by the voltage level set on the control grid. The control grid is a metal cylinder mounted next to the cathodes. If a higher negative voltage is applied to the control grid, it will prevent electron activity and thus cut off the electron beam, so that it will stop passing through the small hole at the end of the control grid. Applying a lower negative voltage to the control grid simply reduces the number of electrons passing through. Since the intensity of light emitted by the fluorescent layer depends on the number of electrons that hit the screen, the intensity of the displayed light can be controlled by changing the voltage of the control grid. We use graphics software commands to set brightness levels at various screen locations, as discussed in Chapter 3
The focus system of the CRT is used to control the electron beam to converge to a small point when it bombards the phosphor layer. Otherwise, the electron beam would spread out as it gets closer to the screen because the electrons repel each other. Focusing can be achieved with either an electric field or a magnetic field. For electrostatic focusing the electron beam passes through a positively charged metal cylinder as shown in Figure 2.2, which forms an electrostatic lens. The purpose of the electrostatic lens is to focus the electron beam at the center of the screen, just as an optical lens focuses the beam at a specified focal length. The lens-like focusing effect can be accomplished by a magnetic field formed around a coil mounted on the outside of the CRT package. A magnetic focusing lens produces the smallest bright spots on the screen.
In high-precision systems, additional focusing hardware is used to keep the electron beam focused to all screen positions. Because the diameter of the curved portion of most CRTs is greater than the distance from the focusing system to the center of the screen, the electron beams travel different distances to different points on the screen. Therefore, the electron beam can only be properly focused at the center of the screen. When the electron beam moves past the borders of the screen, the displayed image becomes blurred. The system compensates for this by adjusting the focus according to the screen position of the electron beam.
The deflection of the electron beam is controlled by an electric or magnetic field. A CRT is usually equipped with a magnetic deflection coil mounted outside its package, as shown in Figure 2.1. Two pairs of coils are used, mounted in pairs on the neck of the CRT package, one pair mounted on the top and bottom of the neck, and the other on both sides of the neck. The magnetic field generated by each pair of coils creates a lateral deflection force that is normal to the direction of the magnetic field and also perpendicular to the direction of travel of the electron beam. One pair of coils provides horizontal deflection and the other pair provides vertical deflection. The proper amount of deflection can be obtained by adjusting the current through the coil. When electrostatic deflection is used, two pairs of row plates are installed in the CRT package. One pair is placed horizontally to control vertical deflection; the other pair is placed vertically to control horizontal deflection (see Figure 2.3).
By transferring the energy of the CRT electron beam to the phosphor layer, bright spots can be formed on the screen. When the electrons of the electron beam hit the phosphor layer and stop moving, their kinetic energy is absorbed by the phosphor layer. Part of the energy of the electron beam is converted into heat energy due to friction, and the remaining part causes the electrons of the fluorescent layer atoms to jump to a higher quantum energy level. After a short period of time, the "activated" phosphor electrons release small quanta of light energy and begin to fall back to their stable state. What we see on screen is all the TV
Combination effect of sub-light emission: After the luminous point transfers to its own basic energy level with all the activated fluorescent layer electrons, it will quickly attenuate the frequency (or color) of the light emitted by the fluorescent layer and the distance between the activated quantum state and the basic state. proportional to the energy level difference. There are different types of fluorescent layers used in CRTs. Aside from color, the main difference between these phosphors is their persistence time: how long they will continue to emit light (i.e., activate electrons to return to the ground state) after the CRT electron beam is removed. Persistence time is defined as the time it takes for a screen to emit light until it decays to one-tenth of its original brightness. A phosphor layer with a short afterglow time requires a high refresh rate to keep the screen graphics from flickering. Short-persistence phosphor layers are suitable for animation, while long-persistence phosphor layers are suitable for displaying high-complexity static graphics. Although the afterglow time of some fluorescent layers is longer than one second, graphics monitors are usually made of materials with an afterglow time of 10-60ms.
Figure 2.4 shows the brightness distribution of a bright spot on the screen. The brightness at the center of the bright spot is the largest, and it decays to the edge of the bright spot according to Gaussian distribution. This distribution depends on the electron density distribution of the CRT electron beam cross-section. The maximum number of points displayed by a CRT without overlapping is called resolution. Although it is often stated simply as the total number of dots per direction, a more precise definition of resolution is the number of drawable dots per centimeter in the horizontal and vertical directions. The intensity of the bright spot satisfies the Gaussian distribution (see Figure 2.4) so the distance between two adjacent bright spots should be greater than the diameter when the bright spot intensity is at 60% of the maximum intensity value. This overlay location is shown in Figure 2.5. Bright spot size also depends on brightness. When more electrons are accelerated to fly to the fluorescent layer per second, the diameter of the CRT electron beam and the area of the bright spot increase. In addition, the increased activation energy tends to spread to neighboring fluorescent atoms rather than directly into the path of the electron beam, further increasing the bright spot diameter. Therefore, the resolution of a CRT depends on the type of phosphor layer, the brightness of the display, the focusing system, and the deflection system. A typical high-quality system resolution is 1280x1024, and higher resolutions are used in many systems. High-resolution systems are often referred to as high-definition systems. The physical size of a graphics monitor is given by the length of the diagonal of the screen. Available from 12 inches (1 inch = 2.54 cm) to 27 inches or larger. A CRT monitor can be connected to various computer systems, so the actual number of screen points that can be drawn depends on the capabilities of the system it is connected to.
1.2 Raster Scan Displays
A common graphics monitor using a CRT is a raster-scan display based on television technology. In a raster-scanning system, an electron beam scans across the screen, one row at a time, from top to bottom. Each line is called a scan line (scan line). As the beam moves laterally along each row, the intensity of the beam varies, creating a pattern of bright spots. Graphics definitions are stored in memory called a refresh buffer or frame buffer, where a frame refers to the entire screen. This memory holds a set of intensity values corresponding to all points on the screen. As the electron beam moves point by point on the screen, its intensity is controlled by the intensity value fetched from the refresh buffer. In this way, as shown in Figure 2.6, "drawing" on the screen is one line at a time. Each screen point that can be illuminated by an electron beam is called a pixel (pixel or pel is an abbreviation for pictureelement). Since the refresh buffer is used to store screen color values, it is also called a color buffer. In addition to color, other information about pixels is also stored in the buffer, so the different buffer areas are sometimes collectively referred to as "frame buffer". Raster scan systems have the ability to store intensity information for every point of the screen, making them well suited for realistic display of scenes containing subtle shadows and color patterns. Home televisions and printers are another example of a raster scan approach.
Raster systems are often characterized by the number of pixels called resolution. Another characteristic of video displays is the aspect ratio, defined as the number of pixel columns that a system can display divided by the number of rows (sometimes the term aspect ratio is used to mean the number of scanned rows divided by the number of pixel columns). The aspect ratio can also be described by the ratio of the number of points required to display a line segment of the same length horizontally and vertically on the screen. Therefore, an aspect ratio of 4/3 means that a horizontal line drawn with 4 points has the same weight as a straight line drawn with 3 points. Physical length, such as the same number of centimeters. Similarly, the aspect ratio of any rectangle (including the entire screen) can be described by dividing its width by its height.
The colors or shades of gray that can be displayed by a raster system depend on the type of phosphor used by the CRT and the number of bits in the frame buffer for each pixel. For a simple black and white system, each screen point is either bright or dark, so only one bit per pixel is needed to control the brightness at that screen location. The value of this bit is 1, which means that the electron beam is turned on when it is at this position, and the value of this bit is 0, which means that the electron beam is turned off when it is at this position. Additional bits need to be provided if the electron beam is to have more intensity levels than the "on" and "off" states. Up to 24 bits per pixel in a high-performance system, a screen with a resolution of 1024 x 1024 would use a refresh cache of 3MB. The number of bits per pixel is also sometimes called the buffer depth or the number of bit planes. A frame buffer with one bit per pixel is usually called a bitmap, and a buffer with multiple bits per pixel is called a pixmap, but these terms are also used to describe an array of arbitrary binary values or a color array of pixmaps.
When the refresh rate is not too low, we will feel that the content of two adjacent frames is a smooth transition during the refresh process. Below 24 frames per second, we can perceive a gap between adjacent images on the screen, that is, the image flickers. For example, early silent films were filmed at 16 frames per second and thus exhibited flickering. When sound systems were developed in the 1920s, film rates had been increased to 24 frames per second, thus eliminating flicker and erratic movements of actors. Early raster computer systems were designed at 30 refreshes per cycle and thus produced good results, but since display technology on monitors is fundamentally different from film, picture quality improvements also depended on higher refresh rates on monitors frequency. Movie projectors can maintain continuity in the display by continuing to show one frame until the next frame starts showing. But on a video monitor, the phosphor dots begin to decay immediately after lighting up. Therefore, most scanning displays now use a refresh rate of 60-80 per second and some systems achieve a refresh rate of 120 per second. Some graphics systems are designed to use variable refresh rates. For example, choose a high refresh rate for stereoscopic applications so that there is no flickering when alternating between two views of the scene. This type of application usually uses the approach of multiple framebuffers.
Sometimes the refresh rate is described in cycles per second, or Hertz (HZ), where one cycle corresponds to one frame. Therefore, we can simply refer to a refresh rate of 60 per second as 60 H. At the end of each scan line, the electrons return to the left side of the screen, and then begin to display the next scan line. After refreshing each scan line, the electron beam returns to the left end of the screen, which is called the horizontal retrace of the electron beam. And at the end of each (1/80 to 1/60 display of one second), the electron beam returns (vertical retrace, vertical retrace) to the upper left corner of the screen, and begins to display the next frame.
In some raster scanning systems and TVs, an interlaced (interlaced) refresh method is used to display each image twice. For the first time, the electron beam scans from top to bottom, row by row. After the vertical retrace, the electron beam scans the other half of the scan lines (see Figure 2.7). This interlacing method allows the entire screen to be viewed in half the time required for progressive scanning. Interlacing technology is mainly used for slower refresh rates. For example, flickering may be noticed on an old 30 fps non-interlaced monitor. However, with interlaced scanning, each of the two scans can be done in 1/60 of a second, ie the refresh rate is close to 60 per second. This is an effective technique to avoid flicker and provide adjacent scan lines containing similar display information
1.3 Random scan display
When CRT is used for random scan display (randoms scandis-play), its electron beam only moves on the part where the graphics are displayed on the screen. The electron beam traces the constituent lines of the figure one by one, creating a line drawing. Therefore, random scan displays are also called vector displays, stroke-writing displays or calligraphic displays. The constituent lines of the graph are drawn and refreshed in any specified order by a random scanning system (see Figure 2.8). A pen plotter works in a similar fashion and is an example of a random scanning, hardcopy device.
The refresh rate of a random scan system depends on the number of lines displayed. At this time, the definition of graphics is a group of line drawing commands stored in the storage area called refresh display file. A refresh display file is called a display list, a refresh display file, a vector file, or a display program. In order to display the specified graphics, the system periodically draws its constituent lines sequentially according to a set of commands in the display file. When all line drawing commands are processed, the system periodically returns to the first line drawing command in the list.
Random scan monitors are designed to draw all the lines of the graph 30 to 60 times per second. A high performance vector system can handle about 100,000 short lines at this refresh rate. When few lines are displayed, each refresh cycle is delayed to avoid the refresh rate exceeding 60 per second. Otherwise, the refresh of the lines is too fast, which may burn out the fluorescent layer. Random scan systems are used for line drawing applications such as architectural and engineering layouts, which cannot display realistic shaded scenes. Because the graphics definition is stored as a set of line drawing commands rather than intensity values for all screen points, vectordisplays generally have higher resolution than raster systems. In addition, the CRT electron beam of the vector display draws the line directly on the line path, thus producing smooth lines. In contrast, raster systems draw lines by displaying a set of discrete points, resulting in jagged lines. However, the great flexibility and increased line-drawing capabilities of raster systems made vector technology obsolete.
1.4 color CRT monitor
CRT monitors display color graphics using a combination of phosphor layers that emit different colors. The light emitted from the different phosphor layers combine to produce a visible color in proportion to it.
One way to display color graphics is to coat the screen with multiple layers of different phosphors. The emission color is determined by the penetration depth of the electron beam in the phosphor layer. This method is called beam-penetration , which is commonly used in red and green two-layer structures. The slow electron beam only activates the outer red layer, and the fast electron beam can pass through the red layer and activate the inner layer. green layer. The medium-speed electron beam produces two additional colors by emitting a combination of red and green light: orange and yellow. The speed of the electrons, which is the color at any point on the screen, is controlled by the accelerating voltage of the electron beam. Electron beam penetration is an inexpensive way to produce color graphics for random scan monitors, but only in a smaller variety of colors and the graphics quality is not as good as other methods.
The shadow-mask method is often used in raster scanning systems (including color televisions) because it can produce a much larger range of colors than the electron beam penetration method. This method is based on the familiar principle of combining colors from the three primary colors of red, green, and blue, which is called the RGB color model (RGB color model). For each pixel location, the shadow mask CRT has three phosphor colored dots: one phosphor dot emits red light, another emits green light, and the third emits blue light. These CRTs have three electron guns, one for each color dot, and a shadow mask grid located just behind the phosphor-coated screen. Since the human eye combines light from three points into one combined color, the light emitted by the three phosphors creates a small point of color at the pixel location. Figure 2.9 shows the delta-delta shadow mask method commonly used in color CRT systems. Three of the electron beams are deflected, focused, and emitted onto the shadow mask together. The shadow mask has a series of holes arranged in a pattern of phosphor dots. When three electron beams pass through holes in the shadow mask, a dot triangle is activated, displaying a small colored dot on the screen. Phosphor dots are arranged in a triangle, and when each electron beam passes through the shadow mask, only the corresponding color dot can be activated. Another configuration of the three electron guns is arranged in-line. Among them, the three electron guns and the corresponding RGB color dots on the screen are arranged along the scan lines rather than in a triangular pattern. The line arrangement of this electron gun is easy to maintain alignment and is usually used in high-resolution color CRTs.
Changing the intensity level of the three electron beams can change the color displayed by the shadow mask CRT. With two of the three guns turned off, we only get the color (red, green, blue) from a single activated fluorescent dot. When activating three points with the same electron beam intensity, we will see white. Yellow is produced by equal intensities of green and red dots, and magenta is produced by equal intensities of blue and red dots. And when the blue dot and the green dot are activated to the same degree, it will appear cyan. In some low-cost systems, the electron beam can only be turned on or off, so only eight colors can be displayed. More advanced systems can set intermediate intensity levels for the electron beam, allowing millions of different colors to be produced.
The color graphic system can be designed according to matching with various CRT display devices. Some inexpensive home computer systems and video game consoles are designed to work with color televisions and RF (radio-frequency) modulators. The role of the RF modulator is to simulate the signal of a broadcast TV station. This means that the color and brightness information of the graphics must be combined and superimposed on the broadcast frequency carrier signal as the input of the TV set. Circuitry in the TV then receives this signal from the RF modulator, extracts the graphics information, and displays it on the screen. As we can expect, due to the additional processing of the graphics information by the RF modulator and the TV circuit, the quality of the displayed image will be degraded.
A composite monitor is a television adaptation device used to allow bypassing of broadcast circuits. These display devices still require combined graphic information, but no carrier signal. It combines the graphics information into a composite signal, which is then separated by the monitor.
The resulting graphics are still of poor quality. People design the color CRT of the graphics system as an RGB monitor (RCB monitor). These monitors use the shadow mask method to obtain the intensity level of each electron gun (red, green and blue) directly from the computer system without any intermediate processing. In the frame buffer of a high-quality raster graphics system, each pixel corresponds to 24 bits, and each electron gun allows 256 voltage settings, so each pixel has nearly 17 million colors to choose from. An RGB color system with 24 storage bits per pixel is generally called a full-color system or a true-color system.
1.5 flat panel display
Although most graphics monitors are still constructed of CRTs, other technologies may soon replace CRT monitors. Flat-panel displays represent a class of video devices that reduce size, weight, and power consumption compared to CRTs. An interesting property of flat panel displays is that they are thinner than CRTs, allowing them to be hung on a wall or worn on the wrist. Some flat-panel displays can even be written on, so they can be used in pocket notebooks. Flat panel displays are also used in small TV monitors, calculators, pocket video game consoles, laptop computers, movie screens on airline seats, signage in elevators, and in less demanding portable monitor applications as a graphics display.
We can divide flat-panel displays into two categories: emissive displays and non-emissive displays. Emissive displays are devices that convert electrical energy into light energy. Plasma display panels, thin film photovoltaic displays, and light emitting diodes are examples of emissive displays. Flat panel CRTs have also been invented, in which a beam of electrons is accelerated parallel to the screen and then deflected 90° to strike the screen. However, flat panel CRTs have not proven to be as practical as other transmitter devices. Non-emissive displays use optical effects to convert sunlight or light from some other light source into a graphic pattern. Liquid crystal devices are the most important examples of non-emissive flat panel displays.
A plasma display panel (plasma panel), also known as a gas-discharge display, is formed by filling the area between two glass plates with a mixed gas that usually contains atmosphere. A series of vertical conductive strips are placed on one glass plate, while a set of horizontal conductive strips are constructed on the other glass plate (see Figure 2.10). The ignition voltage is applied to the paired horizontal and vertical conductive strips, causing the gas at the intersection of the two conductive strips to enter the glow discharge plasma region of electrons and ions. The definition of the pattern is stored in the refresh buffer, and the firing voltage refreshes the pixel positions (intersections of the conductive strips) at a rate of 60 times per second. A brighter display can be obtained by rapidly supplying the ignition voltage with the alternating current method. The separation between pixels is provided by the electric field of the conductive strips. A plasma display panel has the disadvantage that it is a strictly monochromatic device, but plasma displays that can display color and grayscale have now been developed.
A thin-film electroluminescent display has a similar structure to a plasma display panel. The difference is that the area between the glass plates is filled with a phosphor, such as a jelly of zinc sulfide and manganese, instead of a gas (see Figure 2.11). When a sufficiently high voltage is applied to a pair of intersecting electrodes, the fluorescent layer becomes an electrical conductor in the area where the two electrodes intersect. Electric energy is absorbed by the manganese atoms, and then the energy is released to become a bright spot, which is similar to the plasma effect of the glow discharge of the plasma display panel. Optoelectronic displays require more power than plasma display panels, and have difficulty achieving good color and gray scale displays.
The third type of emitting device is light-emitting diode (light-emitting diode LED). Diodes are arranged in a matrix to form the pixel locations of the display, and the definition of the graphics is stored in the refresh cache. As with a CRT's scan line refresh, information is read from the refresh buffer and converted to voltage levels, which are then applied to the diodes to produce a pattern of light on the display.
Liquid-crystal displays (LCDs) are commonly used in small systems such as laptops and calculators (see Figure 2.12). These non-emissive devices generate graphics by passing polarized light from ambient or internal light sources through liquid crystal materials that block or pass light.
The term liquid crystal refers to molecules of these compounds that have a crystalline structure and can flow like a liquid. Flat panel displays typically use linear liquid crystal compounds, which tend to keep the long axes of the rod-shaped molecules aligned. Therefore, flat-panel displays can be made of linear liquid crystals, as shown in Figure 2.13. There are two glass plates, each with a light polarizer, at the right angle to the other, and filled with a liquid crystal material. Horizontal rows of transparent conductors are laid out on one board, while vertical columns of transparent conductors are placed on the other board. The intersection of row and column conductors defines a pixel location. Typically, the molecules are arranged in an "on state" as shown in Figure 2.13. Polarized light passing through the material is twisted so that it passes through the opposite polarizer, which reflects the light back to the viewer. To turn off the pixel, we can put a voltage across the two crossed conductors so that the molecules align so that the polarized light is no longer distorted. Such flat panel display devices can be considered as passive-matrix LCDs. Graphics definitions are stored in the refresh cache, which refreshes the screen at a rate of 60 frames per second, the same as the transmitting device. When using solid-state electronics, backlighting is also often utilized so that the system is not completely dependent on external light sources. Different materials or dyes can be used to display the colors, and a three-in-one colored pixel is placed at each screen location. Another way to construct an LCD is to place a p-transistor at each pixel location and use thin-film transistor technology. Transistors are used to control the voltage at the pixel locations and to stop the liquid crystal cells from chronic leakage. These devices are called active-matrix displays.
1.6 Three-dimensional observation equipment
Graphics monitors that display three-dimensional scenes are designed using technology that reflects CRT images from vibrating flexible mirrors. The principle of operation of such a system is illustrated in Figure 2.14. Changes the focal length when the zoom mirror vibrates. These vibrations are synchronized with the display of objects on the CRT. Thus, each point on that object is reflected from the specular to a spatial position corresponding to the distance of that point from the specified viewing position. This allows us to walk around an object or scene and observe it from different angles. In addition to displaying 3D images, these systems can also display 2D "slices" of cross-sections of selected objects at different depths, for example in medical applications to analyze data from contrast-enhanced sonography and CAT scanning equipment: in geological applications can analyze earthquakes in topography data, introducing solid objects into design applications, and 3D simulation applications of molecular and terrain systems.
1.7 Stereoscopic and virtual reality system
Another technique for representing three-dimensional objects is to display a stereoscopic view. This method does not produce a true 3D image but gives the viewer a different view for each eye to provide a 3D effect, thereby giving depth to the scene. In order to obtain a stereoscopic projection, it is first necessary to obtain two views of the relevant scene from the viewing direction relative to each eye (left eye and right eye). These two views can be obtained by specifying different viewing positions and having the scene generated by a computer, or by photographing some object or scene with a pair of stereo cameras. When we get the left view with the left eye and the right view with the right eye at the same time, the two views are combined into a single image and the scene is perceived as having depth. One way to create a stereoscopic effect is to use a raster system to alternate the two views at different refresh cycles. Looking at the screen through the glasses, each lens is designed as a high-speed alternating shutter that simultaneously blocks the display of another view. Figure 2.15 is a design using liquid crystal shutters and an infrared emitter that synchronizes the glasses with the screen view.
Stereoscopic views are also an integral part of virtual-reality systems. Users can walk into the scene and interact with the environment. Headsets with optical systems that generate stereoscopic views can be used to interface with interactive input devices to locate and manipulate objects in a scene. A sensor system within the headgear tracks the position of the viewer so that the front and back of the subject can be seen as the viewer "walks in" and interacts with the display. Another way to generate a virtual environment is to use a projector to generate a scene on a laid out wall, and the observer interacts with the virtual scene using a headgear and a data glove worn on the right hand (see Section 2.4)
Low-cost interactive virtual reality environments can be built with video monitors, stereo glasses, and head-tracking devices. A tracking device is placed on top of the video display and is used to monitor head movements. Therefore, the observation position of the scene can change following the change of the head position.