The worlds we live in. The worlds we live in

Transcription

1 Introduction The unit aims to: explain the concept of crossing the boundary between the computer s world and our own. explain the digital nature of the computer s world and contrast it with our analogue world of sense and motion. describe in detail how to transform features of our world into computer representation. look at ways in which digital information in the computer can be displayed for our eyes and ears. consider the amazing implications of being able to use the computer to manipulate and transform digital representations of our world. 1

2 The worlds we live in The computer is a tool like any other. The computer s job is to acquire, store, present, control, exchange and manipulate interesting characteristics of the natural world. Storage and presentation keeping copies of all your TMAs or letters or digital films; Control control programs in a washing machine; Exchange sending data between two computers linked by a network. Manipulation computer-aided design systems; The worlds we live in We live in an analogue world (a world of sensation) while the world of the computer is digital (a world of numbers). In order to capture pieces of reality on a computer, you have to move them from an analogue to a digital world inside the computer. you have to cross the boundary between two very different worlds. Humans (and all livings) have the ability to sense or perceive the world around them. We have five senses: vision, hearing, smell, taste and touch. The human perceptual system uses our five senses but is very strongly based on vision and hearing. Humans have learned to enhance their perceptual systems with instruments. Example: microscopes, telescopes, radar, gas sniffers, sonar, X-ray devices, hearing aids, Some of the dangers that can arise from the capture, storage and manipulation of digital information: privacy, liberty and security. 2

3 Analogue information: digital representation Analogue quantities are ones that change continuously. Example1: The analogue thermometer, that uses the length of a column of mercury or colored alcohol to indicate the temperature. There is an infinite number of possible temperatures between the two values 14 and 15 or any other two values. temperature, goes up smoothly and progressively. Temperature is an analogue quantity. Analogue information: digital representation Example2: The volume control of a radio or stereo with a simple knob. When the knob is rotated to increase the volume, the intensity of the sound does not increase in a series of jumps but goes up smoothly and progressively. Volume is an analogue quantity. Other examples on analogue quantities are: Brightness (of light), color, pitch (how high or low a musical note is), pressure, speed. 3

4 Analogue information: digital representation In contrast to analogue quantities, which change continuously, discrete quantities change in a series of clear steps. Example1: The discrete volume control shown figure 3.4. When the control is moved from click to click, you can hear the volume increasing not smoothly, but in a series of steps. Discrete quantities has a fixed number of possible values between any two points on its scale. Example2: The discrete thermometer shown in figure3.5. Temperature and volume are fundamentally analogue quantities in that they are infinitely variable. But we may choose to measure them with discrete instruments for our own convenience. Other examples on fundamentally discrete quantities are: the number of people in a party, the number of bricks in a house. Analogue information: digital representation Exercise 3.1: Which items in the following list are fundamentally analogue and which fundamentally discrete? 1. The price of petrol 2. The amount of heat from a fire 3. The speed of a car 4. The energy of a star 5. The size of the audience at a play 6. The pressure of the atmosphere. Answer: items 2, 3, 4 and 6 are definitely analogue, although we might choose to measure them with discrete instruments. Items 1 and 5 are discrete. 4

5 Analogue information: digital representation The terms discrete and digital are often used interchangeably. Digital: Any communication or computing technology whose data may only have a finite number of discrete values. The interior world of the computer is a discrete world, a digital world, a world of numbers, nothing but numbers! Analogue information: digital representation Base 10 arithmetic, or a decimal system, is used more or less worldwide. Using base 10, we can count to, and write down, any number we want. It: uses ten digits (0 to 9); and each column counts groups ten times bigger than those counted in the column to its right; This can be represented as follows: Examples: 37 = 7 + 3* = 5 + 4* * = 1 + 2* * *10 3 5

6 Analogue information: digital representation Computer scientists sometimes use an octal (base 8) system. This system: uses the first eight digits, 0 to 7. The first column counts units. Each new column counts groups eight times the size of the groups counted by the column immediately to its right. So the table would look like this: Analogue information: digital representation Base 2 arithmetic, or a binary system, is used by the computer. This system: uses two digits only (0 and 1); and each column counts groups that are two times bigger than groups counted in the column immediately to its right, as shown below. 6

7 Analogue information: digital representation Examples: What is the decimal equivalent of the following numbers in the binary system? 101 = = (5) 10 of 4 of 2 1s = = (7) 10 of 4 of 2 1s = of 32 of 16 of 8 of 4 of s = = (41) 10 Analogue information: digital representation of 128 of 64 of 32 of 16 of 8 of 4 of s = = (209) of 256 of 128 of 64 of 32 of 16 of 8 of 4 of s = = (278) 10 7

8 Analogue information: digital representation A bit is short for binary digit and refers to a 1 or a 0 stored in the computer. A byte is a group of eight bits that can be used to represent numbers between 0 and 255. A byte looks like this: The largest number we can store in the a byte is = (255) 10. The smallest number we can store in the a byte is = (0) 10. Analogue information: digital representation A word is a group of four bytes. It is the largest data object a particular computer can process in a single operation. A word is 32 bits (4 x 8), so the largest binary number that a computer using a four-byte word can process in a single operation would be which is decimal 4,294,967,295. Computer s memory and hard drive is measured in: bytes = 8 bits kilobytes (KB) = 1024 (2 10 ) bytes megabytes (MB) = (2 20 ) bytes or 1024KB gigabytes (GB) = (2 30 ) bytes or 1024 MB 8

9 Crossing the boundary The computer handles everything in a binary form, i.e. in numbers. Therefore, to take the features of our analogue experience across the boundary into a computer means transforming these features into numbers. text, pictures, moving pictures, diagrams and sound can all be reduced to numbers and stored inside the boundary in a computer s memory. Crossing the boundary Word processors enable us to enter text into the computer, edit it, store it and then print it out when we are satisfied with the result. Text exists inside the computer in the form of numbers. Each character (alphabets, parentheses, accents, spaces, etc..) has its numerical representation in the computer. If different computers used different numbers to encode the same character, people would not be able to read each other s documents. There have to be standards: ASCII and Unicode 9

10 Crossing the boundary ASCII (American Standard for Computer Information Interchange) was approved in ASCII uses 7 bits for 128 numbers, from 0 to 127, for upper and lowercase alphabetic characters, punctuation marks and some invisible characters, such as a carriage return (start a new line) and a tab. Unicode standard for character representation certified in It preserves the ASCII numbers, but expands it. It uses 16 bits (two bytes) for 65,536 as follows: 8192 numbers for representing characters in the world s main languages, including Hebrew and Sanskrit for punctuation marks, graphics and special symbols for developers to define their own symbols. 27,000 or so for Han Chinese characters. the remainder for characters yet to be invented. Crossing the boundary Text capture devices are those used for transforming text into digital form inside the computer. These are: Keyboard, scanners (which produce an image of a page) and optical character recognizers (OCRs). Keyboards are still the main way of taking text across the boundary into the computer. When typing on a keyboard a program called BIOS (Basic Input/Output System) will transform the signal from the pressed button to its appropriate numerical code And then other software will store it in memory. 10

11 Crossing the boundary Scanners and OCRs provide faster and more efficient way of getting text into a computer than the keyboard. A scanner produces an image of the page. The image is passed to a computer program called OCR, which detects each letter on the page in turn and transforms it into a digital code. Recognizing characters is a difficult task for the computer because the same character can be represented in many different forms. Such as: Crossing the boundary Vision is the most dominant sense for a human being. a visual field includes light, colors, shade, form, etc Such features need to be transformed into numbers in order to cross the boundary and enter the computer world. We ll start with a simple picture (a), place a border around it (b), to indicate the area of interested. 11

12 Crossing the boundary Next, divide the picture into equal-sized squares over it (c). Examine each square (d): If the square is entirely grey make it white. If the square is entirely colored (mauve) make it black. If the square contains both grey and color, if more than the third of the square is mauve then it will become black otherwise it will become white. (c) (d) Crossing the boundary Finally, map the square s color to a number by assigning the number 0 to it if it is colored white and 1 if it is colored black (e). This sort of encoding is referred to as a bitmap. It is also called raster graphics. Each square that we have mapped to a 0 or a 1 is called a pixel (picture element). Bitmap is a method for storing images in computer memory in which the image is divided into rows of pixels and each pixel is represented by certain fixed number of bits. (e) 12

13 Crossing the boundary One way to improve the quality of the image is to increase the number of squares and make each square smaller. This is called increasing the resolution of the picture. Crossing the boundary The simple method for encoding images we ve used is not satisfactory for more sophisticated pictures, such as these shown in figure 4.9. It doesn t handle colors. It is plain black and white, which won t allow us to deal with shade and texture. In our simple method, we dedicated one bit to each pixel in our image. To accommodate a greater range of shades, we need to devote more bits to each pixel 13

14 Crossing the boundary Allocating two bits per pixel gives us four shades. 11 (black), 00 (white), 01 (light grey) and 10 (dark grey). Three bits per pixel will give us eight shades, from black to white; four bits per pixel gives us 16 shades; and so on. n bits give us 2 n shades. This mapping of shades of grey between black and white in a black and white bitmap is known as greyscale. Greyscale: an image that is made of finite number of shades of grey. The range of numbers to which a pixel can be mapped is termed the pixel amplitude. Pixel amplitude: The range of numbers available to represent a pixel digitally. A pixel amplitude of 2 allows only black and while representation. Crossing the boundary Colors are represented in a different way. Any color can be made out of a mixture of three basic shades: Red, Green and Blue (R, G, B). Each shade is represented by a byte (8 bits), giving values ranging from 0 to 255. As a total we have 256 x 256 x 256 possible colors, reflecting every possible mixture of our three basic shades. Red is (255, 0, 0) since it is all Red and 0 Green and 0 Blue. Green is (0, 255, 0). Blue is (0, 0, 255). White is (255, 255, 255), all the color spectrum. Black is (0, 0, 0), no color. Each pixel in the above scheme is stored in 24 bits (or 3 bytes) Large amounts of memory are required. A more efficient way of storing visual information is needed. This way of representing color is called the RGB (red, green, blue) model. 14

15 Crossing the boundary Diagrams can be represented more economically than in a bitmap. For the diagram shown in figure 4.13, all we care about are the objects shown (circle, a rectangle, a line, an arrow and a piece of text) the rest is just empty space. To reconstruct the diagram the only information we need about each of these objects is: what sort of object it is (e.g. line, square); its size and position on the page; details about its coloring, width of line (line weight), and so on. Crossing the boundary The method for encoding diagrams into numbers is easy. assign a number to each type of object. place two axes, x and y at right angles on the page to be able to record the size and position of each object. This depends on the object, as follows: A circle is defined by its radius and the coordinates of its center. A rectangle by the coordinates of its upper left and lower right corners. Lines and arrows by their starting and ending points. The text area by the coordinates of its top left corner. for each object, record details as the line color, fill color, line width, This sort of encoding of visual information is usually known as vector graphics. 15

16 Crossing the boundary Vector graphics: a form of digital representation of images, in which the picture is seen as a collection of geometrical objects, such as lines, squares, circles. Vector format graphic files are much more compact than raster graphics. Vector images are easily altered in scale. They are scalable, we can easily shrink or stretch the size of it without any loss of information. Vector graphics is of little use in representing complex, photorealistic image information. Programs that allow us to draw and display vector graphics are generally referred to as drawing packages. Systems for constructing and displaying raster graphics are usually called painting packages. Crossing the boundary A Video or a movie, is a series of images that slightly differ one from another, passing them one after the other at a certain speed will give the illusion of movement. A picture would be called a frame. Frame rate is the speed of flipping the frames one after the other. Films are usually shot and displayed at 24 frames a second (fps). TV pictures are presented at between 25 and 30 fps, depending on what country you live in. High definition TV, currently only available in Japan, uses 60 fps. 16

17 Crossing the boundary Computers use the same principle to display moving visual information. A series of images is taken from the computer s memory, or direct from a storage device such as a CD, and presented on the computer screen in quick succession. Each image is different from, but correlated with, the previous image; the illusion of smooth movement is created by our own eyes and brains. The problem here is generally with the size of digital encoding of images. a two-hour film that is displayed on a computer at 24 fps, with each frame is 640 X 380 pixels and a 24-bit RGB color encoding is being used would require over 126 GB! (see exercise 4.5). One approach of reducing the amount of storage that moving images need depends on the fact that frames are correlated (similar). Hence, if we fully encode the first frame and then just record the differences between it and the next frame, and so on, we will save huge amounts of space. Crossing the boundary Other ways to reduce image storage space is by compressing it. Whatever compression strategy we adopt we need to have agreements (standards). Other computers need to be able to decompress it again to display it. Most common compression standards are JPEG (Joint Photographic Experts Group) and GIF (Graphics Interchange Format). Both standards reduce the number of bits used to store each pixel. GIF condenses each pixel from 24 bits to 8, by reducing the set of colors used to a smaller set, called a palette. Image data can sometimes be compressed to one twenty-fifth of the original size. For video, the dominant standard is MPEG (Moving Picture Experts Group), which is now used in most digital camcorders. 17

18 Crossing the boundary The main devices used for transforming images and video into digital form inside the computer are scanners, digital cameras and camcorders. 1. Scanners: Can be used for images as well as text. A scanner works by moving a sensing point rapidly across the image, in a series of lines, as illustrated in the figure. This is known as raster scanning. The scanner measures the brightness (luminance) and the coloring (chrominance) of a series of points along each line and converts the readings at each point into a number. The quality of the resulting bitmap depends on the number of lines the scanner follows across the specimen, and the number of measurement points along each line. Software inside the camera converts the bitmap into compressed format, including JPEG and GIF. Crossing the boundary 2. Digital cameras and camcorders: Digital still cameras usually compress their images into JPEG format and store them on a removable memory card inside the camera. Digital camcorders generally produce MPEG format, stored on a special tape. Both devices work by means of an electronic chip called a chargecoupled device (CCD). CCD: an array of sensors inside a digital camera or scanner. Each sensor respond to light by generating a tiny voltage, which is then mapped into a single pixel in the camera s memory. The larger the array is, the higher the definition, and thus better quality, of the image will be. 18

19 Crossing the boundary Second to vision, we rely on sound. It s another analogue feature of the world. If you cry out or hit a piano key, then you set particles of air shaking and any ears will interpret this shake as sound. Vibrating a tuning fork makes pure sound that makes particles of air move backwards and forwards with the fork. One way to visualize this movement is to draw a graph of how far an air particle moves backwards and forwards (we call this its displacement) as time passes. The graph will look like the figure shown. The distance it moves either way is known as the amplitude. The rate at which it completes one backwards and forwards movement is known as the frequency. Figure 4.18 Displacement of air particles over time by vibrating a tuning fork Crossing the boundary As with images, the process of digitizing a sound includes: Sampling and quantization. In order to digitize a sound waveform, we take samples of the sound at small time intervals, such a process is called sampling. An image is sampled into rows of pixels (units of area). A waveform is sampled into a series of discrete time intervals (units of time). The number of times/second we take a sample is called the sampling rate. The smaller the interval is, the better. brings the encoding closer to the original sound. We can never make a perfect digital representation of an analogue quantity because we will always lose information between the times we sample a waveform. (it is also impossible to make a perfect digital coding of an analogue picture, because we will always lose information between the pixels) 19

20 Crossing the boundary Quantization is the second stage in the process of taking an image or sound wave across the boundary. It is the process of converting each sample into a suitable number for computer storage. The smaller the interval of taking the samples, the more accurate our representation of the analogue sound will be. Smaller intervals will mean, more samples and therefore more bits to quantize them, which means more storage space. Several formats exist for sound digitizing, depending on the allowed loss of precision (ex: mp3, wav, mid, etc ) Crossing the boundary A final world to say is that we can take any feature of our analogue world (other than the visual and sound information) across the boundary into the digital area by following the same, two-step process consisting of: 1. Breaking the target into parts (sampling). 2. Mapping each part onto a binary number (quantization ). Digital representations are perfectly flexible once captured, programs can be written that will change them in any way that suits us. 20

21 Going back Digital representations (of pictures, movies, sound, ) are of no value to us unless we can take them back across the boundary into the human area. Sending a digital representation back across the boundary is a three stage process, consisting of: 1. Identifying the output device to be used. Depends on a direct command from the computer user. 2. Arranging the numbers into a form that can be handled by the output device. The necessary arrangements are made by a special class of digital encoding a program. digital world can only be manipulated from inside by other digital things. Inside the computer boundary, a program is a set of binary words (a word is a group of bytes, usually four). Each word of the program stands for an instruction to the machine s central processor (CPU) When a computer s central processor reads one of these words it carries out the instruction that the word stands for. By this, programs prepare digital encodings for output. 3. Interpretation of the code by the output device. Carried out by specialized electronics in the output device. Going back A computer uses different output devices such as: monitors (Screens), printers, plotters, speakers. Monitors: there are two main types of monitor: CRT monitor: cathode ray tube monitor. displays the image by the same means as a television, using a vacuum tube with a raster scanning electron beam. Professional graphic artists still favor CRT monitors for their color accuracy. LCD monitor: liquid crystal display monitor. Displays an image in the same way as the screen of mobile phone or calculator. Light is passed through a special material, the molecules of which change their orientation when an electrical voltage is applied to them. LCDs are to be preferred over CRT for general use. They use much less power than CRTs; they take up little space; and they are much less tiring to work on. Monitors supply digital display. If you look very closely at your screen while it is displaying an image, you will see the pixels. 21

22 Going back Printers: there are two main types in use today: inkjets printers: Produces its output by firing tiny jets of ink through a matrix of holes onto the paper. can print in both color and black and white. Its color cartridge comes in three parts: cyan, magenta and yellow, indicating the color model (CMYK) that the printer uses. Lasers printers: Produces very high quality print by imprinting an image as a series of electrical charges on a selenium-coated drum. The charged drum is then rolled in toner, a dry powder type of ink, which sticks to the charged image on the drum. The toner is transferred onto a paper using heat and pressure. generally print only in black and white but color lasers are available, but are rather expensive for individual users. Both types of printer produce a digital output, as they make graphics and text by firing the laser, or ink, through a square matrix of tiny holes, as illustrated in figure 5.1. Going back Plotters: A plotter is a special type of printing device mostly used by architects, engineers and map makers. The printed output is produced by moving a pen across the paper. (several differently colored pens are also available). Plotters are most suitable for line drawings, which is why architects, for instance, use them. Plotters produce an analogue output directly. Loudspeakers: Speakers also produce an analogue output. The audio program inside the boundary converts the digital encoding of the sound to a series of electrical pulses that are sent to the speaker, where they cause a cone of stiffened paper (or some synthetic material) to vibrate in and out. This makes the air vibrate in the characteristic sound wave. 22

23 What if?... changing the digital world Digital representations are perfectly flexible once captured, programs can be written that will change them in any way that suits us. They help us to range over past, present and future worlds. Using appropriate software, computers will help you simulate what happens in the real world. Simulations help us study problems through models. What if?... changing the digital world Simulation is the use of computer system to imitate a physical system, such as: A weather system, a machine or a stock market. A simulation can be created by fist building a model of the system in question and then writing a program that encodes the rules over time. The rules are applied to the model continuously giving an indication of the evolution of the system over time. Examples of rules: Rules of a model of planetary system would encode the effect of gravity and of the lows of motion. Rules of a stock market simulation would express: Known economic laws such as supply and demand. Patterns of human economic behavior such a preference for lower prices, an acceptance of certain levels of risks and so no 23

24 What if?... changing the digital world Model is a set of digital data items that, taken together, represent a digital encoding of the important properties of the system to be simulate. Any real world system can be modeled but all such models are simplifications of the reality. Simulations applications requires very powerful computers such as supercomputers. Supercomputers are specially designed to carry out billions of numerical calculations a second. Even at these speeds, simulations may take hours, or even days, to run. What if?... changing the digital world 24

25 What if?... changing the digital world What if?... changing the digital world Human imagination associated with appropriate software will allow the creation virtual worlds in a computer memory. An example is AlphaWorld. AlphaWorld does not exist in space. It is a purely digital world, existing in the memory of a powerful computer, or group of computers, somewhere in the physical world. It is a virtual world. virtual is a term used to describe any entity that does not really exist, but is simulated by the action of a computer. Digital representations of real persons or other creatures of AlphaWorlds are called avatar. Virtual reality (VR) is an advanced form of 3D graphics that aims to convince the viewers that they are experiencing a real scene, by creating the illusion of real 3D space, through which the viewer can appear to travel using a steering device such as joystick. Good VR programs require powerful computing resources and sometimes specialized graphics hardware. 25

26 Unit summary In this unit you have learned about: The difference between the analogue world we inhabit and the digital world of the computer. How features of our world can cross the boundary and be represented in the digital world, and then brought back across the boundary to us. How computer programs that manipulate digital representations of our world enable us to: simulate physical and social processes, explain current events, predict future events and create futuristic virtual worlds. 26