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UNIT -1 INTRODUCTION TO COMPUTER GRAPHICS
UNIT STRUCTURE
1.0 Objective 1.1 Definition of Computer Graphics 1.2 History of Computer Graphics 1.3 Applications of Computer Graphics 1.3.1 CAD and CAM processes 1.3.2 Scientific Visualization 1.3.3 Entertainment and photorealism 1.3.4 Medical Content Creation 1.4 Graphics Package 1.4.1 Pixels and frame Buffer 1.4.2 Input Devices 1.4.3 Output Devices 1.4.4 The CPU and GPU 1.4.5 Graphics Pipeline 1.5 Graphics Software Implements 1.6 Numerical Examples 1.7 Summary 1.8 Questions for Exercise 1.9 Suggested Readings
1.0 OBJECTIVE
The objectives of this lesson are to make the student aware of the following concepts
- Describe computer graphics, its features and characteristics;
- Discuss applications of computer graphics in various fields
- Describe various types of hardware, required to work with graphic systems like a) Display systems b) Cathode ray tube c) Random Scan d) Raster Scan and e) Display processor f) Graphics Pipelining Discuss existing Graphic Softwares to assist in areas of Graphical processing
1.1 DEFINITION OF COMPUTER GRAPHICS
Computer Graphics is principally concerned with the generation of images, with wide ranging applications from entertainment to scientific visualisation. In other words, we can say that computer graphics is a rendering tool for the generation and manipulation of images. Thus, by using a computer as a rendering tool for the generation and manipulation of images is called computer graphics. Computer graphics is an art of drawing pictures on computer screens with the help of programming. It involves computations, creation, and manipulation of data. Graphics is a vast field that encompasses almost any graphical aspect like special effects, simulation and training, games, medical imagery, animations and much more. “Computer Graphics refers to any sketch, drawing, special artwork or other material generated with the help of computer to pictorially depict an object or a process or convey information, as a supplement to or instead of written descriptions”. It relies on an internal model of the scene, that is, a mathematical representation suitable for graphical computations. The model describes the 3D shapes, layout, projections to compute a 2D image from a given viewpoint and rendering which involves projecting the objects (perspective), handling visibility (which parts of objects are hidden) and computing their appearance and lighting interactions and materials of the scene.
1.2 HISTORY OF COMPUTER GRAPHICS
In the 1950’s, graphics output were taken via teletypes, line printer, and Cathode Ray Tube (CRT). Using dark and light characters, a picture could be reproduced. In the 1960’s, advent of modern interactive graphics, output were vector graphics and interactive graphics. One of the worst problems was the cost and inaccessibility of machines. In the early 1970’s, output started using raster displays, graphics capability was still fairly chunky. In the 1980’s output were built-in raster graphics, bitmap image and pixel. Personal computers costs decreased drastically; trackball and mouse become the standard interactive devices. In the 1990’s, since the introduction of VGA and SVGA, personal computer could easily display photo-realistic images and movies. 3D image renderings became the main advances and it stimulated cinematic graphics applications. Table 1: gives a general history of computer graphics. YEAR (^) DISCOVERY & FINDINGS (^1950) Ben Laposky created the first graphic images, an Oscilloscope, generated by an electronic machine
The core elements of computer graphics include the following :- Modeling :- Involves representation of choices, geometric processing Rendering :- Executes geometric transformation, visibility, implements simulation of light Interaction :- Synergy of input/output devices, tools Animation :- Simulation of lifelike characters, natural phenomena, their interactions, surrounding environments Computer graphics can be broadly divided into the following classes: Business Presentation Graphics, which refers to graphics, such as bar-charts, histograms, pie-charts, pictograms, x-y charts, etc. Scientific Graphics, such as x-y plots, curve-fitting, contour plots, system or program flowcharts etc. Cartography (Scaled Drawings), such as architectural representations, drawings of buildings, bridges, and machines. Satellite Imaging − Geodesic images. Photo Enhancement − Sharpening blurred photos. Medical imaging − MRIs, CAT scans, etc. - Non-invasive internal examination. Engineering drawings − mechanical, electrical, civil, Replacing the blueprints of the past. Typography − The use of character images in publishing - replacing the hard type of the past. Cartoons and artwork, including advertisements. Simulation and modeling − Replacing physical modeling and enactments Graphics User Interfaces (GUIs) A graphic, mouse-oriented paradigm which allows the user to interact with a computer.The images that appear and are designed to help the user utilise the software without having to refer to manuals or read a lot of text on the monitor. Fig :- Some Application Areas of Computer Graphics
Computer Graphics is used for a broad spectrum of applications, on a large number of different graphical devices and some of the main areas of its applications are :-
1.3.1 CAD and CAM processes
CAD is used to design, develop and optimize products, which can be goods used by end consumers or intermediate goods used in other products. CAD is also extensively used in the design of tools and machinery used in the manufacture of components, and in the drafting and design of all types of buildings, as it enables designers to layout and develop work on screen, print it out and save it for future editing, saving time on their drawings CAD is mainly used for detailed engineering of 3D models and/or 2D drawings of physical components, but it is also used throughout the engineering process from conceptual design and layout of products, through strength and dynamic analysis of assemblies to definition of manufacturing methods of components. Fig:- The Process of Computer Aided Design Fig :- Gear Shaft Design Bench mark Bimodal Function A CAM or Computer-Aided Manufacturing system usually seeks to assist and control the production process through varying degrees of automation. Because each of the many manufacturing processes in a CAM system is computer controlled, a high degree of precision can
a technology which helps to explore and understand scientific phenomena visually, objectively, quantitatively. Scientific visualization allow scientists to think about the unthinkable and visualise the unviable. This concept of scientific visualisation fits well with modeling and simulation. Fig : Visualization in areas of Car Crash, Jurassic Park, Volcano eruption
1.3.3 Entertainment and Photorealism
One of the main goals of today’s special effects producers and animators is to create images with highest levels of photorealism. Volume graphics is the key technology to provide full immersion in upcoming virtual worlds e.g. movies or computer games. Real world phenomena can be realized best with true physics based models and volume graphics is the tool to generate, visualize and even feel these models! Movies like Star Wars Episode I, Titanic and The Fifth Element already started employing true physics based effects.
1.3.4 Medical Content Creation
Medical content creation like virtual anatomical atlas on CD-ROM and DVD have been build on the base of the NIH Visible Human Project data set and different kind of simulation and training
software were build up using volume rendering techniques. Volume Graphics' products like the VGStudio software are dedicated to the used in the field of medical content creation. VGStudio provides powerful tools to manipulate and edit volume data. An easy to use keyframer tool allows to generate animations. Fig : Graphic Representation of a foetus in the womb
1.4 GRAPHICS PACKAGE
A computer graphics system is a computer system which must have all the components of a general-purpose computer system. Considering the high-level view of a graphics system, there are six major elements in the Graphics system:
- Input devices
- Central Processing Unit
- Graphics Processing Unit
- Memory
- Frame buffer
- Output devices This model is general enough to include workstations and personal computers, interactive game systems, mobile phones, GPS systems, and sophisticated image generation systems. Although most of the components are present in a standard computer, it is the way each element is specialized for computer graphics that characterizes this diagram as a portrait of a graphics system. Fig : A Graphics System
Fig :- The Relation between Resolution, Pixels and Aspect Ratio of a Graphical Device
1.4.2 Input Devices
Most graphics systems provide a keyboard and at least one other input device. The most common input devices are the mouse, the joystick, and the data tablet. Each provides positional information to the system, and each usually is equipped with one or more buttons to provide signals to the processor. Often called pointing devices, these devices allow a user to indicate a particular location on the display. Some commercial input devices used in Graphics System are
- 2D mice
- Scanners
- Light Pens
- Digitizers
- Digital Camera
- Video Camera
- Touch-sensitive screens
- Joysticks
- Trackballs
- Thumb wheels
- Microphones (voice data entry)
- Touch Panels The keyboard device is a device that returns character codes. We use the American Standard Code for Information Interchange (ASCII) in our examples. ASCII assigns a single unsigned byte to each character. However in Internet applications, multiple bytes were used for each character, thus allowing for a much richer set of supported characters. Touch Panels allow displayed object or screen positions to be selected with the touch of the finger and is also known as Touch Sensitive Screens (TSS). A typical application of touch panels is for
the selection of processing options that are represented with graphical icons. Touch input can be recorded using optical electrical or acoustical methods. Optical touch panels employ a line of intra red LEDS (light emitting diodes) along one vertical edge and along one horizontal edge of frame. The opposite vertical and horizontal edges contain light detection which are used to record the beams that may have been interrupted when the panel was touched. An electrical touch panel is constructed with two transparent plates separated by a short distance. One of the plates is coated with a conducting material and the other is resistive material. When the outer plate is touched, it is forced into contact with the inner plate. The contact creates a voltage drop that is converted to a coordinate value of the selected screen position. They are not too reliable or accurate, but are easy to use. Fig : Some Input Devices in a Graphics System (Mouse, Trackball, Data tablet, Joystick) Data tablets provide absolute positioning with rows and columns of wires embedded under its surface. The position of the stylus is determined through electromagnetic interactions between signals traveling through the wires and sensors in the stylus. Touch-sensitive transparent screens that can be placed over the face of a CRT have many of the same properties as the data tablet. Small, rectangular, pressure-sensitive touchpads are embedded in the keyboards of many portable computers. One other device, the joystick regulates its motion of the stick in two orthogonal directions by encoding and interpreting as two velocities, and integrated to identify a screen location. The integration implies that if the stick is left in its resting position, there is no change in the cursor position and that the farther the stick is moved from its resting position, the faster the screen location changes. Thus, the joystick is a variable-sensitivity device.
1.4.3 Output Devices
Aspect ratio, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. Display resolution. The number of distinct pixels in each dimension that can be displayed. The formula to calculate the video memory required at a given resolution and bit-depth is :- Memory (in MB) = (X-resolution * Y-resolution * Bit per pixel) / (810241024) Display systems use either random or raster scan: Random scan displays, often termed vector displays, came first and are still used in some applications. Here the electron gun of a CRT illuminates points and/or straight lines in any order. The display processor repeatedly reads a variable 'display file' defining a sequence of X,Y coordinate pairs and brightness or colour values, and converts these to voltages controlling the electron gun. In a Random Scan System, the Display buffer stores the picture information. Further, the device is capable of producing pictures made up of lines but not of curves. Thus, it is also known as “Vector display device or Line display device or Calligraphic display device”. Fig : A Random Scan Display (……. Beam off;____ Beam on) Raster scan displays, also known as bit-mapped or raster displays, are somewhat less relaxed. Their whole display area is updated many times a second from image data held in raster memory. The rest of this handout concerns hardware and software aspects of raster displays. In a raster scan, an image is cut up into successive samples called pixels, or picture elements, along scan lines. Each scan line can be transmitted as it is read from the detector, as in television systems, or can be stored as a row of pixel values in an array in a computer system. Each complete sweep from top left to bottom right of the screen is one complete cycle, called the Refresh Cycle. Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. Using these units, we would describe a refresh rate of 60 frames per second as simply 60 Hz. At the end of each
scan line, the electron beam returns to the left side of the screen to begin displaving the next scan line. The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. And at the end of each frame (displayed in 1/80th to 1/60th of a second), the electron beam returns (vertical retrace) to the top left comer of the screen to begin the next frame. On some raster-scan systems (and in TV sets), each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining scan lines. Interlacing of the scan lines in this way allows us to see the entire image displayed in one-half the time it would have taken to sweep all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. Fig :- Horizontal and Vertical Retrace in a non-Interlaced Raster Display Color CRTs have three different colored phosphors (red, green, and blue), arranged in small groups like in triangular groups called triads, each triad consisting of three phosphors. In the shadow- mask CRT , a metal screen with small holes—the shadow mask—ensures that an electron beam excites only phosphors of the proper color.
Fig : A Drum Plotter
1.4.4 The CPU and the GPU
In a simple system, there may be only one processor, the central processing unit (CPU) of the system, which must do both the normal processing and the graphical processing. The main graphical function of the processor is to take specifications of graphical primitives (such as lines, circles, and polygons) generated by application programs and to assign values to the pixels in the frame buffer that best represent these entities. For example, a triangle is specified by its three vertices, but to display its outline by the three line segments connecting the vertices, the graphics system must generate a set of pixels that appear as line segments to the viewer. The conversion of geometric entities to pixel colors and locations in the frame buffer is known as rasterization, or scan conversion. In early graphics systems, the frame buffer was part of the standard memory that could be directly addressed by the CPU. Today, virtually all graphics systems are characterized by special-purpose graphics processing units (GPUs), to carry out specific graphics functions. The GPU can be either on the mother board of the system or on a graphics card. The frame buffer is accessed through the graphics processing unit and usually is on the same circuit board as the GPU. GPUs are characterized by both special-purpose modules geared toward graphical operations and a high degree of parallelism—recent GPUs contain over 100 processing units, each of which is user programmable. GPUs are so powerful that they can often be used as mini supercomputers for general purpose computing. Fig : The Graphic Processor Architecture
1.4.5 The Graphics Pipeline
Fig :- The :Pipeline Architecture of Graphics Package The functioning of a standard graphics system is typically described by an abstraction called the graphics pipeline. The term “pipeline” is used because the transformation from mathematical model to pixels on the screen involves multiple steps, and in a typical architecture, these are performed in sequence; the results of one stage are pushed on to the next stage so that the first stage can begin processing the next polygon immediately. Each object comprises a set of graphical primitives. Each primitive comprises a set of vertices. We can think of the collection of primitive types and vertices as defining the geometry of the scene. In a complex scene, there may be thousands—even millions—of vertices that define the objects. We must process all these vertices in a similar manner to form an image in the frame buffer. If we think in terms of processing the geometry of our objects to obtain an image, we can employ the four major steps in the imaging process:
- Vertex processing
- Clipping and primitive assembly
- Rasterization
- Fragment processing No matter with which advance graphic software you are working with, if your output device or Graphics hardware is not good, or hardware handling that software is not good, then ultimate result will be not good, as discussed in the previous section to work with graphic packages.
- Often easy to use but it helps to be familiar with remote sensing.
- Numerical Analysis Packages: Software used: Matlab.
- Focus usually on numeric processing.
- Programming or mathematical skills usually helpful.
- Used to build more user-friendly applications.
- Web-based Services: Software used : Protected Area Archive.
- Image display, roam, zoom.
- Image enhancement.
- Simple image processing.
- Distance and area measurement.
- Comparison of old and new images.
- Image annotation (adding text, lines, etc).
- Overlaying vector layers.
- Computer Aided Design and Drafting (CADD): Software used: AutoCad,SolidWorks CAD/CAM software uses CAD drawing tools to describe geometries used by the CAM portion of the program to define a tool path that will direct the motion of a machine tool to machine the exact shape that is to be drawn on the computer. Now-a-days many new machine tools incorporate CNC technologies. These tools are used in every conceivable manufacturing sector, like CNC technology is related to Computer Integrated Manufacturing (CIM), Computer Aided Process Planning (CAPP) and other technologies such as Group Technology (GT) and Cellular Manufacturing. Flexible Manufacturing Systems (FMS) and Just-In- Time Production (JIT) are made possible by Numerically-Controlled Machines.
1.5 NUMERICAL EXAMPLES
- Consider a raster system with resolution 1280 x 1024 and 2560 x 2048. What size frame buffer (in bytes) is needed for the system to store 12 bits/pixel? How much storage is required for each system if 24 bits per pixel are to be stored? Solution : Frame-buffer size for the system is 1280 × 1024 × 12 bits ÷ 8 bits per byte = 1920 KB 2560 × 2048 × 12 bits ÷ 8 bits per byte = 7680 KB For 24 bits of storage per pixel, each of the above values is doubled.
- Consider a raster system with the resolution of 1024 x 768 pixels and the color palette calls for 65,536 colors. What is the minimum amount of video RAM that the computer must have to support the above-mentioned resolution and number of colors? Solution :- Recall that the color of each pixel on a display is represented with some number of bits. Hence, a display capable of showing up to 256 colors is using 8 bits per pixels (i.e. “8-bit color”). Notice first that the color palette calls for 65,536 colors. This number is but 216 , which implies that 16 bits are being used to represent the color of each pixel on the display. The display’s resolution is 1024 by 768 pixels, which implies that there is a total of 786, (1024 × 768) pixels on the display. Hence, the total number of bits required to display any of 65,536 colors on each of the screen’s 786,432 pixels is 12,582,912 (786,432 × 16). Dividing this value by 8 yields an answer of 1,572,864 bytes. Dividing that value by 1, yields an answer of 1,536 KB. Dividing that value by 1,024 yields an answer of 1.5 MB.
- How many Kilobytes does a frame buffer need in a 600 x 400 pixel? Solution : Resolution is 600 x 400 Suppose 1 pixel can store n bits Then, the size of frame buffer = Resolution X bits per pixel = (600 X 400) X n bits = 240000 n bits = 240000 n/1024 X 8 = 29.30 n KB (as 1kb = 1024 bytes)
- Find out the aspect ratio of the raster system using 8 x 10 inches screen and 100 pixel/inch. Solution :- Aspect ratio = Width : Height = (8 x 100) /( 10 x 100) Aspect ratio = 4 : 5
- How much time is spent scanning across each row of pixels during screen refresh on a raster system with resolution of 1280 X 1024 and a refresh rate of 60 frames per second? Solution :- Here, resolution = 1280 X 1024 that means system contains 1024 scan lines and each scan line contains 128 pixels refresh rate = 60 frame/sec. So, 1 frame takes = 1/60 sec. Since resolution = 1280 X 1024 1 frame buffer consist of 1024 scan lines It means then 1024 scan lines takes 1/60 sec Therefore, 1 scan line takes , 1 /(60 X 1024) = 0.058 sec
1.6 SUMMARY
