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Computer Graphics Notes


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Computer Graphics and its applications, Elements of a Graphics, Graphics Systems: Video Display Devices, Raster Scan Systems, Random Scan Systems, Input devices.
: Introduction of Rendering, Raytracing, Antialiasing, Fractals

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Computer Graphics Notes

  1. 1. 1 | P a g e B y E r G u r p r e e t S i n g h Visit: BTCS 504 Computer Graphics OBJECTIVES: Understanding the fundamental graphical operations and the implementation on computer, Get a glimpse of recent advances in computer graphics, Understanding user interface issues that make the computer easy for the novice to use. COURSE CONTENTS: 1. Introduction: Computer Graphics and its applications, Elements of a Graphics, Graphics Systems: Video Display Devices, Raster Scan Systems, Random Scan Systems, Input devices. 2. Basic Raster Graphics: Scan conversion- Point plot technique, Line drawing, Circle generating and Ellipse generating algorithms. 3. Two-dimensional Geometric Transformations : Basic Transformations-Translation, Rotation and Scalling, Matrix Representation and Homogeneous Coordinates, Composite Transformations, Reflection and Shearing transformations. 4. Clipping: Window to viewport transformation, Clipping Operations- Point Clipping, Line Clipping, Polygon Clipping and Text Clipping. 5. Filling Techniques: Scan line algorithms, Boundary-fill algorithm, Flood-fill algorithm, Edge fill and fence fill algorithms, 6. Elementary 3D Graphics: Plane projections and its types, Vanishing points, Specification of a 3D view. 7. Visibility: Image and object precision, Hidden edge/surface removal or visible edge/surface determination techniques; z buffer algorithms, Depth sort algorithm, Scan line algorithm and Floating horizon technique. 8. Advance Topics: Introduction of Rendering, Raytracing, Antialiasing, Fractals, Gourard and Phong shading.
  2. 2. 2 | P a g e B y E r G u r p r e e t S i n g h Visit: Computer graphics are pictures and movies created using computers - usually referring to image data created by a computer specifically with help from specialized graphical hardware and software. It is a vast and recent area in computer science. The phrase was coined by computer graphics researchers Verne Hudson and William Fetter of Boeing in 1960. Another name for the field is computer-generated imagery, or simply CGI Computer graphics are visual representations of data displayed on a monitor made on a computer. Computer graphics can be a series of images (most often called video) or a single image. Computer graphics are very useful. Computer-generated imagery is used for movie making, video game and computer program development, scientific modeling, and design for catalogs and other commercial art. Some people even make computer graphics as art. Computer graphics can be 2D or 3D. They are made differently and used differently. People use different computer programs to make different types of graphics 2D graphics 2D computer graphics are usually split into two categories: vector graphics and raster graphics. Vector graphics Vector graphics use lines, shapes, and text to create a more complex image. If a vector graphic image is made very big on the monitor, it will still be as good as its regular size. This is one of the reasons vector graphics are liked so much. Vector graphics are made with programs like Adobe Illustrator and Inkscape, and were used for some older computer games. Today, they are often used for when computer graphics have to be printed out. Examples of vector graphics  A car.
  3. 3. 3 | P a g e B y E r G u r p r e e t S i n g h Visit:  A compact fluorescent light bulb. Figure: Vector Scan System
  4. 4. 4 | P a g e B y E r G u r p r e e t S i n g h Visit: Raster graphics Raster graphics use pixels to make up a larger image. This does not mean the artist has to change a single pixel at a time - raster programs often have tools like paintbrushes, paint buckets, and erasers to make a picture. Programs used to make these include Adobe Photoshop and Corel Paint Shop Pro. These are often used as part of what the user sees when she uses a computer program. Sometimes people do use only pixels to make an image. This is called pixel art and it has a very unique style. Examples of raster graphics  A pixel art image of "The Gunk."  Photographs are raster images.
  5. 5. 5 | P a g e B y E r G u r p r e e t S i n g h Visit: Figure: Raster Scan System Base of Difference Raster Scan System Random ( Vector ) Scan System Electron Beam The electron beam is swept across the screen, one row at a time, from top to bottom. The electron beam is directed only to the parts of screen where a picture is to be drawn. Resolution Its resolution is poor because raster system in contrast produces zig-zag lines that are plotted as discrete point sets. Its resolution is good because this system produces smooth lines drawings because CRT beam directly follows the line path. Picture Definition Picture definition is stored as a set of intensity values for all screen points, called pixels in a refresh buffer area. Picture definition is stored as a set of line drawing instructions in a display file.
  6. 6. 6 | P a g e B y E r G u r p r e e t S i n g h Visit: Realistic Display The capability of this system to store intensity values for pixel makes it well suited for the realistic display of scenes contain shadow and color pattern. These systems are designed for line- drawing and can’t display realistic shaded scenes. Draw an Image Screen points/pixels are used to draw an image. Mathematical functions are used to draw an image. Cost It is less expensive than Random Scan System. It is Costlier than Raster Scan System. Line Drawing Zig – Zag line is produced because plotted value are discrete. Smooth line is produced because directly the line path is followed by electron beam Refresh Rate Refresh rate is 60 to 80 frame per second. Refresh Rate depends on the number of lines to be displayed i.e 30 to 60 times per second. 3D graphics 3D graphics are graphics that look like objects because they are three-dimensional. This means the computer thinks it has a height, a length, and a depth, and displays them as this. Some programs used to make 3D graphics are Bryce, 3D Studio Max, Maya and Blender, and 3D graphics are used many times in movies and TV shows and video games. Pixel: Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected. The number of bits used to represent each pixel determines how many colors or shades of gray can be displayed. For example, in 8-bit color mode, the color monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colors or shades of gray. Persistence: The major difference between phosphors is their persistence. It decides how long they continue to emit light after the electron beam is removed. Persistence is defined as the time
  7. 7. 7 | P a g e B y E r G u r p r e e t S i n g h Visit: it takes the emitted light from the screen to decay to one-tenth of its original intensity. Lower persistence phosphors require higher refreshing rates to maintain a picture on the screen without flicker. However it is useful for displaying animations. On the other hand higher persistence phosphors are useful for displaying static and highly complex pictures Resolution: For graphics monitors, the screen resolution signifies the number of dots (pixels) on the entire screen, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis. Some common resolutions are: CGA (Color Graphics Adapter): 320 x 200 EGA (Enhanced Graphics Adapter): 640 x 350 VGA (Video Graphics Array): 640 x 480 SVGA (Super VGA): 800 x 600 XGA (Enhanced Graphics Array): 1024 x 768 720p: 1280 x 720 1080p: 1920 x 1080 Aspect Ratio: The aspect ratio of a geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side - the ratio of width to height when the rectangle is oriented as a "landscape". The aspect ratio is expressed as two numbers separated by a colon ( x : y ). The values x and y do not represent actual width and height but, rather, the "relation" between width and height. As an example, 8:5, 16:10 and 1.6:1 are the same aspect ratio. An aspect ratio of 4:5 means that a vertical line plotted with four points has the same length as a horizontal line plotted with five points Input Devices: Following are few of the important input devices which are used in a computer:  Keyboard  Mouse  Joy Stick
  8. 8. 8 | P a g e B y E r G u r p r e e t S i n g h Visit:  Light pen  Track Ball  Scanner  Graphic Tablet  Microphone  Magnetic Ink Card Reader(MICR)  Optical Character Reader(OCR)  Bar Code Reader  Optical Mark Reader(OMR) Video Display Devices: Cathode-Ray-Tubes: A CRT is an evacuated glass tube. An electron gun at the rear of the tube produces a beam of electrons which is directed towards the front of the tube (screen). The inner side of the screen is coated with phosphor substance which gives off light when it is stroked by electrons. It is possible to control the point at which the electron beam strikes the screen, and therefore the position of the dot upon the screen, by deflecting the electron beam. The fig below shows the electrostatic deflection of the electron beam in a CRT.
  9. 9. 9 | P a g e B y E r G u r p r e e t S i n g h Visit: The deflection system of the cathode-ray-tube consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. The voltage applied to vertical plates controls the vertical deflection of the electron beam and voltage applied to the horizontal deflection plates controls the horizontal deflection of the electron beam. There are two techniques used for producing image on the CRT screen: Vector scan / random scan and Raster scan. Rendering: Rendering is the process of generating an image from a 2D or 3D model (or models in what collectively could be called a scene file), by means of computer programs. Also, the results of such a model can be called a rendering. A scene file contains objects in a strictly defined language or data structure; it would contain:  geometry,  viewpoint,  texture,  lighting,  shading information as a description of the virtual scene. The data contained in the scene file is then passed to a rendering program to be processed and output to a digital image or raster graphics image file. When the pre-image (a wireframe sketch usually) is complete, rendering is used, which adds in bitmap textures or procedural textures, lights, bump mapping and relative position to other objects. The result is a completed image the consumer or intended viewer sees. For movie animations, several images (frames) must be rendered, and stitched together in a program capable of making an animation of this sort. Most 3D image editing programs can do this. A rendered image can be understood in terms of a number of visible features:  shading – how the color and brightness of a surface varies with lighting  texture-mapping – a method of applying detail to surfaces  bump-mapping – a method of simulating small-scale bumpiness on surfaces
  10. 10. 10 | P a g e B y E r G u r p r e e t S i n g h Visit:  fogging/participating medium – how light dims when passing through non-clear atmosphere or air  shadows – the effect of obstructing light  soft shadows – varying darkness caused by partially obscured light sources  reflection – mirror-like or highly glossy reflection  transparency (optics), transparency (graphic) or opacity – sharp transmission of light through solid objects  translucency – highly scattered transmission of light through solid objects  refraction – bending of light associated with transparency  diffraction – bending, spreading and interference of light passing by an object or aperture that disrupts the ray  indirect illumination – surfaces illuminated by light reflected off other surfaces, rather than directly from a light source (also known as global illumination)  motion blur – objects appear blurry due to high-speed motion, or the motion of the camera  non-photorealistic rendering – rendering of scenes in an artistic style, intended to look like a painting or drawing RayTracing: In computer graphics, ray tracing is a technique for generating an image by tracing the path of light through pixels in an image plane and simulating the effects of its encounters with virtual objects. The technique is capable of producing a very high degree of visual realism, usually higher than that of typical scanline rendering methods, but at a greater computational cost. This makes ray tracing best suited for applications where the image can be rendered slowly ahead of time, such as in still images and film and television visual effects, and more poorly suited for real-time applications like video games where speed is critical. Ray tracing is capable of simulating a wide variety of optical effects, such as reflection and refraction, scattering, and dispersion phenomena (such as chromatic aberration). Ray tracing's popularity stems from its basis in a realistic simulation of lighting over other rendering methods (such as scanline rendering or ray casting). Effects such as reflections and shadows, which are difficult to simulate using other algorithms, are a natural result of the ray
  11. 11. 11 | P a g e B y E r G u r p r e e t S i n g h Visit: tracing algorithm. The computational independence of each ray makes ray tracing amenable to parallelization. A serious disadvantage of ray tracing is performance. Scanline algorithms and other algorithms use data coherence to share computations between pixels, while ray tracing normally starts the process anew, treating each eye ray separately. However, this separation offers other advantages, such as the ability to shoot more rays as needed to perform spatial anti-aliasing and improve image quality where needed. Although it does handle inter reflection and optical effects such as refraction accurately, traditional ray tracing is also not necessarily photorealistic. True photorealism occurs when the rendering equation is closely approximated or fully implemented. Implementing the rendering equation gives true photorealism, as the equation describes every physical effect of light flow. However, this is usually infeasible given the computing resources required AntiAliasing: In computer graphics, antialiasing is a software technique for diminishing jaggies - stairstep-like lines that should be smooth. Jaggies occur because the output device, the monitor or printer, doesn't have a high enough resolution to represent a smooth line. Antialiasing reduces the prominence of jaggies by surrounding the stairsteps with intermediate shades of gray (for gray- scaling devices) or color (for color devices). Although this reduces the jagged appearance of the lines, it also makes them fuzzier. Another method for reducing jaggies is called smoothing, in which the printer changes the size and horizontal alignment of dots to make curves smoother. Antialiasing is sometimes called oversampling. Fractals: The term "fractal" was coined by Benoit Mandelbrot in 1975. It comes from the Latin fractus, meaning an irregular surface like that of a broken stone. Fractals are non-regular geometric shapes that have the same degree of non-regularity on all scales. Just as a stone at the base of a foothill can resemble in miniature the mountain from which it originally tumbled down, so are fractals self-similar whether you view them from close up or very far away.
  12. 12. 12 | P a g e B y E r G u r p r e e t S i n g h Visit: Fractals are the kind of shapes we see in nature. We can describe a right triangle by the Pythagorean theorem, but finding a right triangle in nature is a different matter altogether. We find trees, mountains, rocks and cloud formations in nature, but what is the geometrical formula for a cloud? How can we determine the shape of a dollop of cream in a cup of coffee? Fractal geometry, chaos theory, and complex mathematics attempt to answer questions like these. Science continues to discover an amazingly consistent order behind the universe's most seemingly chaotic phenomena. Mathematicians have attempted to describe fractal shapes for over one hundred years, but with the processing power and imaging abilities of modern computers, fractals have enjoyed a new popularity because they can be digitally rendered and explored in all of their fascinating beauty. Fractals are being used in schools as a visual aid to teaching math, and also in our popular culture as computer-generated surfaces for landscapes and planetary
  13. 13. 13 | P a g e B y E r G u r p r e e t S i n g h Visit: