Computer graphics lecture notes

Computer graphics lecture notes in Computer graphics uploaded at P.R.Engineering college

Computer graphics lecture notes in Computer graphics at P.R.Engineering college uploaded by P.R.Engineering college Staff in lecturenotes.net

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CHAPTER I

INTRODUCTION
LEARNING OBJECTIVES

• To understand the different primitives like line, circle and their construction using different methods.
• To learn different clipping algorithms and their importance in visualization
 OVERVIEW OF GRAPHICS SYSTEMS
Construction of pictures / drawings through computers and various manipulations are known as computer graphics. An image is nothing but 2d quantity of pixels, arranged in the form of matrix. The comparison between image and graphics is as given in table 1.1 below. Tools and techniques like rotation, scaling and translation are common for both graphics and image processing. Graphics find applications like building drawing, machine drawing, air craft design, simulation and modeling, presentation, advertisement, entertainment and game applications. Apart from that it has applications in critical domains like accident analysis, earth quake analysis, weather forecasting and atomic radiation pattern analysis.
Table 1.1 Image Vs Graphics

Image Graphics
Real picture Artificially created
Captured through camera / scanner Drawn using computer
Editable only as a sub picture level Individual elements are editable
Two dimensional Any number of dimensions

RasterAlgorithms
The goal of these raster algorithms is to convert graphical primitives into a set of pixels as efficiently as possible. This process of converting from primitives to pixels is known as scan conversion. These algorithms typically use incremental methods in order to minimise the number of calculation (particularly multiplications and divisions) that are
 
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performed during each iteration. These techniques use integer arithmetic rather than floating- point arithmetic.
 Video Display Devices
 CRT monitors

A beam of electrons emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.
The maximum number of points that can be displayed without overlap on a CRT is referred to as the resolution. Typical resolution on high-quality systems is 1280 by 1024, with higher resolutions available on manysystems. High resolution systems are often referred to as high-definition systems. Another property of video monitors is aspect ratio. This number gives the ratio of vertical points to horizontal points necessary to produce equal- length lines in both directions on the screen. An aspect ratio of 3/4 means that a vertical line plotted with three points has the same length as a horizontal line plotted with four points.

Raster Scan Displays

The most common type of graphics monitor employing a CRT is the raster-scan display, based on television technology. In a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and “painted” on the screen one row (scan line) at a time. Each screen point is referred to as a pixel or pel
 
(shortened form of picture element). The capabilityof a raster-scan systemto store intensity information for each screen point makes it well suited for the realistic display of scenes containing subtle shading and color patterns. Home television sets and printers are examples of other systems using raster-scan methods.
A system with 24 bits per pixel and a screen resolution of 1024 by 1024 requires 3 megabytes of storage for the frame buffer. On a black-and-white system with one bit per pixel, the frame buffer is commonly called a bitmap. For systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.

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 displaying 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, 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 (Fig. 2-8). Interlacing of the scan lines in this way allows us to see the entire s m n displayed in one-half the time it would have taken to sweep across all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. On an older, 30 frame per-second, non-interlaced display, for instance, some flicker is noticeable. But with interlacing, each of the two passes can be accomplished in 1/60th of a second, which brings the refresh rate nearer to 60 frames per second. This is an effective technique for avoiding flicker, providing that adjacent scan lines contain similar display information.
Color CRTs in graphics systems are designed as RGB monitors. These monitors use shadow-mask methods and take the intensity level for each electron gun (red, green, and blue) directly from the computer system without anyintermediate processing. High-quality
 
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raster-graphics systems have 24 bits per pixel in the frame buffer, allowing 256 voltage settings for each electron gun and nearly 17 million color choices for each pixel. An RGB color system with 24 bits of storage per pixel is generally referred to as a full-color system or a true-color system.
 Random Scan Displays
Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays. The component lines of a picture can be drawn and refreshed by a random-scan sys- tem in any specified order. Apen plotter operates in a similar way and is an example of a random-scan, hard-copy device. Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition is now stored as a set of line drawing commands in an area of memory referred to as the refresh display file. Sometimes the refresh display file is called the display list, display program, or simply the refresh buffer. To displaya specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all line drawing commands have been processed, the system cycles back to the first line command in the list.
Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. Random-scan systems are designed for line drawing applications and cannot display realistic shaded scenes. Since picture definition is stored as a set of line drawing instructions and not as a set of intensity values for all screen points, vector displays generally have higher resolution than raster systems. Also, vector displays produce smooth line drawings because the CRT beam directly follows the line path. A raster system, in contrast, produces jagged lines that are plotted as discrete point sets.
 Flat panel displays

Title:
Computer graphics lecture notes

Subject:
Computer graphics

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P.R.Engineering college

Computer graphics lecture notes

Computer graphics lecture notes in Computer graphics uploaded at P.R.Engineering college

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