1 Computer Graphics Part 2: Images

2 What is an image?  An image is the graphical and visual representation of some information that can be displayed on a computer screen or printed out  Images come in a variety of forms:  Photographs  Drawings  Paintings  Television and motion pictures  Maps etc.

3  Images show us the prominent features of the objects that they represent.  Images play an important part in multimedia Navigation User interface components Help systems Clip art What is an image?

4 Image media types  Images can be generally divided into two formats:  Bitmapped or raster images  Vector graphics or Metafile images  Bitmapped images are stored as an array of pixels  Vector graphics are stored as the set of graphic primitives required to represent the image

5 Bitmaps Image  A pixel is the smallest element of resolution on a computer screen  A pixel is the basic unit of a digital images. Digital image is a picture that may be stored in, displayed on, processed by a computer.  As mentioned, bitmap is composed of a matrix elements called pixels  Each pixel can be in a specific colour and each pixel consists of two or more colours.

6 Bitmaps Image  The range of these colours is known as the colour depth.  The colour depth determine “How much data in bits used to determine the number of colours”.  Colour depth is measured in bits per pixel  Remember: a bit (binary digit) is either 1 or 0 and that there are eight bits in a byte

7 Colour depth 1 bit per pixel = 2 colours (monochrome) 2 bits per pixel = 4 colours 4 bits per pixel = 16 colours 8 bits per pixel = 256 colours Generally good enough for colour images 16 bits per pixel = colours Better quality for photograph-like images, also known as high colour 24 bits per pixel = >16 million possible colours Used to recreate photo realistic images, also known as true colour

8 Bitmaps Image  The number of pixels is related to the size of file that required to store an image.  Remember, two factors effect the size of file bitmap :  Resolution  Color Depth

9 Bitmapped images Original image Shownmagnified

10 Calculating the size of a raster image  Where:  Width of the images measured in pixels  Height of the images measured in pixels  Colour depth is the number of bits used for color measured in bits per pixel  Remember:  1024 bytes = 1 kilobyte (KB)  1024 kilobytes = 1 megabyte (MB)

11 Example  A 640 x 480 pixel image in 24-bit colour would require how much disk space?

12 Popular bitmap formats  Microsoft bitmap (.bmp)  Used in Microsoft windows  TIFF - Tagged Image File Format (.tif)  Used for faxing images  JPEG - Joint Photographic Expert Group (.jpg)  Useful for storing photographic images

13 Popular bitmap formats  GIF - Graphics Interchange Format (.gif)  Used a lot on web sites  PNG - Portable Network Graphics (.png)  A new format for web graphics  PCD – Kodak photo CD  A new format for store image in a compressed form on a CD

14 Advantages and Disadvantages of using bitmap images  Advantages  Convey(يوصل, ينقل) detail of information quickly  Real life  Disadvantages  Depend on a Resolution  Effect to the image quality  Size file is big

15 Vector images  Vector images are stored as the set of graphic primitives required to represent the image  A graphic primitive is a simple graphic based on drawing elements or objects such as shape  square, line, arc, etc.  The image consists of a set of commands (mathematical equations) that are drawn the object when needed.

16 Vector images  Storing and representing images by mathematical equations is called vector graphics or Object Oriented graphics.  Each primitive object has various attributes that go to make up the entire image  x-y location, fill colour, line colour, line style, etc.  Example:  RECTANGLE : rectangle top, left, width, height, colour is ( 0, 0, 300, 200, red)

17 Vector images  CIRCLE : circle top, left, radius, colour  LINE : Line x1, y1, x2, y2, colour  Vector image or vector graphics can be resized without losing the integrity of the original image.  Scaling a vector is a mathematical operation - only the attributes change, the image is unaffected

18 Primitive geometric drawing objects  Basic  Line  Polyline  Arc  Bezier curve  Text  Font  Shapes  Circle  Rectangle  Square  Triangle  Pentagon, hexagon, heptagon, octagon, etc

19 Scaling vector graphics Original image Shownmagnified

20 Advantages and Disadvantages of using vector image  Advantages  Relatively small amount of data required to represent the image.  Therefore, it does not required a lot of memory to store  Easier to manipulate  Disadvantages  Limited level of details that can be presented in an image

21 Vector formats  Windows metafile (.wmf)  Used by Microsoft Windows  SVG - Scalable Vector Graphics (.svg)  A new format devised for the web  CGM - Computer Graphics Metafile (.cgm)  Older format commonly used for clip art  Adobe PostScript (.ps)  A page description language used to control printers

22 Vector formats  Adobe Portable Document Format (.pdf)  A page description language common on the web  Drawing Exchange Format (.dfx)  Store 3D image created by design program AutoCAD  Encapsulated PostScript (.epf)  Professional printing: Illustration program, Adobe Systems, Desktop Publishing programs

23 3-Dimensional Graphic models  A 3D model is a variation on the vector format  The location of a 3-dimensional object is specified using x, y and z co-ordinates  Further primitives can be found in 3D models  Cube, sphere, pyramid, etc.  Camera, spotlight, texture, shading etc. 3D model X Y Z

24 3-Dimensional Graphic models  3D graphics offer the photo-realistic effects that have you seen in TV, Computer Games  Examples, Motion Picture films such as:  Jurassic Park, Terminator 2, Lost World and Toy Story  Examples 3D programs:  Carigali Truespace  3D Studio Max  Infini-D

25 3-Dimensional Graphic models

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27 Resolution Resolution (def1): number of points that can be displayed on a CRT ( cathode ray tube). l Resolution (def2): number of points per centimeter that can be plotted horizontally and vertically. l Typical resolution on high-quality systems 1280 X l Higher resolution systems : also called high- definition systems. l The higher resolution gives the better picture quality.

28 Raster Scan Displays (RSD) l Graphics monitors employing CRT technology is called raster scan displays, based on TV technology. l In RSD the beam is swept across the screen one row at a time from top to bottom, and from left to right. l Picture definition is stored in a memory area called Refresh Buffer or Frame Buffer. l Refresh Buffer contains the intensities of each point on the screen

29 Frame Buffer l Each screen point is referred to a pixel or pel (Picture element). l In simple black and white systems we need 1 bit per pixel. l High resolution systems use 24 bit(3 byte ) per pixel. l Systems with resolution of 1024 x 1024, and 24 bit per pixel requires approximately 3 MByte of memory to store only a single frame. l Black and white systems that require 1 bit /pixel are called bitmap. l Colored systems that uses multibits/pixel are called pixmap.

30 Refresh rate l Refresh rate for RSD carried out at the rate of 60 to 80 frames per second, although there might be some systems with higher refresh rates. l Refresh rates are described in units of cycles per second or Hertz (HZ). l So when we refer to a system with refresh rate of 60 frame per second we simply say 60 Hz.

31 How RSD works l The RSD starts from the upper-left corner of the screen, and continues at the same scan line (as shown in fig3). At the end of each scan line ( a single row of pixels in a raster graphics image ), the electron beam ( Cathode rays )returns to the left side of the screen to begin displaying the next line. l The return to the left side of the screen to begin displaying the next scan line is called horizontal retrace of the electron beam. l At the end of each frame the electron beam must return to the top left corner of the screen to begin a new frame this is called vertical retrace.

32 Figure 3: The raster scan displays an object as a set of discrete point across each scan line

33 Color raster images l The most common way to represent the colors of pixels in a color raster image is using an order triple (red,green,blue). l Color Depth : is the number of bits used to represent the color of each pixel.

34 Color raster images displayedColor value black0,0,0 blue0,0,1 green0,1,0 cyan0,1,1 red1,0,0 magenta1,0,1 yellow1,1,0 white1,1,1 Table 1: colors displayed with a color depth of 3 bits

35 Color raster images l If a color depth of a byte represents 256 colors.in this system we allow the red and green components to have three bits each and the blue component has two bits. l True color systems as already defined use color depth of 3 bytes, 1 byte per each color.