ch3

• OpenGL


  Graphics library developed by Silicon Graphics, an API that provides a large set of functions to manipulate graphics and images
• OpenGL


  • Platform independent, but not windows-system independent
  • Provides the core library
• glut


  • GL Utilities Toolkit, contains extra routines for drawing 3D objects and other primitives
  • Enables writing windows-system independent code
• glu


  • OpenGL Utilities, contains extra routines for projections and rendering complex 3D objects
• glui


  • Contains extra routines for creating user-interfaces
• Function naming convention


  • Prefix is gl, glu, or glut depending on the library
  • Main part indicates the purpose of the function
  • Suffix indicates the number and type of arguments
• Symbolic constants


  Predefined identifiers defined using #define, always written in capital letters
• Getting started with OpenGL

  1. Include gl.h for core OpenGL

  2. Include glu.h for glu library

  3. Include glui.h for glui library

  4. Include glut.h for glut library
• Writing OpenGL code


  1. Perform GLUT initialization with glutInit

  2. Set initial window position with glutInitWindowPosition

  3. Set initial window size with glutInitWindowSize

  4. Set display mode options with glutInitDisplayMode

  5. Create display window with glutCreateWindow
• Event-driven


  OpenGL executes code in response to events, defined by programmer-defined callback functions
• Display event


  Most important event, occurs whenever OpenGL needs to redraw the display window
• Structure of OpenGL programs

  1. Perform initialization tasks

  2. Define callback functions and associate them with events

  3. Start the event loop
• OpenGL programs consist of a number of parts: initialization, defining callback functions, and starting the event loop
• Coordinate system


  System that uses one or more numbers, or coordinates, to uniquely determine the position of points or other geometric elements
• Number line


  • Simplest example of a coordinate system, identifies points on a line with real numbers
• Cartesian coordinate system


  • Prototypical example, uses perpendicular lines in 2D or mutually orthogonal planes in 3D to define coordinates
• Synthetic camera


  Programmer's reference model for specifying 3D view projection parameters, including position, orientation, field of view, depth of field, and projection type
• View volume


  Contains everything visible from the camera's point of view, can be approximated by a rectangular cone (frustum) for efficient clipping
• Primitive attribute


  Parameter that affects the way a primitive is displayed, OpenGL maintains a list of current state variables representing these attributes
• Color representation


  RGB: color defined by red, green, and blue components

  RGBA: color defined by red, green, blue, and alpha (transparency) components
• Frame buffer


  Used by raster graphics systems to store the image ready for display, can use direct storage or color look-up table
• Direct storage


  Color values of each pixel are stored directly in the frame buffer
• Color look-up table


  Stores an index for each pixel, the actual colors are stored in a separate look-up table
• OpenGL uses direct storage by default, but the programmer can choose to use a color look-up table
• Setting OpenGL color attributes

  Use glColor*() to set the drawing color

  Use glClearColor() to set the background color

  Drawing color can be set using RGB or RGBA representation

  Background color is always set using RGBA representation
• Defining the frame buffer for color drawing

  Use glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB) for RGB color

  Use glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA) to store alpha values
• Down the rasterization process, an extra look-up operation is required
• Color look-up tables were commonly used in the early days of computer graphics when memory was expensive, but these days memory is cheaper, so most systems use direct storage
• OpenGL uses direct storage by default, but the programmer can choose to use a color look-up table if they wish
• glColor*


  Function to change the current drawing colour
• glClearColor


  Function to change the current background colour
• The background colour is always set using an RGBA colour representation
• The drawing colour can be set using either RGB or RGBA representations
• Setting drawing and background colours

  • glColor3f(1.0,0.0,0.0);
  • glColor4f(0.0,1.0,0.0,0.05);
  • glClearColor(1.0,1.0,1.0,1.0);
• Defining a frame buffer before drawing in OpenGL

  1. Specify a color frame buffer

  2. To store alpha values in the frame buffer, use GLUT_RGBA

  3. To use a color look-up table, use GLUT_INDEX
• Point primitives


  The only attributes we can modify are the color and size
• Changing point size


  Use glPointSize(size)
• Changing point color


  Use glColor3f(colorCode)
• Line primitives


  • Attributes that can be modified: line color, line width, line style (solid/dashed etc.)
• setcolor


  Function to set the color of a line