Basic 3D Graph in OpenGL

Box and Sphere

• Perspective Mode
  In Perspective Mode, parallel lines are not really drawn parallel but are drawn such that they appear to converge at some point on the horizon. By using Perspective, scenes will look more realistic since objects in the distance appear smaller than ones close to the viewer even though they are defined to have the same dimensions. In Orthographic Mode, however, parallel lines are always drawn parallel and identical linear measures will always be drawn the same size regardless of their distance from the viewer.

  In OpenGL, some of the important Perspective commands are the following. Read about them in more detail in your textbook, but here are a few we will be emphasizing in this program:
  
  

  1. glMatrixMode(GL_PROJECTION)
     Sets up the matrix that will be used to appy to objects drawn in Perspective Mode
     
     
  2. gluPerspective(Angle, Aspect Ratio, Near Clipping, Far Clipping)
     This determines how the window is viewed. The Angle, represented in degrees, refers to the amount of the scene that the eye can actually view. The Aspect Ratio is the ratio between width and height of the viewing field. The Near Clipping Plane instructs the graphic engine to avoid drawing anything that is closer than the indicated number. The Far Clipping Plane is similar in concept, but instructs the program to avoid wasting time on anything that is farther away than the specified number.
     
     
  3. gluLookAt(Eye_X, Eye_Y, Eye_Z, View_X, View_Y, View_Z, Up_X, Up_Y, UpZ)
     Determines how the user will be viewing the scene. There are three coordinates for the position of the viewer's "Eye", three for the center of the person's focus or "View", and three to determine which way is "Up".
  
  
  
• Depth Buffering
  Depth buffering, also known as "Z-Buffering", controls the order in which objects are drawn by calculating their distance away from the viewer along the z-axis. Items that are far away are drawn first, and items close to the viewer are drawn last. Without Depth Buffering, it would be much more difficult to draw objects in a scene that might overlap because whichever one is drawn last will hide portions of previously drawn images. For instance, if the box and the sphere were drawn without Depth Buffering, whichever one was drawn last would be drawn over top of the previous image. It would be much more difficult to have the sphere appear to be within the box, overlapping the back edge but being obscured by the front edge as shown in the example.

  To activate depth buffering, the following commands are used. In main, there is a call to:
  

  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );

  Then in the init() function there is a call to:
  

  glEnable(GL_DEPTH_TEST);

  
  This sets up depth buffering, so that the program handles the concerns of overlapping objects and which one should be drawn first.
  
  
• Lighting
  Lighting is a very complex subject. We will look at in more detail later but for now we will be using the following commands to make our image look nice. First there are two global arrays related to light.

  float light_diffuse[] = {2.0, 2.0, 1.0, 1.0}; // strong light yellow light
  float light_position[] = {10.0, 5.0, 5.0, 0.0}; // Position of the light
  

  In the init() function, the following are used:

  glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
  glLightfv(GL_LIGHT0, GL_POSITION, light_position);
  glEnable(GL_LIGHTING);
  glEnable(GL_LIGHT0);

  
  
  
• The Source Code
  Take a look at the source code: basic3D.c