计算机图形学（八）Modeling and Hierarchy

1. Example I: Car Model

• DAG Model
  • Directed acyclic graph 有向无环图
• Modeling with Tree
  • Nodes: what to draw, pointers to children
  • Edges: may have information on incremental changes to transformation matrices (can also store in nodes)
• 层级（hierarchical）

2. Example II: Robot Arm

• Articulated Model 铰链模型

• Possible Node Structure

  

• OpenGL code for robot

  mat4 ctm;
  robot_arm()
  {
  ctm = RotateY(theta);
  base();
  ctm *= Translate(0.0, h1, 0.0) * RotateZ(phi);
    lower_arm();
  ctm *= Translate(0.0, h2, 0.0) * RotateZ(psi);
    upper_arm();
  }

3. Example III: Humanoid Figure

• Tree with Matrices

  

• Display and Traversal

  • Stack-based Traversal code

  figure() {
  mvstack.push( model_view );//save present model-view matrix 
  torso();
  //update model-view matrix for head
  model_view = model_view*Translate(...)*Rotate(...); 
  head();
  
  model_view = mvstack.pop();//recover original model-view matrix 
  mvstack.push( model_view );//save it again
  //update model-view matrix for left upper arm
  model_view= model_view*Translate(...)*Rotate(...); 
  left_upper_arm();
  
  model_view = mvstack.pop();//recover original model-view matrix again ......
  }

  • General Tree Data Structure

    • left-child right sibling structure

      • Used linked lists
      • Each node in data structure is two pointers
      • Left: next node
      • Right: linked list of children

      

    • Tree node structure

      • Pointer to sibling / child
      • Pointer to a function that draws the object represented by the node
      • Homogeneous coordinate matrix to multiply on the right of the current model-view matrix

4. OpenGL and Objects

• OpenGL lacks an object orientation

5. Scene Graph

• Scene graph

• Group Nodes
  • Necessary to isolate state chages
    • Equivalent to Push/Pop
  • Note that as with the figure model
    • We can write a universal traversal algorithm
    • The order of traversal can matter
• Events and the Scene Graph
  • It is not difficult to let any part of the scene graph vary
  • Events could be used to identify the part of the scene graph and how it is to change
    • Change geometry
    • Change transformations
    • Change appearance

6. Events in OpenGL

• Event Processing in OpenGL

• code example

  int main( int argc, char **argv){
      glutInit( &argc, argv); 
      glutInitWindowSize( 400, 400); 
      glutInitWindowPosition( 200, 100);
      glutCreateWindow( “Sample”);
  
      //specify callback functions
      glutDisplayFunc( display);
      glutIdleFunc( animation); 
      glutKeyboardFunc( kboard);
      glutSpecialFunc(special_key); 
      glutMouseFunc( mousef);
      glutMotionFunc( mousePos); 
      glutCreateMenu( menuf ); 
      glutTimerFunc(300, timer, -1); glutReshapeFunc(reshape);
  
      init(); 
      glutMainLoop();
      return 0;
  }
  
  timer() {
    ... ; 
    glutPostRedisplay(); 
    // call display() right now
  }
  
  void mousef(int button, int state, int x, int y){
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) {
      drag = true; 
    }
    if (button == GLUT_LEFT_BUTTON && state == GLUT_UP) { 
      xx = x/100.-2.; yy = 2.-y/100.;
      drag = false;
      glutPostRedisplay();
    }
  }
  
  void mousePos( int x, int y) { 
    if ( drag ) {
      xx = x/100.-2.; yy = 2.-y/100.;
      glutPostRedisplay(); 
    }
  }
  
  glutCreateMenu( menuf ); //callback menuf( int i) is called if triggered
  glutAddMenuEntry("Level 1: Beginner", 1);
  glutAddMenuEntry("Level 2: amateur", 2);
  glutAddMenuEntry("Level 3: Professional", 3);
  glutAttachMenu(GLUT_RIGHT_BUTTON);
  
  void  menuf( int i) { 
    if (i == 1) {
      ispeed = 0.002; rw = .125;
      }
      else if (i == 2) {
          ... 
    }
      else if (i == 3) { 
      ...
      }
      ...
  }
  
  //Create the main menu
  glutCreateMenu( mainf ); 
  glutAddMenuEntry("Restart", 1);
  //Link the sub-menu to the main menu 
  glutAddSubMenu("Set Level", levelMenu); 
  glutAddMenuEntry("Quit", 3); 
  glutAttachMenu( GLUT_RIGHT_BUTTON);
  //Create a sub-menu
  int levelMenu = glutCreateMenu( levelf); 
  glutAddMenuEntry("Learner", 1); 
  glutAddMenuEntry("Average", 2); 
  glutAddMenuEntry("Racing God", 3);
  
  //define Callback function for the main menu
  void mainf( int i) {
    if (i == 1) { restart(); . . . }
    else if (i == 3) { . . . }
    //If i == 2, levelf() will be called automatically
  }
  
  //define Callback for //the sub-menu 
  void levelf( int i) {
    if ( i == 1) . . .
    ... 
  }

• MUI

  • MUI is a particular user interaction toolkit
  • It’s very simple, but very easy to use