week of September 28
 

Quiz

• Assignment 5
• For this assignment you can use Java or Javascript.  Javascript is probably easier (mainly because you don't have to use all those annoying access functions to get at fields of a node, or do any of the event decoding stuff).
• You can make a transparent fish bowl (cylinder whatever) by using the transparency field for the material node.
• You can "pass a node" to your javascript like so:

 

 

DEF fish1 Transform{  // This transform node positions the fish
  .
  .
  .
  children[
      // The fish is defined in here
  ]
}

DEF the_script  Script{
 url "javascript:

   function some_function (frac, eventTime) {
      .
      .
      .
      fish_1_translation = ifish1.translation;
      .
      .
      .
    }
        "

   field SFNode ifish1 USE fish1  //This makes an SFNode variable
                                  //initialized to the Transform
                                  //for fish
   eventIn SFFloat some_function
}

# Route the time signal to the some_function
ROUTE MainClock.fraction_changed TO the_script.some_function
 
 

• How to make the fish move:
• All the fish move based more or less on the idea that they attempt to be in some location.
• The lead fish should always be swimming around in circles, but should occasionally decide to change its depth and radius.  The professor suggests that you use a random variable to decide when to make a change in desired depth or radius, and then simulate some force acting on the fish to get it to that depth and radius as it continues to circle the tank.  To do this you need to come up with some formula which determines a force based on how far the fish is from where it wants to be.  Note that it is important that the fish not approach its desired location too fast, so as the fish approaches you should probably have some force retarding its motion so that it does not overshoot the desired location (at least not by too much).  Also note that you do not actually have to compute the force all at once, but could compute a force (actually an acceleration) for the radius seperately from an acceleration up and down, and again for angular acceleration (the angle of the fish around the tank should not always change at the same rate).  You could see this as computing the force on the fish in a different (cylindrical) coordinate system.  Finally you can avoid doing the second order dynamics (basically skip a step) by simply making the fish change its radius rapidly towards the desired radius at first, and then more slowly as it approaches.  This simulates the effect of makeing some force which at first pushed the fish towards the desired radius, and then slows it down.
• The following fish work along similar lines, but attempt to stay in a fixed position relative to the lead fish.  By giving each following fish a significantly different desired position relative to the lead fish you can make collisions unlikely.  In addition you should prevent the following fish from staying exactly in their desired locations relative to the lead fish.  (Optional: You could also do collision detection between all of the fish, or use some technique to apply repulsive forces to the fish if they get too close)
• Animation (Two ends of the animation spectrum)
• Key Framing:  Specify the location and orientation or other parameters at discrete times and interpolate between them.
• Provides maximum control
• May require too much input
• Dynamic Simulation:  Specify rules for how objects move and interact and let these animate objets given initial conditions.
• Possibly a great deal less input for very detailed and long animation sequences
• Possibility of greater physical reality
• It is often difficult (or impossible) to make things do specifically what you want
• Kinematics
• Forward
• Specify Joint angles (or more generally parameters) and get back position, shape, etc.
• In an dynamic simulation this might be useful as joint angles could be detemined by the interaction of forces on a model.  While making keyframes an animator might want to specify specific joint angles (but more commonly would use IK).
• Inverse
• Specify location of end effector or desired shape, position, etc and get back joint angles or parameters which will result in  that location for the end effector or that position or shape, etc. 
• In key framing an animator might use IK to position a hand or finger in a particular position without having to figure out the joint angles herself.   

• Particle Systems
• Particle systems span a large range from bits of rain falling to fish swimming in schools.  They are usefull in the simple cases because by modeling small simple particles we can generate complex shapes (like a water fountain over time) and in the complex cases (like fish) we can model a complex animation without minimal work.
• It is useful to look at particle systems in terms of the following criteria:
• What are the particles?
• How do they move?
• Do they interact and if so, how?
• What are tthe initial conditions for the particles?
• What is the a terminating (or extinction) condition for the particles?