Gravity simulation
Exercise

Description

In this exercise you will build a ball shape, then animate its position to cause the shape to bounce up and down, as if under the effect of gravity. Next, you will expand on the exercise and add nodes to sense the viewer's touch and start and stop the animation.

In visualization, animations are often based upon data computed by simulation applications. This example uses a simple gravity simulation program to generate a series of 3D positions. Your task will be to incorporate these positions into your animation to cause a shape to bounce.

This exercise focuses upon use of TimeSensor, PositionInterpolator, and TouchSensor nodes.

VRML Concepts

This exercise requires you to understand and make use of the following VRML features and concepts:
• Basics
  • VRML file structure
  • Comments
  • DEF and USE
• Shapes and appearance
  • Shape node
  • Appearance node
  • Material node
  • Box geometry node
  • Sphere geometry node
• Grouping
  • Transform node
• Animation and touch
  • TimeSensor node
  • PositionInterpolator node
  • ROUTE statement
  • TouchSensor node

Files

Here are the files you may use to get started with this exercise:
• gravity.c
  This is the source code file for a C program which generates key fractional times and key position values. In the first task you will compile this program and run the resulting application to produce the raw data. In subsequent tasks, you will use the data within an animated VRML world according to each task's instructions.

• Makefile
  This is the make file which contains instructions for the make program. This will enable you to easily build the applications you will create throughout this exercise.

Tasks

• First, compile and run the gravity program, redirecting the output into a data file called "gravity.dat":
  % make gravity
  % gravity > gravity.dat

  You will need to use this data for subsequent tasks.

  SOLUTION: gravity.dat

1. Create and edit a new file in "gravity1.wrl":
   % vi gravity1.wrl

   Your task is to make ball and ground shapes, then make the ball bounce up and down forever. You'll need a TimeSensor node and a PositionInterpolator node. Use the data from the "gravity.dat" file for the key fractional times and key positions for the interpolator.

   HINT: When a TimeSensor node's start time is later than its stop time, the sensor never stops.

   HINT: You will need to route the fractional times out of the timer into the interpolator.

   HINT: You will need to route the position values from the position interpolator into a Transform node.

   SOLUTION: gravity1.wrl

   ---

2. Make a copy of the "gravity1.wrl" world:
   % cp gravity1.wrl gravity2.wrl

   The animation from task 1 runs forever. Your task is to modify gravity2.wrl to make the ball bounce just once each time the ball is touched. You'll need a TouchSensor node.

   HINT: TouchSensor nodes sense the viewer's touch on any shape in the same group of children nodes.

   HINT: The start time of a TimeSensor node can be set by another node.

   SOLUTION: gravity2.wrl

   ---

3. Make a copy of the "gravity2.wrl" world:
   % cp gravity2.wrl gravity3.wrl

   The animation from task 2 bounces the ball once, then stops. Your task is to modify gravity3.wrl to make the ball start bouncing when the viewer touches a button, and stop bouncing when the viewer touches another button.

   HINT: Use more than one TouchSensor node.

   HINT: The stop time of a TimeSensor node can be set by another node.

   SOLUTION: gravity3.wrl

   ---

4. Make a copy of the "gravity3.wrl" world:
   % cp gravity3.wrl gravity4.wrl

   Extra credit: The previous tasks enabled you to make the ball bounce forever, just once, or between a starting and stopping time. Another common animation need is to cause an animation to play back for a specific number of cycles, then stop. Your task is to modify gravity4.wrl to make the ball bounce exactly twice each time the viewer touches the start button. Pressing the stop button stops the animation prematurely.

   HINT: Use more than one TimeSensor node.

   HINT: A TimeSensor node can be enabled and disabled using output from another node.

   HINT: A TimeSensor node outputs TRUE when the sensor becomes active, and FALSE when it's done.

   SOLUTION: gravity4.wrl