Extrusion
Exercise

Description

In this exercise you will build an extrusion shape, then animate a ball sliding up the extrusion. Next, for extra credit, you will reverse the direction of the animation so that the ball slides down the extrusion instead of up!

In visualization, shapes and animations are often created by using data generated by programs. This example uses an extrusion path generator program. Your task will be to take the output of this program and build VRML nodes around it to make the extrusion shape and ball animation.

This exercise focuses upon use of Extrusion, TimeSensor, PositionInterpolator, and ScalarInterpolator 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
  • Sphere geometry node
  • Extrusion geometry node
• Grouping
  • Transform node
• Animation and touch
  • TimeSensor node
  • PositionInterpolator node
  • ScalarInterpolator node
  • ROUTE statement
  • TouchSensor node

Files

Here are the files you may use to get started with this exercise:
• extrude.c
  This is the source code file for a C program which generates the extrusion cross-section, extrusion spine, key fractional times, and key positions for an animation. 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 extrude program, redirecting the output into a data file called "extrude.dat":
  % make extrude
  % extrude > extrude.dat

  You will need to use this data for subsequent tasks.

  SOLUTION: extrude.dat

1. Create and edit a new file in "extrude1.wrl":
   % vi extrude1.wrl

   Your task is to create an extrusion shape using the cross-section and extrusion spine values output by the extrude program and saved in extrude.dat. You will need to create a Shape, set it's Appearance and Material, then build the Extrusion geometry. In addition to the cross-section and spine for the extrusion, set these fields:

   creaseAngle 0.785
   solid TRUE

   You'll need to read the extrusion data into your VRML file. In vi you can do this by typing, in command mode:

   :r extrude.dat

   This will read the extrude.dat file contents into your VRML file starting on the line right after the line the cursor is on.

   HINT: After you've read the extrusion data in, add this line before the cross-section data output by the program:
   crossSection [

   Then go to the end of the cross-section data and add this after it, and before the extrusion spine data:

   ]
   spine [

   Then scroll down the page a bit to the end of the spine data and add this:

   ]

   Then delete the rest of the data in the file and finish the Extrusion node (closing curly-braces).

   SOLUTION: extrude1.wrl

   ---

2. Make a copy of your "extrude1.wrl" world:
   % cp extrude1.wrl extrude2.wrl

   Your task is to add a ball that slides up the extrusion path. You'll need a TimeSensor node and a PositionInterpolator node. Use the data from the "extrude.dat" file for the key fractional times and key positions for the interpolator.

   HINT: Read the extrude.dat file back in to the end of your VRML world, then delete the cross-section and spine values from that data, keeping your VRML nodes and the key times and positions from the data file.

   HINT: To turn the data into a PositionInterpolator node, add this just before the key times:

   DEF Mover PositionInterpolator {
   
   key [

   Add this after the times and before the positions:

   ]
   keyValue [

   And add this to the end of the positions:

   ]
   }

   HINT: You will need to create a ball shape within a Transform node that you can animate.

   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: extrude2.wrl

   ---

3. Make a copy of the "extrude2.wrl" world:
   % cp extrude2.wrl extrude3.wrl

   Extra credit: The animation from the previous task slides the ball up the slide. Your task is to make it slide down the slide instead! There are at least two ways to do this:

   • Reverse the order of the positions in the PositionInterpolator

   • Reverse time!

   The first method above is easy. Your task is to do the second method: reverse time!

   HINT: You need to make the times that go into the PositionInterpolator node vary from 1.0 to 0.0 instead of from 0.0 to 1.0. There's no way to make a TimeSensor node do this. BUT, you can use another interpolator to convert 0.0 to 1.0 times into 1.0 to 0.0 times!

   HINT: Use a ScalarInterpolator node. Use two just two key times and values:

   DEF Reverser ScalarInterpolator {
   
   key [ 0.0, 1.0 ]
   keyValue [ 1.0, 0.0 ]
   
   }
   Route the scalar interpolator between the TimeSensor and PositionInterpolator nodes.

   SOLUTION: extrude3.wrl