Terrain
Exercise

Description

In this exercise you will work with a program that computes a mathematical function that generates height values for a terrain. You will modify the program to add color to the terrain, varying colors with elevation.

In visualization, real and mathematical surfaces are often computed on a 2D regular grid. Calculated values are mapped to heights above the grid, creating mathematical terrains. Similarly, terrain surfaces can be created using real-world elevation information for parts of the Earth.

This exercise focuses upon use of ElevationGrid and Color nodes.

VRML Concepts

This exercise requires you to understand and make use of the following VRML features and concepts:
• Basics
  • VRML file structure
  • Comments
• Shapes and appearance
  • Shape node
  • Appearance node
  • Material node
  • ElevationGrid node
  • Color node
• Grouping
  • Transform node

Files

Here are the files you may use to get started with this exercise:
• terrain.c
  This is the source code file for a C program which generates a simple un-colored terrain. In the first task you will compile this program and run the resulting application to produce and view the simple graph. In subsequent tasks, you will make copies of the source code and modify it 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 terrain program, redirecting the output into a VRML file called "terrain.wrl":
  % make terrain
  % terrain > terrain.wrl

  View the resulting VRML scene by loading the "terrain.wrl" file into your browser.

  The program generates a terrain using an ElevationGrid node with terrain elevations set in the node's height field.

  SOLUTION: terrain.wrl

1. Make a copy of the "terrain.c" program:
   % cp terrain.c terrain1.c

   In complex visualizations, the height and color of a terrain may each vary with the output of a mathematical function or simulation. Your task is to modify the terrain1.c program to vary the terrain color with the height. You will need to add one color to each vertex in the terrain. For simplicity, compute the color based upon the height, though you could use a separate function if you like.

   HINT: You will need to add a Color node to the ElevationGrid node.

   HINT: You will need to add an additional field to the ElevationGrid node so that it colors each vertex with your list of colors.

   HINT: You will need to declare variables at the top of the program to contain the color values:
   

   float r, g, b;

   HINT: You will need to output the correct number of colors in the correct order. In C you would use two nested loops like:

   for ( sample_x=0; sample_x<samples; sample_x++ )
   {
   
   for ( sample_z=0; sample_z<samples; sample_z++ )
   {
   
   something . . .
   }
   
   }

   HINT: You will need to compute colors based on terrain height. A nice color scheme is a ramp from blue to red:
   

   r = fabs( f[sample_x][sample_z] );
   g = 0.0;
   b = 1.0 - r;

   Recompile and run the new terrain1 program, and view the resulting VRML scene (call it "terrain1.wrl").
   % make terrain1
   % terrain1 > terrain1.wrl

   SOLUTION: terrain1.c, terrain1.wrl