Bar Plot
Exercise

Description

In this exercise you will create and place a series of boxes to create a 3D bar plot. Next you will expand on the bar plot and add colors and text labels for each bar.

In visualization, the size and position of shapes are often set using the results of a mathematical function or simulation. This example uses a simple C program that computes a sine wave. Your task will be to modify this program to output VRML to build a bar at each step along the sine wave.

This exercise focuses upon use of Shape, Box, Text, Appearance, Material, FontStyle, Transform, and Billboard 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
  • Box geometry node
  • Text geometry node
  • FontStyle node
• Grouping
  • Transform node
  • Billboard grouping node

Files

Here are the files you may use to get started with this exercise:
• barplot.c
  This is the source code file for a C program which generates a simple un-colored bar graph. 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 barplot program, redirecting the output into a VRML file called "barplot.wrl":
  % make barplot
  % barplot > barplot.wrl

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

  HINT: To load the world, choose "Open File..." from the browser's "File" menu and pick "barplot.wrl" from the file browser list.

  SOLUTION: barplot.wrl

1. Make a copy of the "barplot.c" program:
   % cp barplot.c barplot1.c

   The bar plot program generates bars that are all white. Your task is to modify the barplot1.c program to add color to each bar. Make the color of each bar a function of the bar's height.

   HINT: A Material node selects the diffuse (shading) color of a shape.

   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 compute colors based on bar height. A nice color scheme is a ramp from blue to red:
   

   r = fabs( f ); /* NOTE: Color must be >= 0 */
   g = 0.0;
   b = 1.0 - r; /* Ramp from red to blue */

   Recompile and run the new barplot1 program, and view the resulting VRML scene (call it "barplot1.wrl").

   % make barplot1
   % barplot1 > barplot1.wrl

   SOLUTION: barplot1.c, barplot1.wrl

   ---

2. Make a copy of the "barplot1.c" program:
   % cp barplot1.c barplot2.c

   The bars on a bar plot give a quick over-all impression of the changing values of a function, like a sine wave. Numeric labels on bars are often used to show the actual value for each bar. Your task is to modify the barplot2.c program to add a numeric text label for each bar. Put the label above the bars that go up from the horizontal zero-line, and put the labels below the bars that go down.

   HINT: You will need to add a Transform node to place the text.

   HINT: When the function goes negative, you need to account for the negative offset when positioning the text labels. Here's a piece of code that computes a "nice" text offset:

   if ( y >= 0 )
   
   printf( "\ttranslation %f %f %f\n", w*sample, y+(w/2.1), 0 );
   
   else
   
   printf( "\ttranslation %f %f %f\n", w*sample, y-(w/2.1), 0 );
   

   HINT: The default text font size is too large. You will need to add a FontStyle node to the label's Text node.

   HINT: To center the text, you can set the justify field of a FontStyle node to:

   justify [ "MIDDLE", "MIDDLE" ]

   SOLUTION: barplot2.c, barplot2.wrl

   ---

3. Extra credit: Make a copy of the "barplot2.c" program:
   % cp barplot2.c barplot3.c

   When you spin the bar plot around, the flat text labels disappear when viewed edge-on. Your task is to make the text labels always face the viewer.

   HINT: Think of text labels like advertising text that a billboard advertiser would always want the viewer to see.

   HINT: To enable the labels to orient themselves without reguard to any viewer orientation, you can specify a special axis of rotation in the Billboard node:

   axisOfRotation 0.0 0.0 0.0

   SOLUTION: barplot3.c, barplot3.wrl