Modeling a Colored Cube

Tom Kelliher, CS 320

Mar. 14, 2001

Administrivia

Announcements

Book is confused on this section!!!

Do we want to keep you-know-who???

Assignment

Read 4.1--3, Appendices B and C as necessary.

From Last Time

Pong project.

Outline

1. Rotating cube program.

2. Cube representation.

3. Depth buffering.

4. Non-Commutivity of rotations.

Coming Up

Linear algebra basis (ha ha) of transformations.

Prelude

If you want to rotate an object about its center, in what order do you apply the three transformations? Does the order matter?

A Rotating, Color-Interpolated Cube

• Assign a color to each vertex and see what happens.

• Note dimensions of cube and clipping volume.

• Note that the reshape function maintains the aspect ratio:

  void myReshape(int w, int h)
  {
      glViewport(0, 0, w, h);
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      if (w <= h)
          glOrtho(-2.0, 2.0, -2.0 * (GLfloat) h / (GLfloat) w,
              2.0 * (GLfloat) h / (GLfloat) w, -10.0, 10.0);
      else
          glOrtho(-2.0 * (GLfloat) w / (GLfloat) h,
              2.0 * (GLfloat) w / (GLfloat) h, -2.0, 2.0, -10.0, 10.0);
      glMatrixMode(GL_MODELVIEW);
  }
  

  (Note use of glOrtho.)

Representation

There's a hard way and an easy way to do this. Which way is this?

GLfloat vertices[][3] = {{-1.0,-1.0,-1.0},{1.0,-1.0,-1.0},
{1.0,1.0,-1.0}, {-1.0,1.0,-1.0}, {-1.0,-1.0,1.0}, 
{1.0,-1.0,1.0}, {1.0,1.0,1.0}, {-1.0,1.0,1.0}};

GLfloat colors[][3] = {{0.0,0.0,0.0},{1.0,0.0,0.0},
{1.0,1.0,0.0}, {0.0,1.0,0.0}, {0.0,0.0,1.0}, 
{1.0,0.0,1.0}, {1.0,1.0,1.0}, {0.0,1.0,1.0}};

1. Coordinate system: +x to right, +y up, +z towards us. Right-hand system.

2. Vertex list and a numbering of the cube's vertices:

   

3. Color interpolation: bilinear interpolation.

   Let p be the way from to . p's color is:

   

   (for each color)

   What about points on interior of polygon?

4. Enumerating the vertices on each of the faces:

   void colorcube(void)
   {
   
   /* map vertices to faces */
   
      polygon(0,3,2,1);
      polygon(2,3,7,6);
      polygon(0,4,7,3);
      polygon(1,2,6,5);
      polygon(4,5,6,7);
      polygon(0,1,5,4);
   }
   
   
   void polygon(int a, int b, int c , int d)
   {
   
   /* draw a polygon via list of vertices */
   
      glBegin(GL_POLYGON);
         glColor3fv(colors[a]);
         glVertex3fv(vertices[a]);
         glColor3fv(colors[b]);
         glVertex3fv(vertices[b]);
         glColor3fv(colors[c]);
         glVertex3fv(vertices[c]);
         glColor3fv(colors[d]);
         glVertex3fv(vertices[d]);
      glEnd();
   }
   

   Are we following the righthand rule for the outer side of each face? Does order of rendering faces matter?

   Why represent a cube this way?

   Why the normals in the program code?

5. Display function:

   void display(void)
   {
   /* display callback, clear frame buffer and z buffer,
      rotate cube and draw, swap buffers */
   
      glLoadIdentity();
      glRotatef(theta[0], 1.0, 0.0, 0.0);
      glRotatef(theta[1], 0.0, 1.0, 0.0);
      glRotatef(theta[2], 0.0, 0.0, 1.0);
   
      colorcube();
   
      glutSwapBuffers();
   }
   

Rotating One and Two Faces

Been here, done this. One face:

1. The face is rotating about the origin.

2. Perspective is not maintained.

Two faces:

1. An unexpected result? Why?

2. Fixing it: the depth buffer and hidden surface removal. Idea: associate a z-value with each pixel in the frame buffer and only conditionally write new pixels.

Non-Commutivity of Rotations

Will these give the same result?

   glLoadIdentity();
   glRotatef(theta[0], 1.0, 0.0, 0.0);   /* Rotate about x axis. */
   glRotatef(theta[2], 0.0, 0.0, 1.0);   /* Rotate about z axis. *.

   glLoadIdentity();
   glRotatef(theta[2], 0.0, 0.0, 1.0);   /* Rotate about z axis. *.
   glRotatef(theta[0], 1.0, 0.0, 0.0);   /* Rotate about x axis. */