Aim:

Program to draw a color cube and spin it using OpenGL transformation matrices.

Algorithm

1. Choose eight 3 dimensional coordinate points such that will make a cube
2. Group the vertices together such that they make 6 faces.
3. Identify the button that was last pressed.
4. Rotate the cube by a small angle, along a particular axis; by associating one mouse button to one axis.

The following values assume vertices as show below:

   5-------6
  /|      /|
 / |     / |
1-------2  |
|  4----|--7
| /     | /
|/      |/
0-------3

Code: spinCube.c

#include<GL/glut.h>

GLfloat vertices[]={-0.5f,-0.5f,-0.5f,    -0.5f,0.5f,-0.5f,    0.5f,0.5f,-0.5f,   0.5f,-0.5f,-0.5f,
                    -0.5f,-0.5f,0.5f,     -0.5f,0.5f,0.5f,     0.5f,0.5f,0.5f,    0.5f,-0.5f,0.5f};

GLfloat colors[] = {0,0,0,   0,0,1,    0,1,0,    0,1,1,
                    1,0,0,   1,0,1,    1,1,0,    1,1,1};

GLbyte faces[] = {0,1,2,3,   2,3,7,6,   4,5,6,7,  4,5,1,0,   5,6,2,1,   0,3,7,4};

GLint currentBtn = GLUT_MIDDLE_BUTTON;

void mouse(int btn, int state, int x, int y) {
  currentBtn = btn;
}

void display() {
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glRotated(0.06,
            currentBtn == GLUT_LEFT_BUTTON,
            currentBtn == GLUT_MIDDLE_BUTTON,
            currentBtn == GLUT_RIGHT_BUTTON);
  glDrawElements(GL_QUADS, 24, GL_UNSIGNED_BYTE, faces);
  glFlush();
}

void glInit(int w, int h) {
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

  glEnableClientState(GL_COLOR_ARRAY);
  glEnableClientState(GL_VERTEX_ARRAY);
  glVertexPointer(3, GL_FLOAT, 0, vertices);
  glColorPointer(3, GL_FLOAT, 0, colors);
  glEnable(GL_DEPTH_TEST);

  glViewport(0, 0, w, h); // Resize viewport to width and height.
  if (h > w) //Maintain proper aspect ratio; otherwise the image would appear stretched
    glOrtho(-1.0, 1.0, (GLfloat) -h / w, (GLfloat) h / w, -1.0, 1.0);
  else
    glOrtho((GLfloat) -w / h, (GLfloat) w / h, -1.0, 1.0, -1.0, 1.0);
}

int main(int argc, char *argv[]) {
  glutInit(&argc, argv);
  glutInitWindowSize(720, 720);
  glutCreateWindow("Spin a cube");
  glutDisplayFunc(display);
  glutIdleFunc(display);
  glutReshapeFunc(glInit);
  glutMouseFunc(mouse);
  glutMainLoop();
}

Output:

Commands for execution:-

• Open a terminal and Change directory to the file location in both the terminals.
• compile as gcc -lGLU -lGL -lglut spinCube.c -o spincube
• If no errors, run as ./spincube

Screenshots:-