Star Animation

star_animation.cpp

#include <GL/gl.h>
#include <GL/glut.h>
#include <cmath>

static GLfloat spin = 0.0;

void init(void)
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glShadeModel(GL_FLAT);
}

void star(void)
{
    GLfloat angle, r, r_big_circle, r_small_circle, step = 3.14 / 5.0;
    r_big_circle = 25;
    r_small_circle = 10;

    // draw star
    glBegin(GL_LINE_LOOP);
    for (int i = 0; i < 10; i++)
    {
        r = (i % 2 == 0 ? r_small_circle : r_big_circle);
        angle = step * i;
        glVertex3f(r * cos(angle), r * sin(angle), 0);
    }
    glEnd();

    // draw big circle
    int circle_points = 100;
    glBegin(GL_LINE_LOOP);
    for (int i = 0; i < circle_points; i++)
    {
        angle = 2 * 3.14 * i / circle_points;
        glVertex2f(r_big_circle * cos(angle), r_big_circle * sin(angle));
    }
    glEnd();

    // draw small circle
    glBegin(GL_LINE_LOOP);
    for (int i = 0; i < circle_points; i++)
    {
        angle = 2 * 3.14 * i / circle_points;
        glVertex2f(r_small_circle * cos(angle), r_small_circle * sin(angle));
    }
    glEnd();
}


void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glPushMatrix();
    glRotatef(spin, 0.0, 0.0, 1.0);
    glColor3f(1.0, 1.0, 1.0);
    star();
    glPopMatrix();
    glutSwapBuffers();
}

void reshape(int w, int h)
{
    glViewport(0, 0, (GLsizei) w, (GLsizei) h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-50.0, 50.0, -50.0, 50.0, -1.0, 1.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

void spinDisplay(void)
{
    spin = spin + 2.0;
    if (spin > 360.0)
        spin = spin - 360.0;
    glutPostRedisplay();
}

/* Press M to spin */
void keyboard(unsigned char key, int x, int y)
{
    switch (key)
    {
    case 'm':
        glutIdleFunc(spinDisplay);
        break;
    case 'M':
        glutIdleFunc(spinDisplay);
        break;
    default:
        glutIdleFunc(NULL);
        break;
    }
}

void mouse(int button, int state, int x, int y)
{
}

/*
* Request double buffer display mode.
* Register mouse input callback functions
*/
int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowSize(400, 400);
    glutInitWindowPosition(100, 100);
    glutCreateWindow(argv[0]);
    init();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutKeyboardFunc(keyboard);
    glutMouseFunc(mouse);
    glutMainLoop();
    return 0;
}