randoming translation in GLUT turns bad - c++

Question

this is my cube. Once created, it has a random x position on either -2, -1, 0, 1, or 2.
void cube(void)
{
srand (time(0));
int cube_posX;
int lowv = -2;
int highv = 2;
cube_posX = rand() % (highv - lowv + 1) + lowv;
glTranslatef(cube_posX, 0.0, cube_angle);
glRotatef(cube_angle, 90.0, 0.0, 1.0);
glutSolidCube(0.25);
}
and this is how I move the cube slowly forward
void MOVE_CUBE(int value)
{
cube_posZ = cube_posZ - 0.01;
glutPostRedisplay();
glutTimerFunc(25, MOVE_CUBE, 0);
}
and finally putting them in display:
void init(void)
{
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
}
float cam_eyeX = 0.0, cam_eyeY = 1.5, cam_eyeZ = 5.0;
float cam_centerX = 0.0, cam_centerY = 0.0, cam_centerZ = 0.0;
void display(void)
{
glClearColor(1.0,1.0,1.0,1.0); //to add background color (white)
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
gluLookAt(cam_eyeX, cam_eyeY, cam_eyeZ, cam_centerX, cam_centerY, cam_centerZ, 0.0, 1.0, 0.0); //camera! (cam position X, cam position Y, cam position Z, cam target X, cam target Y, cam target Z, up position X, up position Y, up position Z)
cube();
glutSwapBuffers();
angle += 0.05; //to affect the glRotate function
glFlush();
}
void reshape(int w, int h)
{
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0);
glMatrixMode (GL_MODELVIEW);
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH); // Set up display buffer
glutInitWindowSize(750, 500); //window's size
glutInitWindowPosition(100, 100); //window's position
glutCreateWindow("Hendra Ganteng!"); //window's title
init();
glutDisplayFunc(display);
glutIdleFunc (display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard_Handler);
MOVE_CUBE(0);
glutMainLoop();
return 0;
}
But when I see it in action, the cube moves forward flawlessly, but keeps changing x position onto those 5 possibilities (-2,-1,0,1,2) every 0.5 to 1 second. If I disable the srand(time(0)), the cube changes its x position rapidly. I just want to make it stay in 1 x position so then I can call more cubes in different x position. Could someone please kindly what's wrong in my code?

Answer 1

How is that behaviour not expected? You are generating a random X position every time you display your cube. If you re-seed the random number generator using the time, then it will be start a different sequence whenever the time changes (once per second).
Instead, you should pre-generate your cube(s). How about this:
// Global cube data
struct Cube {
int x;
double angle;
};
vector<Cube> cubes;
const int num_cubes = 1;
// Example initialisation...
void InitCubes()
{
cubes.reserve(num_cubes);
for( int i = 0; i < num_cubes; i++ )
{
Cube cube;
cube.x = rand() % (highv - lowv + 1) + lowv;
cube.angle = 0.0;
cubes.push_back(cube);
}
}
Now the update/display cycles simply need to modify the angle, but not the x-position.
void UpdateCube( Cube & cube )
{
cube.angle += 0.05;
}
void DisplayCube( Cube & cube )
{
glTranslatef((double)cube.x, 0.0, cube.angle);
glRotatef(cube.angle, 90.0, 0.0, 1.0);
glutSolidCube(0.25);
}
In your main function, call InitCubes() during startup.
In your display function, do this:
void display(void)
{
glClearColor(1.0,1.0,1.0,1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
gluLookAt(cam_eyeX, cam_eyeY, cam_eyeZ, cam_centerX, cam_centerY, cam_centerZ, 0.0, 1.0, 0.0);
// Display cubes
for( int i = 0; i < cubes.size(); i++ ) DisplayCube( cubes[i] );
glutSwapBuffers();
glFlush();
// Update cubes for next render cycle.
for( int i = 0; i < cubes.size(); i++ ) UpdateCube( cubes[i] );
}