Make object disappear and appear in OpenGL - c++

Question

#include <stdio.h> // this library is for standard input and output
#include "glut.h" // this library is for glut the OpenGL Utility Toolkit
#include <math.h>
// left square
void drawShape1(void) {
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(82, 250);
glVertex2f(82, 200);
glVertex2f(140, 200);
glVertex2f(140, 250);
glEnd();
}
// right square
void drawShape2(void) {
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(232, 250);
glVertex2f(232, 200);
glVertex2f(290, 200);
glVertex2f(290, 250);
glEnd();
}
void initRendering() {
glEnable(GL_DEPTH_TEST);
}
// called when the window is resized
void handleResize(int w, int h) {
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, (float)w, 0.0f, (float)h, -1.0f, 1.0f);
}
void display() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
drawShape1();
drawShape2();
glutSwapBuffers();
glutPostRedisplay();
}
// the timer code
void update(int value) {
// add code here
glutPostRedisplay();
glutTimerFunc(5, update, 0);
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(400, 400);
glutCreateWindow("Squares");
initRendering();
glutDisplayFunc(display);
glutReshapeFunc(handleResize);
glutTimerFunc(5, update, 0);
glutMainLoop();
return(0);
}
I have two squares in the middle. One square is on the left and the other square is on the right (see screenshot below). I am trying to make the left square disappear/appear every 5 seconds. I have added the timer code, but I am struggling on how to make the object disappear/appear.
Preview:

Answer 1

The unit of the first parameter of glutTimerFunc milliseconds rather than seconds. So a 5 seconds equals the value 5000.
Create a variable (square1_visible) of type bool which states whether the left square is visible:
bool square1_visible = true;
Change the state of the variable square1_visible every 5 seconds in the timer function update:
void update(int value) {
glutTimerFunc(5000, update, 0);
square1_visible = !square1_visible;
}
Draw the left square dependent on the state of the variable square1_visible:
void display() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
if ( square1_visible )
drawShape1();
drawShape2();
glutSwapBuffers();
glutPostRedisplay();
}