How do I make cube faces opaque on OpenGL?

I'm writing a program to draw a cube on OpenGL and rotate it continuously on mouse clicks. At particular angles, I'm able to see through the cube (transparent). I've enabled Depth Test, so I don't know why this is happening. I am not sure if I have enabled it correctly.

In this image, the hidden faces are still visible

#include <math.h>
#include <vector>
#include <Windows.h>
#include <gl\glut.h>

using namespace std;


void myInit() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glClearColor(0, 0, 0, 1);
    glOrtho(-2, 2, -2, 2, 2, -2);
    glMatrixMode(GL_MODELVIEW);
}

float Cube[][3] = { {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1}, {-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1} };
float Colors[][3] = { {0, 0, 0}, {1, 0, 0}, {0, 1, 0}, {0, 0, 1}, {1, 1, 0}, {0, 1, 1}, {1, 0, 1}, {1, 1, 1} };

int axis = 0, theta[3] = {0, 0, 0};

void draw_face (int a, int b, int c, int d) {
    glBegin(GL_QUADS);
        glColor3fv(Colors[a]);
        glVertex3fv(Cube[a]);
        glColor3fv(Colors[b]);
        glVertex3fv(Cube[b]);
        glColor3fv(Colors[c]);
        glVertex3fv(Cube[c]);
        glColor3fv(Colors[d]);
        glVertex3fv(Cube[d]);
    glEnd();
}

void draw_cube () {
    draw_face(0, 3, 2, 1);
    draw_face(2, 3, 7, 6);
    draw_face(0, 4, 7, 3);
    draw_face(1, 2, 6, 5);
    draw_face(4, 5, 6, 7);
    draw_face(0, 1, 5, 4);
}

void spin_cube() {
    theta[axis] += 2;
    if (theta[axis] > 360)
        theta[axis] = -360;
    glutPostRedisplay();
}

void idle_func() {
    Sleep(10);
    spin_cube();
}

void mouse_func(int button, int state, int x, int y) {
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
        axis = 0;
    else if (button == GLUT_MIDDLE_BUTTON && state == GLUT_DOWN)
        axis = 1;
    else if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN)
        axis = 2;
}

void myDrawing() {
    glClear(GL_COLOR_BUFFER_BIT);
    glPushMatrix();
    glRotatef(theta[0], 1, 0, 0);
    glRotatef(theta[1], 0, 1, 0);
    glRotatef(theta[2], 0, 0, 1);
    draw_cube();
    glPopMatrix();
    glFlush();
    glutSwapBuffers();
}

int main(int argc, char *argv[]) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glEnable(GL_DEPTH_TEST);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(0, 0);
    glutCreateWindow("sample");
    glutDisplayFunc(myDrawing);
    glutIdleFunc(idle_func);
    glutMouseFunc(mouse_func);
    myInit();
    glutMainLoop();
}

Solution

Multiple issues:

You aren't requesting a depth buffer from GLUT:
```
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
```
Solution: OR in GLUT_DEPTH to make sure GLUT requests some depth buffer bits from the OS during GL context creation:
```
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
```

You're calling glEnable(GL_DEPTH_TEST) before GLUT has created a GL context:

glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
// no GL context yet
glEnable(GL_DEPTH_TEST);

Solution: Move the glEnable() to after glutCreateWindow() so it has a current GL context to work with:

glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0, 0);
glutCreateWindow("sample");
glEnable(GL_DEPTH_TEST);
...

You never clear the depth buffer:

void myDrawing() {
    // where's GL_DEPTH_BUFFER_BIT?
    glClear(GL_COLOR_BUFFER_BIT);
    ...

Solution: OR in GL_DEPTH_BUFFER_BIT to your glClear() argument:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

All the fixes together:

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

using namespace std;

void myInit() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glClearColor(0, 0, 0, 1);
    glOrtho(-2, 2, -2, 2, 2, -2);
    glMatrixMode(GL_MODELVIEW);
}

float Cube[][3] = { {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1}, {-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1} };
float Colors[][3] = { {0, 0, 0}, {1, 0, 0}, {0, 1, 0}, {0, 0, 1}, {1, 1, 0}, {0, 1, 1}, {1, 0, 1}, {1, 1, 1} };

int axis = 0, theta[3] = {0, 0, 0};

void draw_face (int a, int b, int c, int d) {
    glBegin(GL_QUADS);
        glColor3fv(Colors[a]);
        glVertex3fv(Cube[a]);
        glColor3fv(Colors[b]);
        glVertex3fv(Cube[b]);
        glColor3fv(Colors[c]);
        glVertex3fv(Cube[c]);
        glColor3fv(Colors[d]);
        glVertex3fv(Cube[d]);
    glEnd();
}

void draw_cube () {
    draw_face(0, 3, 2, 1);
    draw_face(2, 3, 7, 6);
    draw_face(0, 4, 7, 3);
    draw_face(1, 2, 6, 5);
    draw_face(4, 5, 6, 7);
    draw_face(0, 1, 5, 4);
}

void spin_cube() {
    theta[axis] += 2;
    if (theta[axis] > 360)
        theta[axis] = -360;
    glutPostRedisplay();
}

void idle_func() {
    Sleep(10);
    spin_cube();
}

void mouse_func(int button, int state, int x, int y) {
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
        axis = 0;
    else if (button == GLUT_MIDDLE_BUTTON && state == GLUT_DOWN)
        axis = 1;
    else if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN)
        axis = 2;
}

void myDrawing() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glPushMatrix();
    glRotatef(theta[0], 1, 0, 0);
    glRotatef(theta[1], 0, 1, 0);
    glRotatef(theta[2], 0, 0, 1);
    draw_cube();
    glPopMatrix();
    glFlush();
    glutSwapBuffers();
}

int main(int argc, char *argv[]) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(0, 0);
    glutCreateWindow("sample");
    glEnable(GL_DEPTH_TEST);
    glutDisplayFunc(myDrawing);
    glutIdleFunc(idle_func);
    glutMouseFunc(mouse_func);
    myInit();
    glutMainLoop();
}