How do I make cube faces opaque on OpenGL? - c++

Question

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.
#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();
}

Answer 1

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();
}