title: Reflection and depth in OpenGL
body: <p>I'm trying to create a mirror in OpenGL. Most references I've found recommend using the stencil buffer to define the boundaries of the mirror itself and using a combination of translation and scaling matrices for the actual reflection. I managed to get this to work when the scene is viewed so that no objects are in front of the mirror. However, when an object is in front of the mirror the overlapping part is not shown, making it look as if it's behind the mirror. There's a good chance that I'm misunderstanding how the stencil buffer works, as this is the first time I've tried using it, but I could also be making some mistake regarding depth or something else. I'd appreciate any suggestions. Here is the code.</p>
  
  <pre><code>#include &lt;gl/glut.h&gt;
  
  // perspective
  void view(void) {
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      glOrtho(-2.0, 2.0, -2.0, 2.0, -2.0, 2.0);
  
      glMatrixMode(GL_MODELVIEW);
      glLoadIdentity();
      glRotatef(-85.0, 1.0, 0.0, 0.0);
      glScalef(0.25, 0.25, 0.25);
  }
  
  void init(void) {
      glEnable(GL_DEPTH_TEST);
      glClearStencil(0);
      glEnable(GL_STENCIL_TEST);
  }
  
  void mirror(GLboolean inside, GLfloat vertSet[4][3]) {
      GLint i;
      if(inside)
          glBegin(GL_QUADS); // draw inside of mirror
      else
          glBegin(GL_LINE_LOOP); // draw frame of mirror
              for(i = 0; i &lt; 4; i++)
                  glVertex3fv(vertSet[i]);
      glEnd();
  }
  
  void scene(void) {
      glMatrixMode(GL_MODELVIEW);
      glPushMatrix();
      glRotatef(90.0, 1.0, 0.0, 0.0);
      glutSolidTeapot(1.0);
      glPopMatrix();
  }
  
  void display(void) {
      GLfloat vertSet1[4][3] = {{-3.0, 3.0, 0.0}, {2.0, 3.0, 0.0}, 
          {2.0, 3.0, 2.0}, {-3.0, 3.0, 2.0}};
  
      view();
  
      // store mirror shape in the stencil buffer.
      glClear(GL_STENCIL_BUFFER_BIT);
      glStencilFunc(GL_ALWAYS, 1, 1);
      glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);
      mirror(true, vertSet1);
  
      glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  
      // draw mirror frame.
      glColor3f(0.0, 0.0, 0.0);
      mirror(false, vertSet1);
  
      // draw scene outside mirror
      glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
      glStencilFunc(GL_NOTEQUAL, 1, 1);
      scene();
  
      // draw reflection of scene in mirror
      glStencilFunc(GL_EQUAL, 1, 1);
      glTranslatef(0.0, 3.0, 0.0);
      glScalef(1.0, -1.0, 1.0);
      glTranslatef(0.0, -3.0, 0.0);
      scene();
  
      glFlush();
      glutSwapBuffers();
      glutPostRedisplay();
  }
  
  int main (int argc, char **argv) {
      glutInit(&amp;argc, argv);
      glutInitWindowSize(500, 500);
      glutInitWindowPosition(0, 0);
      glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_STENCIL | GLUT_DEPTH);
      glutCreateWindow("");
  
      glClearColor(1.0, 1.0, 1.0, 0.0);
  
      init();
  
      glutDisplayFunc(display);
  
      glutMainLoop();
  }
  </code></pre>