An example of how to draw reflection in OpenGL from 16 years ago

oldman_reflections.cpp

void Viewer::do_reflections()                                                                                           
{                                                                                                                       
  if (Settings::reflections)                                                                                            
  {                                                                                                                     
    Settings::cel_reflection = true;                                                                                    
    glColorMask(0, 0, 0, 0);                                                                                            
                                                                                                                        
    glEnable(GL_STENCIL_TEST);                                                                                          
    glStencilFunc(GL_ALWAYS, 1, 1);                                                                                     
    glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);                                                                          
                                                                                                                        
    glDisable(GL_DEPTH_TEST);                                                                                           
    level->draw_reflective_surfaces();                                                                                  
                                                                                                                        
    glEnable(GL_DEPTH_TEST);                                                                                            
    color_mask();                                                                                                       
    glStencilFunc(GL_EQUAL, 1, 1);                                                                                      
    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);                                                                             
                                                                                                                        
    double eqr[] = {0.0, -1.0, 0.0, -1.0 };                                                                             
    glEnable(GL_CLIP_PLANE0);                                                                                           
    glClipPlane(GL_CLIP_PLANE0, eqr);                                                                                   
                                                                                                                        
    glPushMatrix();                                                                                                     
    glTranslatef(0.0, -2.0f, 0.0f);                                                                                     
    glScalef(1.0f, -1.0f, 1.0f);                                                                                        
    //glTranslatef(0.0, -1.0f, 0.0f);                                                                                   
    game->draw();                                                                                                       
    p_engine.draw();                                                                                                    
    glPopMatrix();                                                                                                      
                                                                                                                        
    glDisable(GL_CLIP_PLANE0);                                                                                          
    glDisable(GL_STENCIL_TEST);                                                                                         
                                                                                                                        
    glEnable(GL_BLEND);                                                                                                 
    glColor4f(1.0f, 1.0f, 1.0f, 0.5f);                                                                                  
    //glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA);                                                                
    level->draw_reflective_surfaces();                                                                                  
    glDisable(GL_BLEND);                                                                                                
  }                                                                                                                     
  else                                                                                                                  
    level->draw_reflective_surfaces();                                                                                  
                                                                                                                        
  Settings::cel_reflection = false;                                                                                     
}