title: dll injection: drawing simple game overlay with opengl
body: <p>I'm trying to draw a custom opengl overlay (steam does that for example) in a 3d desktop game.
  This overlay should basically be able to show the status of some variables which the user
  can affect by pressing some keys. Think about it like a game trainer.
  The goal is in the first place to draw a few primitives at a specific point on the screen. Later I want to have a little nice looking "gui" component in the game window.
  The game uses the "SwapBuffers" method from the GDI32.dll. 
  Currently I'm able to inject a custom DLL file into the game and hook the "SwapBuffers" method. 
  My first idea was to insert the drawing of the overlay into that function. This could be done by switching the 3d drawing mode from the game into 2d, then draw the 2d overlay on the screen and switch it back again, like this:</p>
  
  <pre><code>//SwapBuffers_HOOK (HDC)
  glPushMatrix();
  glLoadIdentity();
  glMatrixMode(GL_PROJECTION);
  glPushMatrix();
  glOrtho(0.0, 640, 480, 0.0, 1.0, -1.0);
  
  //"OVERLAY"
  glBegin(GL_QUADS);
  glColor3f(1.0f, 1.0f, 1.0f);
  glVertex2f(0, 0);
  glVertex2f(0.5f, 0);
  glVertex2f(0.5f, 0.5f);
  glVertex2f(0.0f, 0.5f);
  glEnd();
  
  glPopMatrix();
  glMatrixMode(GL_MODELVIEW);
  glPopMatrix();
  
  SwapBuffers_OLD(HDC);
  </code></pre>
  
  <p>However, this does not have any effect on the game at all.</p>
  
  <ul>
  <li>Is my approach correct and reasonable (also considering my 3d to 2d switching code)?</li>
  <li>I would like to know what the best way is to design and display a       custom overlay in the hooked function. (should I use something like windows forms or should I assemble my component with opengl functions - lines, quads 
  ...?)</li>
  <li>Is the SwapBuffers method the best place to draw my overlay?</li>
  </ul>
  
  <p>Any hint, source code or tutorial to something similiar is appreciated too.
  The game by the way is counterstrike 1.6  and I don't intend to cheat online.</p>
  
  <p>Thanks.</p>
  
  <p>EDIT:</p>
  
  <p>I could manage to draw a simple rectangle into the game's window by using a new opengl context as proposed by 'derHass'. Here is what I did:</p>
  
  <pre><code>//1. At the beginning of the hooked gdiSwapBuffers(HDC hdc) method save the old context
  GLboolean gdiSwapBuffersHOOKED(HDC hdc) {
      HGLRC oldContext =  wglGetCurrentContext();
  //2. If the new context has not been already created - create it   
  //(we need the "hdc" parameter for the current window, so the initialition 
  //process is happening in this method - anyone has a better solution?)
  //Then set the new context to the current one.
      if (!contextCreated) {
          thisContext = wglCreateContext(hdc);
          wglMakeCurrent(hdc, thisContext);
          initContext();
      }
      else {
          wglMakeCurrent(hdc, thisContext);
      }
      //Draw the quad in the new context and switch back to the old one.
      drawContext();
      wglMakeCurrent(hdc, oldContext);
      return gdiSwapBuffersOLD(hdc);
  }
  
  GLvoid drawContext() {
      glColor3f(1.0f, 0, 0);
      glBegin(GL_QUADS);
      glVertex2f(0,190.0f);
      glVertex2f(100.0f, 190.0f);
      glVertex2f(100.0f,290.0f);
      glVertex2f(0, 290.0f);
      glEnd();
  }
  
  GLvoid initContext() {
      contextCreated = true;
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      glOrtho(0.0, 640, 480, 0.0, 1.0, -1.0);
      glMatrixMode(GL_MODELVIEW);
      glLoadIdentity();
      glClearColor(0, 0, 0, 1.0);
  }
  </code></pre>
  
  <p>Here is the result:
  <a href="http://i.stack.imgur.com/Gq9ft.png" rel="nofollow">cs overlay example</a>
  It is still very simple but I will try to add some more details, text etc. to it.</p>
  
  <p>Thanks.</p>