How to add OpenGL support to ATL controls

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Introduction

This tutorial will demonstrate how to add OpenGL support to an ATL control, and constitutes a cut-down, organized version of the ATL OpenGL demo that comes with Visual C++ in a more printable and easier-to-digest format. In order to work with this tutorial, you must create an ATL control in your project. What follows below is a step-by-step guide to adding the most basic OpenGL support to an ATL control.

Note: though I prefix function declarations with CMyControl::, I would recommend writing these functions inline (in the header file) and if you do that, the prefix must be removed.

1. Add support for headers <gl/gl.h> and <gl/glu.h>; Add support for opengl32.lib and glu32.lib libraries.

   This can be done by adding the following four lines at the end of your stdafx.h file:

   #include <gl/gl.h>
   #include <gl/glu.h>
   #pragma comment(lib, "opengl32.lib")
   #pragma comment(lib, "glu32.lib")
   

2. Add an OpenGL rendering context variable to your control class:

   // OpenGL rendering context
   HGLRC m_hRC;
   

   Now make a function to choose the OpenGL pixel format. This function will take a device context handle as a parameter and would set pixel format specifically for this device context. The function should look like the following:

   // Set OpenGL pixel format for given DC
   BOOL MyControl::SetupPixelFormat(HDC hdc)
   {
       static PIXELFORMATDESCRIPTOR pfd =
       {
           sizeof(PIXELFORMATDESCRIPTOR),   // size of this pfd
               1,                           // version number
               PFD_DRAW_TO_WINDOW |         // support window
               PFD_SUPPORT_OPENGL |         // support OpenGL
               PFD_DOUBLEBUFFER,            // double buffered
               PFD_TYPE_RGBA,               // RGBA type
               24,                          // 24-bit color depth
               0, 0, 0, 0, 0, 0,            // color bits ignored
               0,                           // no alpha buffer
               0,                           // shift bit ignored
               0,                           // no accumulation buffer
               0, 0, 0, 0,                  // accum bits ignored
               32,                          // 32-bit z-buffer
               0,                           // no stencil buffer
               0,                           // no auxiliary buffer
               PFD_MAIN_PLANE,              // main layer
               0,                           // reserved
               0, 0, 0                      // layer masks ignored
       };
       int pixelformat;
   
       if ((pixelformat = ChoosePixelFormat(hdc, &pfd)) == 0)
       {
           ATLASSERT(FALSE);
           return FALSE;
       }
   
       if (SetPixelFormat(hdc, pixelformat, &pfd) == FALSE)
       {
           ATLASSERT(FALSE);
           return FALSE;
       }
   
       return TRUE;
   }

3. Make a function that would create an OpenGL rendering context and initialize the viewport. Note that after setting the current rendering context, we can begin calling OpenGL functions such as glViewport.

   // Create rendering context given device context and control bounds
   void MyControl::CreateContext(HDC hdc, RECT& rc)
   {
       PIXELFORMATDESCRIPTOR pfd;
       if (!SetupPixelFormat(hdc))
           return;
   
       int n = GetPixelFormat(hdc);
       DescribePixelFormat(hdc, n, sizeof(pfd), &pfd);
       m_hRC = wglCreateContext(hdc);
       wglMakeCurrent(hdc, m_hRC);
   
       // OpenGL code starts here - below is an example
       int width = rc.right - rc.left;
       int height = rc.bottom - rc.top;
   
       glViewport(0, 0, width, height);
       glMatrixMode(GL_PROJECTION);
       glLoadIdentity();
       gluOrtho2D(-width/2, width/2, -height/2, height/2);
       glMatrixMode(GL_MODELVIEW);
   }

4. Add a call to the above function from OnCreate. To do that, you first have to create a WM_CREATE handler. Use the Messages button on the Properties toolbar to add a handler. Then, find the function and add code similar to the following:

   LRESULT CMyControl::OnCreate(UINT /*uMsg*/, WPARAM /*wParam*/,
       LPARAM /*lParam*/, BOOL& /*bHandled*/)
   {
       HDC hdc = GetDC();
       RECT rc;
       GetClientRect(&rc);
       CreateContext(hdc, rc);
   
       return 0;
   }

5. In a similar fashion, add code for OnDestroy (WM_DESTROY) to clear the rendering context and delete it.

   LRESULT CMyControl::OnDestroy(UINT /*uMsg*/, WPARAM /*wParam*/, LPARAM /*lParam*/, BOOL& /*bHandled*/)
   {
     wglMakeCurrent(NULL, NULL);
   
     if (m_hRC)
     {
       wglDeleteContext(m_hRC);
       m_hRC = NULL;
     }
   
     return 0;
   }
   

6. Handling WM_SIZE is also important. Whenever the window resizes, we must nullify the current OpenGL rendering context and recreate it from scratch.

   LRESULT CMyControl::OnSize(UINT /*uMsg*/, WPARAM /*wParam*/,
       LPARAM /*lParam*/, BOOL& /*bHandled*/)
   {
       // when resized, recreate the device context
       wglMakeCurrent(NULL,    NULL);
       if (m_hRC)
       {
           wglDeleteContext(m_hRC);
           m_hRC = NULL;
       }
       HDC hdc = GetDC();
       RECT rc;
       GetClientRect(&rc);
       CreateContext(hdc, rc);
   
       return 0;
   }

   All done. Remember! You can use glaux library (glaux32.lib, <glaux.h>) to implement functionality equivalent to the GLUT library (though you probably won't need the windowing functions). For example, you can use auxDIBImageLoad to load bitmaps to use as textures. However, I would recommend using more advanced methods of texturing, for example using PNG files for textures that have an alpha channel.

7. Rendering can be done in OnDraw. Here is an example:

   HRESULT OnDraw(ATL_DRAWINFO& di)
   {
       HDC hdc = di.hdcDraw;
       RECT& rc = *(RECT*)di.prcBounds;
   
   
       wglMakeCurrent(hdc, m_hRC);
   
       glClearColor(1.0f, 0.0f, 0.0f, 10.0f);
       glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   
   
       glPushMatrix();
   
       // compute dimensions of a quarter of the control
       SIZE qtrSize = { 
           (rc.right - rc.left) / 2, 
           (rc.bottom - rc.top) / 2 };
   
       glBegin(GL_QUADS);
           glColor3ub(255, 0, 0);
           glVertex3s(-qtrSize.cx, -qtrSize.cy, 0);
   
           glColor3ub(0, 255, 0);
           glVertex3s(qtrSize.cx, -qtrSize.cy, 0);
   
           glColor3ub(0, 0, 255);
           glVertex3s(qtrSize.cx, qtrSize.cy, 0);
   
           glColor3ub(255, 255, 255);
           glVertex3s(-qtrSize.cx, qtrSize.cy, 0);
       glEnd();
   
       glPopMatrix();
   
       glFinish();
       SwapBuffers(wglGetCurrentDC());
   
       return S_OK;
   }

Possible Improvements

I believe it is possible to create a mixin class/abstract superclass that will add OpenGL functionality to an ATL control in a much cleaner fashion than a bunch of functions/message handlers inside every control class. Writing all this code for each ATL control (if you have many in your project) is tedious: it would be better if you could specify that your control inherits from some CControlOpenGL<YourControl> class. This functionality, as well as more advanced OpenGL modes and options may make up material for another tutorial.

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)