How to make an OpenGL rendering context with transparent background?

Question

Rendering contexts usually have a solid color on the background (black or whatever, see the image below):

I'm wondering if it's possible to setup a window, with no decorations AND with the transparent background, while allowing me to render OpenGL stuff on it.

This would give the illusion that the triangle is floating on the screen. The transparent background should allow you to see the desktop or other applications that might be behind it.

Could you please exemplify with source code?

Platform: Windows (win32 only)

Answer 1

After spending some reputation on a unsuccessful bounty to get some help on this issue, I finally realized how complex was the problem I was interested in.

The few individuals that have accomplished this task don't share much. During my research I found different ways to achieve what I was looking for. One of the most interesting ones is AeroGL, and it shows snippets of code using a technique that was not mentioned so far, which is rendering the graphics to a device-independent bitmap (DIB).

To close this thread permanently, the source code below implements that technique. The code itself is a slight modification of an application presented here (big thanks to Andrei Sapronov Y.).

The end result can be seen in the image below:

The code has been tested on Windows XP (32-bits) and Windows 8.1 (32-bits). Enjoy!

#define _WIN32_WINNT 0x0500  #include <windows.h> #include <windowsx.h> #include <GL/gl.h> #include <GL/glu.h>  #pragma comment (lib, "opengl32.lib") #pragma comment (lib, "glu32.lib")  #include <assert.h> #include <tchar.h>  #ifdef  assert #define verify(expr) if(!expr) assert(0) #else verify(expr) expr #endif  const TCHAR szAppName[]=_T("TransparentGL"); const TCHAR wcWndName[]=_T("WS_EX_LAYERED OpenGL");  HDC hDC;             HGLRC m_hrc;         int w(240); int h(240);   HDC pdcDIB;                  HBITMAP hbmpDIB;             void *bmp_cnt(NULL);         int cxDIB(0);  int cyDIB(0);    BITMAPINFOHEADER BIH;          BOOL initSC() {     glEnable(GL_ALPHA_TEST);             glEnable(GL_DEPTH_TEST);             glEnable(GL_COLOR_MATERIAL);      glEnable(GL_LIGHTING);               glEnable(GL_LIGHT0);                  glEnable(GL_BLEND);                  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);     glClearColor(0, 0, 0, 0);      return 0; }  void resizeSC(int width,int height) {     glViewport(0,0,width,height);     glMatrixMode(GL_PROJECTION);     glLoadIdentity();      glMatrixMode(GL_MODELVIEW );     glLoadIdentity(); }  BOOL renderSC() {        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );      glPushMatrix();      glColor3f(0, 1, 1);     glBegin(GL_TRIANGLES);                              // Drawing Using Triangles         glColor3f(1.0f,0.0f,0.0f);                      // Set The Color To Red         glVertex3f( 0.0f, 1.0f, 0.0f);                  // Top         glColor3f(0.0f,1.0f,0.0f);                      // Set The Color To Green         glVertex3f(-1.0f,-1.0f, 0.0f);                  // Bottom Left         glColor3f(0.0f,0.0f,1.0f);                      // Set The Color To Blue         glVertex3f( 1.0f,-1.0f, 0.0f);                  // Bottom Right     glEnd();      glPopMatrix();     glFlush();      return 0; }  // DIB -> hDC void draw(HDC pdcDest) {     assert(pdcDIB);      verify(BitBlt(pdcDest, 0, 0, w, h, pdcDIB, 0, 0, SRCCOPY)); }  void CreateDIB(int cx, int cy) {     assert(cx > 0);      assert(cy > 0);      cxDIB = cx ;     cyDIB = cy ;      int iSize = sizeof(BITMAPINFOHEADER);        memset(&BIH, 0, iSize);      BIH.biSize = iSize;     BIH.biWidth = cx;        BIH.biHeight = cy;       BIH.biPlanes = 1;        BIH.biBitCount = 24;         BIH.biCompression = BI_RGB;      if(pdcDIB)          verify(DeleteDC(pdcDIB));      pdcDIB = CreateCompatibleDC(NULL);     assert(pdcDIB);      if(hbmpDIB)          verify(DeleteObject(hbmpDIB));      hbmpDIB = CreateDIBSection(         pdcDIB,                  (BITMAPINFO*)&BIH,           DIB_RGB_COLORS,              &bmp_cnt,                NULL,         0);      assert(hbmpDIB);     assert(bmp_cnt);      if(hbmpDIB)         SelectObject(pdcDIB, hbmpDIB); }  BOOL CreateHGLRC() {     DWORD dwFlags = PFD_SUPPORT_OPENGL | PFD_DRAW_TO_BITMAP;      PIXELFORMATDESCRIPTOR pfd ;     memset(&pfd,0, sizeof(PIXELFORMATDESCRIPTOR)) ;     pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);      pfd.nVersion = 1;                            pfd.dwFlags =  dwFlags ;                     pfd.iPixelType = PFD_TYPE_RGBA ;             pfd.cColorBits = 24 ;                        pfd.cDepthBits = 32 ;                        pfd.iLayerType = PFD_MAIN_PLANE ;            int PixelFormat = ChoosePixelFormat(pdcDIB, &pfd);    if (PixelFormat == 0){       assert(0);       return FALSE ;    }     BOOL bResult = SetPixelFormat(pdcDIB, PixelFormat, &pfd);    if (bResult==FALSE){       assert(0);       return FALSE ;    }     m_hrc = wglCreateContext(pdcDIB);    if (!m_hrc){       assert(0);       return FALSE;    }     return TRUE; }  LRESULT CALLBACK WindowFunc(HWND hWnd,UINT msg, WPARAM wParam, LPARAM lParam) {     PAINTSTRUCT ps;      switch(msg)      {         case WM_ERASEBKGND:             return 0;         break;          case WM_CREATE:         break;          case WM_DESTROY:             if(m_hrc)             {                 wglMakeCurrent(NULL, NULL);                 wglDeleteContext(m_hrc) ;             }             PostQuitMessage(0) ;         break;          case WM_PAINT:             hDC = BeginPaint(hWnd, &ps);             renderSC(); // OpenGL -> DIB             draw(hDC);  // DIB -> hDC             EndPaint(hWnd, &ps);         break;          case WM_SIZE:             w = LOWORD(lParam); h = HIWORD(lParam);                      wglMakeCurrent(NULL, NULL);             wglDeleteContext(m_hrc);              CreateDIB(w, h);             CreateHGLRC();             verify(wglMakeCurrent(pdcDIB, m_hrc));              initSC();             resizeSC(w, h);             renderSC();         break;          default:              return DefWindowProc(hWnd,msg,wParam,lParam);     }      return 0; }  int WINAPI _tWinMain(HINSTANCE hThisInst, HINSTANCE hPrevInst, LPSTR str,int nWinMode) {        WNDCLASSEX wc;     memset(&wc, 0, sizeof(wc));     wc.cbSize = sizeof(WNDCLASSEX);     wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);     wc.style = CS_HREDRAW | CS_VREDRAW;     wc.lpfnWndProc = (WNDPROC)WindowFunc;     wc.cbClsExtra  = 0;     wc.cbWndExtra  = 0;     wc.hInstance = hThisInst;     wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);     wc.hCursor = LoadCursor(NULL, IDC_ARROW);     wc.hbrBackground = (HBRUSH) (COLOR_WINDOW);     wc.lpszClassName = szAppName;      if(!RegisterClassEx(&wc))     {         MessageBox(NULL, _T("RegisterClassEx - failed"), _T("Error"), MB_OK | MB_ICONERROR);         return FALSE;     }      HWND hWnd = CreateWindowEx(WS_EX_LAYERED, szAppName, wcWndName,                     WS_VISIBLE | WS_POPUP, 200, 150, w, h,                     NULL, NULL, hThisInst, NULL);     if(!hWnd){         MessageBox(NULL, _T("CreateWindowEx - failed"), _T("Error"), MB_OK | MB_ICONERROR);         return FALSE;     }      verify(SetLayeredWindowAttributes(hWnd, 0x0, 0, LWA_COLORKEY));      MSG msg;     while(1)      {         while (PeekMessage(&msg,NULL,0,0,PM_NOREMOVE)){             if (GetMessage(&msg, NULL, 0, 0))             {                 TranslateMessage(&msg);                 DispatchMessage(&msg);             }             else return 0;         }     }       return (FALSE);  }