begin
    MessageBox(0, 'Unable to unregister window class!', 'Error', MB_OK or MB_ICONERROR);
    hInstance := 0;
  end;
end;


{--------------------------------------------------------------------}
{  Creates the window and attaches a OpenGL rendering context to it  }
{--------------------------------------------------------------------}
function glCreateWnd(Width, Height : Integer; Fullscreen : Boolean; PixelDepth : Integer) : Boolean;
var
  wndClass : TWndClass;         // Window class
  dwStyle : DWORD;              // Window styles
  dwExStyle : DWORD;            // Extended window styles
  dmScreenSettings : DEVMODE;   // Screen settings (fullscreen, etc...)
  PixelFormat : GLuint;         // Settings for the OpenGL rendering
  h_Instance : HINST;           // Current instance
  pfd : TPIXELFORMATDESCRIPTOR;  // Settings for the OpenGL window
begin
  h_Instance := GetModuleHandle(nil);       //Grab An Instance For Our Window
  ZeroMemory(@wndClass, SizeOf(wndClass));  // Clear the window class structure

  with wndClass do                    // Set up the window class
  begin
    style         := CS_HREDRAW or    // Redraws entire window if length changes
                     CS_VREDRAW or    // Redraws entire window if height changes
                     CS_OWNDC;        // Unique device context for the window
    lpfnWndProc   := @WndProc;        // Set the window procedure to our func WndProc
    hInstance     := h_Instance;
    hCursor       := LoadCursor(0, IDC_ARROW);
    lpszClassName := 'OpenGL';
  end;

  if (RegisterClass(wndClass) = 0) then  // Attemp to register the window class
  begin
    MessageBox(0, 'Failed to register the window class!', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit
  end;

  // Change to fullscreen if so desired
  if Fullscreen then
  begin
    ZeroMemory(@dmScreenSettings, SizeOf(dmScreenSettings));
    with dmScreenSettings do begin              // Set parameters for the screen setting
      dmSize       := SizeOf(dmScreenSettings);
      dmPelsWidth  := Width;                    // Window width
      dmPelsHeight := Height;                   // Window height
      dmBitsPerPel := PixelDepth;               // Window color depth
      dmFields     := DM_PELSWIDTH or DM_PELSHEIGHT or DM_BITSPERPEL;
    end;

    // Try to change screen mode to fullscreen
    if (ChangeDisplaySettings(dmScreenSettings, CDS_FULLSCREEN) = DISP_CHANGE_FAILED) then
    begin
      MessageBox(0, 'Unable to switch to fullscreen!', 'Error', MB_OK or MB_ICONERROR);
      Fullscreen := False;
    end;
  end;

  // If we are still in fullscreen then
  if (Fullscreen) then
  begin
    dwStyle := WS_POPUP or                // Creates a popup window
               WS_CLIPCHILDREN            // Doesn't draw within child windows
               or WS_CLIPSIBLINGS;        // Doesn't draw within sibling windows
    dwExStyle := WS_EX_APPWINDOW;         // Top level window
    ShowCursor(False);                    // Turn of the cursor (gets in the way)
  end
  else
  begin
    dwStyle := WS_OVERLAPPEDWINDOW or     // Creates an overlapping window
               WS_CLIPCHILDREN or         // Doesn't draw within child windows
               WS_CLIPSIBLINGS;           // Doesn't draw within sibling windows
    dwExStyle := WS_EX_APPWINDOW or       // Top level window
                 WS_EX_WINDOWEDGE;        // Border with a raised edge
  end;

  // Attempt to create the actual window
  h_Wnd := CreateWindowEx(dwExStyle,      // Extended window styles
                          'OpenGL',       // Class name
                          WND_TITLE,      // Window title (caption)
                          dwStyle,        // Window styles
                          0, 0,           // Window position
                          Width, Height,  // Size of window
                          0,              // No parent window
                          0,              // No menu
                          h_Instance,     // Instance
                          nil);           // Pass nothing to WM_CREATE
  if h_Wnd = 0 then
  begin
    glKillWnd(Fullscreen);                // Undo all the settings we've changed
    MessageBox(0, 'Unable to create window!', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Try to get a device context
  h_DC := GetDC(h_Wnd);
  if (h_DC = 0) then
  begin
    glKillWnd(Fullscreen);
    MessageBox(0, 'Unable to get a device context!', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Settings for the OpenGL window
  with pfd do
  begin
    nSize           := SizeOf(TPIXELFORMATDESCRIPTOR); // Size Of This Pixel Format Descriptor
    nVersion        := 1;                    // The version of this data structure
    dwFlags         := PFD_DRAW_TO_WINDOW    // Buffer supports drawing to window
                       or PFD_SUPPORT_OPENGL // Buffer supports OpenGL drawing
                       or PFD_DOUBLEBUFFER;  // Supports double buffering
    iPixelType      := PFD_TYPE_RGBA;        // RGBA color format
    cColorBits      := PixelDepth;           // OpenGL color depth
    cRedBits        := 0;                    // Number of red bitplanes
    cRedShift       := 0;                    // Shift count for red bitplanes
    cGreenBits      := 0;                    // Number of green bitplanes
    cGreenShift     := 0;                    // Shift count for green bitplanes
    cBlueBits       := 0;                    // Number of blue bitplanes
    cBlueShift      := 0;                    // Shift count for blue bitplanes
    cAlphaBits      := 0;                    // Not supported
    cAlphaShift     := 0;                    // Not supported
    cAccumBits      := 0;                    // No accumulation buffer
    cAccumRedBits   := 0;                    // Number of red bits in a-buffer
    cAccumGreenBits := 0;                    // Number of green bits in a-buffer
    cAccumBlueBits  := 0;                    // Number of blue bits in a-buffer
    cAccumAlphaBits := 0;                    // Number of alpha bits in a-buffer
    cDepthBits      := 16;                   // Specifies the depth of the depth buffer
    cStencilBits    := 0;                    // Turn off stencil buffer
    cAuxBuffers     := 0;                    // Not supported
    iLayerType      := PFD_MAIN_PLANE;       // Ignored
    bReserved       := 0;                    // Number of overlay and underlay planes
    dwLayerMask     := 0;                    // Ignored
    dwVisibleMask   := 0;                    // Transparent color of underlay plane
    dwDamageMask    := 0;                     // Ignored
  end;

  // Attempts to find the pixel format supported by a device context that is the best match to a given pixel format specification.
  PixelFormat := ChoosePixelFormat(h_DC, @pfd);
  if (PixelFormat = 0) then
  begin
    glKillWnd(Fullscreen);
    MessageBox(0, 'Unable to find a suitable pixel format', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Sets the specified device context's pixel format to the format specified by the PixelFormat.
  if (not SetPixelFormat(h_DC, PixelFormat, @pfd)) then
  begin
    glKillWnd(Fullscreen);
    MessageBox(0, 'Unable to set the pixel format', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Create a OpenGL rendering context
  h_RC := wglCreateContext(h_DC);
  if (h_RC = 0) then
  begin
    glKillWnd(Fullscreen);
    MessageBox(0, 'Unable to create an OpenGL rendering context', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Makes the specified OpenGL rendering context the calling thread's current rendering context
  if (not wglMakeCurrent(h_DC, h_RC)) then
  begin
    glKillWnd(Fullscreen);
    MessageBox(0, 'Unable to activate OpenGL rendering context', 'Error', MB_OK or MB_ICONERROR);
    Result := False;
    Exit;
  end;

  // Initializes the timer used to calculate the FPS
  SetTimer(h_Wnd, FPS_TIMER, FPS_INTERVAL, nil);

  // Settings to ensure that the window is the topmost window
  ShowWindow(h_Wnd, SW_SHOW);
  SetForegroundWindow(h_Wnd);
  SetFocus(h_Wnd);

  // Ensure the OpenGL window is resized properly
  glResizeWnd(Width, Height);
  glInit();

  Result := True;
end;


{--------------------------------------------------------------------}
{  Main message loop for the application                             }
{--------------------------------------------------------------------}
function WinMain(hInstance : HINST; hPrevInstance : HINST;
                 lpCmdLine : PChar; nCmdShow : Integer) : Integer; stdcall;
var
  msg : TMsg;
  finished : Boolean;
  DemoStart, LastTime : DWord;
begin
  finished := False;

  // Perform application initialization:
  if not glCreateWnd(800, 600, FALSE, 32) then
  begin
    Result := 0;
    Exit;
  end;

  DemoStart := GetTickCount();            // Get Time when demo started

  // Main message loop:
  while not finished do
  begin
    if (PeekMessage(msg, 0, 0, 0, PM_REMOVE)) then // Check if there is a message for this window
    begin
      if (msg.message = WM_QUIT) then     // If WM_QUIT message received then we are done
        finished := True
      else
      begin                               // Else translate and dispatch the message to this window
  	TranslateMessage(msg);
        DispatchMessage(msg);
      end;
    end
    else
    begin
      Inc(FPSCount);                      // Increment FPS Counter

      LastTime :=ElapsedTime;
      ElapsedTime :=GetTickCount() - DemoStart;     // Calculate Elapsed Time
      ElapsedTime :=(LastTime + ElapsedTime) DIV 2; // Average it out for smoother movement

      glDraw();                           // Draw the scene
      SwapBuffers(h_DC);                  // Display the scene

      if (keys[VK_ESCAPE]) then           // If user pressed ESC then set finised TRUE
        finished := True
      else
        ProcessKeys;                      // Check for any other key Pressed
    end;
  end;
  glKillWnd(FALSE);
  Result := msg.wParam;
end;


begin
  WinMain( hInstance, hPrevInst, CmdLine, CmdShow );
end.