Your client area might not actually be 800x600. When you call CreateWindow (or CreateWindowEx), the values passed in for x, y, w, h, are used to determine the size of the entire window, so the client area will be smaller because of the window's border.

If you then don't set the BackBufferWidth/Height members of the D3DPRESENT_PARAMS struct used to create the device, it will take the client area's size by default as the size of the backbuffer.

To remedy this, you need to resize the window after creating it, but before creating the device, so that the client area is exactly 800x600 (or 1280x720, etc). There's a handy Win32 API function for this, called AdjustWindowRect. Here's an example in my window creation code


HWND CreateGameWindow(int width, int height)
{
WNDCLASS wc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hbrBackground = (HBRUSH)COLOR_WINDOW+1;
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hInstance = NULL;
wc.lpfnWndProc = MsgProc;
wc.lpszClassName = "xeWndCls";
wc.lpszMenuName = "menu";
wc.style = CS_HREDRAW | CS_VREDRAW;
if (!RegisterClass(&wc))
{
return NULL;
}

DWORD style = WS_OVERLAPPEDWINDOW &~ WS_THICKFRAME &~ WS_MAXIMIZEBOX;
RECT wnd = { 0, 0, width, height };
RECT dt;
GetClientRect(GetDesktopWindow(), &dt);
AdjustWindowRect(&wnd, style, FALSE);
int w = (wnd.right - wnd.left);
int h = (wnd.bottom - wnd.top);
int x = ((dt.right - dt.left) - w) / 2;
int y = ((dt.bottom - dt.top) - h) / 2;
HWND hWnd = CreateWindow("xeWndCls", "XEngine", style, x, y, w, h, NULL, NULL, NULL, NULL);
if (hWnd == NULL)
{
return NULL;
}
ShowWindow(hWnd, SW_SHOW);
return hWnd;
}


Get the size from the current back buffer :


IDirect3DSurface9* pSurface;
pD3D9Device->GetBackBuffer( 0, 0, D3DBACKBUFFER_TYPE_MONO, &pSurface );
D3DSURFACE_DESC SurfaceDesc;
pSurface->GetDesc( &SurfaceDesc );
UINT Width = SurfaceDesc.Width;
UINT Height = Surface.Height;
SAFE_RELEASE( pSurface );