Set a window transparent

Question

I have been trying to set BlueStacks window transparent:

DWORD MakeWindowTransparent(HWND hWnd, unsigned char factor)
{
    /* First, see if we can get the API call we need. If we've tried
     * once, we don't need to try again. */
    if (!initialized)
    {
        HMODULE hDLL = LoadLibrary(L"user32");

        pSetLayeredWindowAttributes =
            (PSLWA)GetProcAddress(hDLL, "SetLayeredWindowAttributes");

        initialized = TRUE;
    }

    if (pSetLayeredWindowAttributes == NULL)
        return FALSE;

    /* Windows need to be layered to be made transparent. This is done
     * by modifying the extended style bits to contain WS_EX_LAYARED. */
    SetLastError(0);

    auto winlong = SetWindowLong(hWnd,
    GWL_EXSTYLE,
    GetWindowLong(hWnd, GWL_EXSTYLE) | WS_EX_LAYERED);

    if ((winlong == 0) && (GetLastError() != 0)) { 
        auto error = GetLastErrorAsString(); 
        return FALSE;
    } 

    if (!pSetLayeredWindowAttributes(hWnd,RGB(255, 255, 255),factor, LWA_COLORKEY | LWA_ALPHA))
    { 
        auto error = GetLastErrorAsString(); 
        return FALSE; 
    } 

    return TRUE;
}


int main() {
  HWND hWnd = FindWindowA(NULL, L"BlueStacks");
  MakeWindowTransparent(hWnd, 0);
}

BlueStacks can run in opengl and directx, I have tested the code above, using both modes.

MakeWindowTransparent is returning 0

pSetLayeredWindowAttributes auto error = GetLastErrorAsString(); error returned is: wrong parameter.

I have tested the code with other OpenGL windows, and it did not pause in any of the errors, also the window got transparent correctly.

Some information I have collected about the window:

Appreciate any help.

Answer 1

Why are you using both flags LWA_COLORKEY | LWA_ALPHA? If you are specifying a color RGB(255, 255, 255), just use LWA_COLORKEY.

Also, why are you messing with GetProcAddress? SetLayeredWindowAttributes is available starting with Windows 2000; are you targeting platforms older than that?

I am not familiar with BlueStacks, but SetLayeredWindowAttributes to make a window transparent is only working part of the time suggests that SetLayeredWindowAttributes doesn't work with Direct3D; do you know if BlueStacks uses Direct3D?

For OpenGL see this: How to make an OpenGL rendering context with transparent background?

UPDATE I was playing with that BlueStacks, and figured that you can re-parent it to another window (that's not recommended, as you will need to handle some edge cases). Anyway, I used an existing Notepad window and was able to set alpha on it, and that affected the child BlueStacks window:

int main() {
  HWND hWnd = FindWindow(NULL, L"BlueStacks");
  HWND hWndN = FindWindow(NULL, L"Untitled - Notepad");
  ::SetParent(hWnd, hWndN);

  auto winlong1 = SetWindowLongPtr(hWnd, GWL_EXSTYLE, 0);
  auto winlong2 = SetWindowLongPtr(hWnd, GWL_STYLE, WS_CHILD | WS_VISIBLE);
  ::SetWindowPos(hWnd, 0, 0, 0, 0, 0, SWP_FRAMECHANGED | SWP_NOSIZE | SWP_NOZORDER | SWP_NOREDRAW | SWP_NOACTIVATE);

  MakeWindowTransparent(hWndN, 200);
}

I've also cleaned up your function to get rid of GetProcAddress:

DWORD MakeWindowTransparent(HWND hWnd, unsigned char factor)
{
  auto winlong = SetWindowLong(hWnd,
    GWL_EXSTYLE,
    GetWindowLong(hWnd, GWL_EXSTYLE) | WS_EX_LAYERED);

  if ((winlong == 0) && (GetLastError() != 0)) {
    return FALSE;
  }

  if (!SetLayeredWindowAttributes(hWnd, 0, factor, LWA_ALPHA))
  {
    return FALSE;
  }

  return TRUE;
}