Black screen when im trying to make double buffered GDI overlay

Question

Im trying to create double buffered gdi overlay over desktop but all what i get is black screen with red line

    HDC hdc = GetDC(0);

    HDC mem_handle = CreateCompatibleDC(hdc);
    HBITMAP bitmap_hndl = CreateCompatibleBitmap(hdc, 1920, 1080);

    HBITMAP old0 = (HBITMAP)SelectObject(mem_handle, bitmap_hndl);

    HPEN pen = CreatePen(PS_SOLID, 5, RGB(255, 0, 0));
    HGDIOBJ oldpen = SelectObject(mem_handle, pen);

    POINT Dots[2];
    Dots[0] = { 500, 500 };
    Dots[1] = { 200, 200 };
    DWORD NumDots = 2;
    PolyPolyline(mem_handle, Dots, &NumDots, 1ull);

    BitBlt(hdc, 0, 0, 1920, 1080, mem_handle, 0, 0, SRCCOPY | CAPTUREBLT);

    SelectObject(mem_handle, oldpen);
    SelectObject(mem_handle, old0);

    DeleteObject(pen);
    DeleteObject(bitmap_hndl);
    DeleteDC(mem_handle);
    ReleaseDC(0, hdc);

Im tried to use alphabland and transparentblt but nothing changed

Answer 1

You can't create a "gdi overlay over the desktop" by just painting to the screen's device context. Painting to GetDC(NULL) will, maybe, get some pixels displayed for a few milliseconds before something else paints over them.

Windows is called "Windows" for a reason. If you want something to persist on the screen you need to paint it in the WM_PAINT handler of a window you create. Otherwise, you can't control when what you paint will be overwritten.