winapi – Win32：如何在窗外画画？

使用SpyXX – 我可以获得工具提示的窗口rect和类样式：

Rectangle:     (440,229)-(544,249),104x20
Restored Rect: (440,104x20
Client Rect:   (0,0)-(104,20),104x20

您会注意到,您看到的投影实际上位于正在绘制的窗口之外.当我在窗外时如何在窗户外面画一个阴影？

注意：The shadow is not drawn using the standard CS_DROPSHADOW class style.我已经通过实验证实了这一点,并且还可以在SpyXX中看到窗口的类样式;它不使用CS_DROPSHADOW：

Windows Styles:     94000001

    WS_POPUP        80000000
    WS_VISIBLE      10000000
    WS_CLIPSIBLINGS  4000000
    TTS_ALWAYSTIP          1

Extended Styles:    00080088

    WS_EX_LAYERED      80000
    WS_EX_TOOLWIN         80
    WS_EX_TOPMOST          8

那我怎么能在窗外画画？

注意：尝试在桌面上绘制DC已经完成.来自Greg Schechter的Redirecting GDI,DirectX,and WPF applications：

Drawing To and Reading From the Screen
— Baaaad!

Lastly,since we’re on the redirection
topic,one particularly dangerous
practice is writing to the screen,
either through the use of GetDC(NULL)
and writing to that,or attempting to
do XOR rubber-band lines,etc. There
are two big reasons that writing to
the screen is bad:

It’s expensive… writing to the
screen itself isn’t expensive,but it
is almost always accompanied by
reading from the screen because one
typically does read-modify-write
operations like XOR when writing to
the screen. Reading from the video
memory surface is very expensive,
requires synchronization with the DWM,
and stalls the entire GPU pipe,as
well as the DWM application pipe.
It’s unpredictable… if you somehow
manage to get to the actual primary
and write to it,there can be no
predictability as to how long what you
wrote to the primary will remain on
screen. Since the UCE doesn’t know
about it,it may get cleared in the
next frame refresh,or it may persist
for a very long time,depending on
what else needs to be updated on the
screen. (We really don’t allow direct
writing to the primary anyhow,for
that very reason… if you try to
access the DirectDraw primary,for
instance,the DWM will turn off until
the accessing application exits)