Jwno

SetWindowRgn Shenanigans

Jwno »Devlog

140 days ago byAgent Kilo

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Recently I did some experiments on cropping a window, to see if I can cleanly fit it into a “viewport” defined by Jwno.Along the way, I found an interesting API function:SetWindowRgn.

Warning: We’re gonna cut holes in windows in this post, and it’s going to be GRAPHIC.

So What’s a Rgn?

Rgn stands forregion, which is some data structure used by windows to… define regions in a window’s client area.There’re a bunch of API functions to create and manipulate regions, such asCreateRectRgn.

Regions can be used for various purposes, and one of them is to be used inSetWindowRgn, to define where a window candraw its content. It acts like a mask: only stuff that’s drawn inside the marked area will be shown to the user. And yes,you can impolitely change the mask of a window you don’t own 🤔.

An Example

Here’s some code to cut a diamond-shaped hole in a chosen window:

(use jw32/_winuser)(use jw32/_wingdi)(def hwnd 0xe0732)(def hrgn1 (CreatePolygonRgn [[300 200] [200 400] [300 600] [400 400]] WINDING))(def hrgn2 (CreateRectRgn 0 0 10000 10000))(CombineRgn hrgn1 hrgn2 hrgn1 RGN_DIFF)(SetWindowRgn hwnd hrgn1 true)

If you’re going to try it out in Jwno’s REPL, remember to changehwnd to point to the window of your choice.

And it works like this:

Can It Be Used To Crop a Window?

Unfortunately, no. Not without some hacks, at least. If we move the hole near the border of a window:

You can see the thin window border is still there. That’s because the border is actually rendered by DWM (DesktopWindow Manager, a compoment of Windows), just like Wayland’s server-side window decorations. Of course we can try toremove the window borders entirely, or hook theWM_NCPAINT message, but I think most windows won’t expect thesehacks, and may just freak out.

And there’re these odd-ball windows (File Explorer being one of them):

Instead of the content behind the window, the hole just shows a solid backdrop. It may have something to do withthe glassy backdrop effects rendered by the window. I’ll need to carry out more experiments to figure it out.

Here’s a Funny Usecase, Though

When doing the research, I came acrossan interesting AHK script.The script simply digs a hole in the window beneath the mouse cursor (by eventually callingSetWindowRgn), so thatone can drag-n-drop stuff from places covered by that window. Pretty creative I’d say 😅.