The key to transparent windows is the gl context creation. Since allegro doesn't support using a custom context, you can't currently do it with Allegro.

However, allegro can still give you a HWND on Windows, which you can use to set the window properties and the color key which allows shaping the window with a single transparent color. This does not work for translucency though, for that, again, you need a custom gl context.

So that gray square is your transparent window? Neat!

Do you think you could do the same for Windows? I don't know the specifics of making a WGL context transparent, but it would be awesome if Allegro could do this for all platforms. For OpenGL at least.

EDIT
I found this :
https://stackoverflow.com/questions/4052940/how-to-make-an-opengl-rendering-context-with-transparent-background

Which shows a few different ways. I never used dhpoware's demo code because I didn't understand pbuffers. It looks like all you have to do is change the PIXELFORMATDESCRIPTOR a little bit before you pass it to the rest of the routine.

Maybe I can make a patch for this, not sure.

I've made transparent mice windows before using DIBs but that is no good for anything larger than an icon. And it's way too messy trying to bitblt to the HDC all the time.

I found datenwolf's wglarb demo, and I'm trying to get it to work as we speak. Right now it draws a white window, not a transparent one, so it's not working yet.

As for the PR, for X11, you'd need to incorporate the changes into the window creation system. Can't just enable translucency for every window.

On #allegro on IRC on freenode, I was talking to SiegeLord, and he said adding ALLEGRO_TRANSLUCENT_WINDOW to the new display flags was okay. But the meat of the matter still needs to be implemented.

EDIT
Some progress.

This (https://pastebin.com/BfaHf1NJ) is one way to do it using PFD_SUPPORT_COMPOSITION, and it works on Vista up, simply by asking for a different pixel format.

You can see it here :

{"name":"WNhv2if.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/5\/a5a937eda446b908fdffe4292fd0a591.png","w":1403,"h":509,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/5\/a5a937eda446b908fdffe4292fd0a591"}

EDIT2
You can get identical results using WGL_TRANSPARENT_ARB. I got datenwolf's layered.c demo to work, but I had to edit it. He made the window style WS_OVERLAPPED_WINDOW, which doesn't work. It needs to be WS_POPUP style to work properly.

I'm in the process of modifying a local fork of Allegro 5.