Windows app with opacity

[vadbut]

I'm wondering if it is possible to create a Windows app with an opaque (not fully transparent) background that can be captured and overlayed on top of other sources in OBS? Any examples of such apps?
What I mean is:

If yes, what kind of development tools can I use to create such an app (GUI frameworks with OpenGL, DirectX etc.)?
From the OBS side: are there any filters (like color keys) that can help me to get such an effect? (e.g. somehow using a special color in my app that will be partially filtered in OBS or smth like that)

 

[pubpub-zz]

Hi,
+1!

I'm developping an annotation program https://github.com/pubpub-zz/ppInk which provides allows drawing on screen. Some users are enjoying it with OBS. Up to now I've found only a solution to use filters to convert black to transparency, but I would like to upgrade to capture also transparency through game capture but can not find any example which will propose so. Any examples would be welcomed

 

[vadbut]

I managed to pull it off with an OpenGL library. I assume you can use DirectX/Direct3D in Windows as well.
More specifically, I used the SFML library. You can search for threads about it on the SFML forum.