Highlighting objects in 2d game like in Baldur's Gate/Pillars of Eternity possible in HLSL+DX11 shader?

Hi all,

I was wondering if it is possible to create a HLSL shader to highlight objects like in baldur's gate or pillar of eternity, ie:

Or if this is done using a semi transparent texture on top of the object.

I'm not an expert with shaders, so I dont know if it is possible to detect edges like that.

Thanks for any help!