Check out an incredibly in-depth guide by Alex Ameye.
Rendering outlines is a technique that can be used in games to add a specific stylized look, help players find things and more. What are your favorite examples of using outlines?
The first technique described in the article is the fresnel effect which can be used to render an outline on the rim/edge of an object. The developer also discussed Vertex Extrusion, Blurred Buffer, and more.