Shader to bevel the edges of a cube?

This question relates to using shaders (probably in the Unity3D milieu, but Metal or OpenGL is fine), to achieve rounded edges on a mesh-minimal cube.

enter image description here

I wish to use only 12-triangle minimalist mesh cubes,
and then via the shader,
Achieve the edges (/corners) of each block being slightly bevelled.

In fact, can this be done with a shader?

Solution

I recently finished creating such shader. The only way it can work is by providing 4 normal vectors instead of one for each vertex (smooth, sharp and one for each edge of the triangle for the given vertex). You will also need one float3 to detect edges.

To add such data in a mesh I made a custom mesh editor, comes with Playtime Painter Asset from Unity Asset Store. Will post the shader with the next update. Also will post to public GitHub.

You can see some dark lines, it's because it starts to interpolate to a normal vector which facing away from light source, but since there are no additional triangles, the result is visible on a triangle which is facing the camera.

Update (2/12/2018) Realised that by clipping pixels that end up having a normal facing away from the camera, it is possible to smooth the outline shape. It wasn't tested for all possible scenarios but works great for simple shapes:

As per request added a comparison cube:

Currently, Playtime Painter has a simplified version of that shader, which interpolates between 2 normal vectors and gives ok results on some edges.

Wrote an article.