Something to try at a later date:

Create raytracer without lighting or normals.
Surface colour product of distance travelled by light rays before they intersect surface.

I’m envisaging it being like a kind of spotlight effect, because the rays are cast radially, with points on the surface nearer the screen plane being lighter, and further-away points being darker.

Offsets could be applied to change the centre-position of the shading effect.

Something to ponder…