Outline Rendering of 3D Models (POV-Ray)
Here are some images:
The interesting thing about these is that they are computer rendered 3D models (in POV-Ray), not created in 2D drawing programs such as photoshop or Inkscape. Am surprised to learn, that this is not possible with POV-Ray (See: Intro to POV-Ray) Instead, you have to use a special add-on software.
The outline rendering is significant, because it has a lot applications, especially in illustration or technical drawing. The essence is that this illustration-style shows the important aspects and hides away irrelevant details, such as complex meshes in wireframe rendering or the realism detail from ray-tracing. (much like black'n'white photos make you see the composition) My need for 3D modeling is mostly illustration of geometry, so i really want to render 3D objects like this. (See: Illustrating Geometry with POV-Ray • Geometry: Requirements for a Visualization System for 2020)
Here's a example of illustration of so-called “stereographic projection”.
It is rendered in a wireframe mode. Note, a desired rendering, is to actually hide the meshes behind the sphere.
It's somewhat surprising that POV-Ray can't do it, thinking about all the trouble it went thru with ray-tracing. If you think about it, it does make sense, because the essence of illustration-style drawing is a parallel projection from the camera, but realistic 3D rendering is about tracing every light ray to render every point on the object. However, i think the 2 issues are still quite related and is not something totally separate. I think the fact POV-Ray cannot do it is just a particular historical or idiosyncratic reason. Because, for example, when rendering in POV-Ray or any 3D modeler, you have to have algorithms that hide invisible parts. (that is, you shouldn't see what's behind a box) The hiding invisible object algorithm is part of the parallel projection from the camera, same with rendering in outline mode.
Note that Second Life (a virtual world 3D “game” application), is capable of outline rendering, in real time. See: Toons in Second Life.
(thanks to namejuseijin for mentioning the POV-Ray outline-rendering add-on.)