This was a survey paper for glyphs. It collected a good number of guidelines, which should ideally be examined and discussed in detail in this forum.
The following discussions are NOT guidelines but some speculations based on my experience of 3D visualization and my knowledge of perception.
Back to your question, perhaps one needs to consider the main visual channel(s) and a secondary channel for continuity or alike. For example, would the main visual channel show a long tube if the object is continuous (i.e., the secondary channel opacity alpha§ = opaque for all p, where p is a point on a path? OR is it a set of discrete objects such as spheres or irregular shapes.
In VR, 3D shading is expected to provide some visual cues for opaque objects as different objects at the overlapping parts in a view will be shown with discontinuous shading. When a viewer moves his/her head slightly (i.e., motion parallax), the discontinuous patterns changes, offering self-error correction in depth and continuity perception [Chen & Jaenicke, TVCG, 2010]. Irregular shapes exhibit more variations of discontinuous patterns than randomly placed tube segments, which have more variations than spheres. Spheres have more of such variations than segments of tubes parallelly along the same path.
With head movement, specular highlighting may offer additional visual cues as it is view-dependent in addition to light-dependent.Furthermore, the signals and the changes of discontinuous patterns can be further strengthened using relatively continuous and predictable texture mapping such as line-patterns, wood effects, etc.
The challenge is perhaps with translucent objects, especially amorphous objects without an outer surface layer in volume rendering. Perhaps I should let other colleagues to help find a solution.
