I'm trying to add volume to closed path. Simple, use extrude. However, what if I want a somewhat more rounded or irregular top surface?
Let's say I hade a closed polyline silhouette of a simple tree. I can extrude it for a flat top, play with fillet caps, but how can I literally round out the trunk and branches making them look more realistic; it would be as if the tree were sliced down the middle. How can I achieve that?
By the way, I'm a 3d printer, so I need to use a displacer to rough up the trunk etc using a bark texture if possible. Commentary in that regard would be appreciated as well.
I'm not sure I understand the extrusion part of your question (can you post a scribble?), but I hear you on the trunk texture for 3D printing.
Get a good fine mesh on your object then use a displacement deformer in mograph to add a texture for displacement. Then use the "current state to object" function in the right click menu to collapse all of the geometry effects into an actual mesh.
Back to the extrude. That's not the best way to create a tree. There are lots of ways though (even with mograph since you have to get there anyway for the deformation effect).
You can use splines and sweep a profile tapering them to 0 for the branches and then bool all of them together. You can let mograph scatter and warp those splines for you from a master or a few splines.
You can use subdivision modelling to extrude polys off a core shape (and keep extruding and pulling and scaling down for all the branches) and then add a subD Surface as a parent and let it subdivide and soften the boxy model.
You can emit particles with turbulence and use the tracer to get the paths and then sweep those.
You could get a couple of profiles of trees going and then extrude them, Rotate them all differently about the y so you have branches going off in all directions. Make each extrude a single object in the extrude attributes and then make them editable, bool all that together then use the SubD as a parent to smooth it all out.
There are a few dedicated plug ins that make tree like things as well. Lots of free models too.