I'm currently designing a very large and complicated model, because it's large and should be strong enough completely hollow I want to print it with no infill to save material.
However, there are some points in the model where there are indentations in the top shell.
The bottom of those indentations are lower then the surrounding shell (pretty mush the dictionary definition of indentation) and so they are printed earlier with nothing to connect to.
For example, in the image below from Cura's layer view you can see two round surfaces just hanging in mid air
Obviously this can't be printed.
What I'm currently doing is designing internal spaces inside the model where the internal space shell is holding the "detached" surfaces.
What I want is a mostly automatic process that will generate a support structure inside where needed or that will generate infill but only between the floating surface and the nearest solid shell below it.
Reorienting the model is not an option because a. it wouldn't fit on the build plate and b. even if it did it has a lot of internal parts with different orientations so it would just move the problem from one part of the model to another.
I usually use SketchUp for designing and Cura for slicing so I prefer something I can do in Cura - but since I strongly suspect this can't be done in Cura I'll accept answers that work with other tools
In Cura 2 you can do several things:
Design internal support pillars, and make them an Infill Mesh via the Per-object settings. I suggest you also reduce the top/bottom thickness and Wall thickness of the infill mesh.
Enable Hollow Out Objects and enable support.
Enable gradual infill: increase the Gradual Infill Steps. Although this will still make infill everywhere it will drastically reduce the infill where it's less needed.