I also struggled with the same problem, before understanding (maybe) how it works, and
maybe I can help you.
I think I understand that Outerra renders an object only if its center is included within the actual point of view.
If you approach a side part of a very large object and the center of the object is not in the frame ... the object disappears completely.
This is very useful in order not to overload the engine with unnecessary rendering processes if the object is not in the current frame (perhaps behind the observer) but annoying in the case of large objects to be explored in every small detail even far from its own. center.
In reality it is not considered the only central point but the "boundig box" of the central object.
Outerra's FBX importer has a limit: it cannot import objects with a number of vertices greater than a certain number (64K?)
In most cases, a model does not exceed this limit, it can be imported as a single object and does not suffer the disadvantage of the disappearance of its lateral parts because they are always included within its bounding-box.
If, on the other hand, the model is very complex it must be divided into single objects which, individually, do not exceed the vertex limit.
Usually all these objects are "parented" to the central object which constitutes the reference point of the model and assume its common center.
It can therefore happen that, if the central object has a small bounding-box, and this is not inside the frame, the lateral objects that have assumed the same center are not rendered.
The inconvenience can be easily remedied.
In the objects that can present this problem I define, as the main object, a cube or a parallelepiped or a sphere that completely contains all the connected objects (even those that could move with the animations and move away from the center).
All other objects are related to this cube which forms the core of the model.
Any exploration, even of the most external parts, will always contain a part of this cube and there will be no more rendering deficiencies.
However, this central "convenient" object must be made to disappear because it is not part of the model.
So I equip the object with its own material and, in the mtl file, I define it as completely transparent.
"color": "0.0,0.0,0.0,0.0",
"f0": "0.0",
"roughness": "1.0",
This is sufficient to carry out the various preliminary tests but still leaves a slight shadow that remains visible.
For a complete disappearance it is necessary to define, in the model script, the meches of the cube as "false" and only the model will remain visible.
As always there is a "downside" ...
If it is a model called "vehicle" this meches will become the collision meches of the object and could be exaggeratedly large and, in any case, not realistic.
Fortunately, in the case of an "aircraft" object, the collision points are defined in the JsbSim XML file and there are no problems.
In the objects that you see disappearing in your screen-shots, probably, even if they are single objects, the setting of a "center" other than their physical center of mass has remained, and the exclusion of this "center" from the shot makes them disappear ...
As I said at the beginning, all this reasoning derives from my very personal experimental deductions and could be totally denied by an authoritative indication of the developers ...I hope, however, that I have been useful to you.
P.S. Just complimenting you on how you are building your model is absolutely an understatement !!!
It is, without a doubt, a
WONDERFUL job !!!!!!!!
P.P.S. I apologize for the post in English and not in German ...
I use Google Translator and I can "slightly" understand if an English translation is faithful enough to my original thought but I have no knowledge of the German language and I could not check if the translation is faithful ...
