You can access the Scene.InternalOctreeVisibleTriangles or Scene.InternalOctreeStaticCollisions. They are non-nil if you include ssVisibleTriangles or ssStaticCollisions in the Scene.Spatial, see TCastleSceneCore.Spatial docs and ssXx flags docs.
These already contain all shapes of the scene “summed up”.
To access octree for given shape, there is TShape.InternalOctreeTriangles. It is available if you add ssDynamicCollisions to Scene.Spatial.
Up to you whether to use octrees for the whole scene, or the ones per-shape. Using the ones per-shape is more complicated… but if the result of your operation must preserve appearance (materials, textured) then it may be better. Otherwise, in the “whole scene” octree you wlll have a big set of all triangles, with all possible appearances, and you will later need to organize them into new shapes, grouping the resulting triangles with the same appearance.
Looking at the ancestor of TTriangleOctree (TBaseTrianglesOctree) it has methods for colliding various primitives, and I’m interested in BoxCollision:
TBaseTrianglesOctree.BoxCollision(): PTriangle;
This only returns a pointer to a single triangle. How would collision with multiple triangles be handled?
The purpose of this method is to return any triangle that collides with the box. From what I recall, and see in code, it doesn’t have any logic to select a particular colliding triangle. It just returns any colliding triangle. The callers (like TCastleSceneCore.LocalBoxCollision) are using it to have a Boolean knowledge “there is some collision with this box / there is none”.
So it’s not really what you need. I think you’d like to add a similar method like AllBoxCollisions that lists all the colliding triangles, maybe by calling some callback. It could use a similar logic as current BoxCollides, just not exit early – instead find all triangles.
It is possible you can implement it without changing CGE, and just traversing the octree yourself. But admittedly I’m not sure – so far, such operations have been implemented by extending existing CGE octree classes. If need be, go ahead and modify CGE – we can make it included 