If you want Bifrost to "see" your geometry, then yes, you will have to bring it into a graph.
Bifrost cannot currently select anything. But maybe you can use the output of a graph and feed that to a MEL script command for select or something?
In any event, there are several ways to bring in many geometries and check if they're inside a cube or sphere or some big bounding object. First of all - the best way to bring in many geometries is using regular expression on an input node. For more info on this, please take a look at my little FAQ https://forums.autodesk.com/t5/bifrost-forum/faq-for-new-bifrosters/td-p/8920248
Once you have an array of geometries coming into you graph, you'll need some way to check if they're inside the bounding geometry. You can do this in two ways: with polygons or by turning the bounding geometry into a volume.
The polygon way: Iterate on the incoming geometry array, and for each geometry, get the point_positions. Then, for each point_position, check that it is on the negative side of the plane defined by ALL polygon faces of the bounding geometry. This method will be lightweight and fast, assuming the bounding geometry is simple, like a cube. It will become slow if the bounding geometry has a lot of faces like a high-res sphere or a highly detailed mesh.
The volume way: Convert the bounding geometry to a volume, via mesh_to_volume, operating in "solid" mode. The trick here is that, depending on how exact you want the result to be, you can use quite large values for the detail-size to make it fun faster and use less RAM. Now, once you have the volume representation of the bounding geo, iterate on every geometry and get the point_positions. Then use sample_volume, using the property called "voxel_signed_distance". If this compound returns a positive number, that position is outside the volume. This method will be faster than the other one, if the bounding geometry has a lot of faces.
There may be other ways of doing this, perhaps clever use of closest_locations_on_surface, but the two methods above will work.
Marcus
