Carrying: ferrying objects on a moving target¶

bCarryObjects lets a Group attach whatever is resting on or inside its moving target for the duration of the motion: a platform lift carries the player up with it, a drawer carries its contents as it slides open. The mechanism is simple: one scan, one instant, fixed for the whole run.

Decided once, at group start¶

bCarryObjects scans for what to ferry when the group starts playing, and that set is then fixed for the whole motion:

        graph LR
    Start[Group.StartPlayback] --> Scan[Scan bounds + CarryHeight ONCE]
    Scan --> Attach[Attach whatever overlapped]
    Attach --> Play[Play timeline: set is now fixed]
    Play --> End[Group ends]
    End --> Detach[Detach all carried objects]

Drop something into a drawer that is already opening and it will not be picked up, it only rides if it was there the instant the drawer began to move. Need a late passenger? Retrigger the group; StartPlayback re-runs the scan from scratch.

Why once, not continuous¶

The reasons:

Predictable: what a group carries is decided at one instant you can reason about and reproduce, not on whichever frame something drifted inside the volume mid-animation.
Cheap: a continuous re-scan needs an overlap query per moving object, per frame. A single scan at trigger time doesn't, and most games never need the continuous version anyway.

The scan volume itself is the target's bounds extended upward by CarryHeight (cm): tall enough to pick up a standing player or a stack of loose props, not so tall it catches the ceiling above.

Requirements for a carried object¶

Requirement	Why
Mesh must be Movable	A Static mesh's transform can't be driven at runtime: carrying one is skipped and the log says so, instead of doing nothing.
Replicated actor + replicated movement (multiplayer)	Without both, the object attaches correctly on the server but remote clients never see it move: the two must agree on where the carried object ends up.

See Multiplayer for the full replication story. The short version: bCarryObjects doesn't replicate the attach decision separately from the actor's own movement replication, so the carried actor has to be able to replicate itself.

Use cases¶

Scenario	What's carried
Lift carries the player	Whoever is standing on the platform when it starts rising, including a pawn.
Drawer carries its contents	Any loose Movable prop resting inside the drawer's bounds when it starts sliding open.
Elevator floor carries a crate	A pushed-in crate rides up with the elevator, as long as it was inside the scan volume at trigger time.
Turntable carries a display prop	The prop turns with the platform instead of sliding off as it spins.

Worked example¶

Group "Rise":
  bCarryObjects: true
  CarryHeight: 20.0

A player standing on the lift's platform mesh when Rise starts is caught by the bounds-plus-20cm scan, attached for the whole ascent, and detached the instant the group ends, so they land on the platform at its new height instead of staying attached to a component that has finished moving.