Groups, activation and triggers
Large structures should not all fall at once. Three kinds of nodes let you control which pieces do what, and when.
Impact Group
Section titled “Impact Group”An Impact Group selects a set of pieces. Other nodes (Constraint, Rebar, Debris, Activation, Trigger) take a Group input and act only on those pieces. An unlinked Group input means every piece.
| Select | Chooses pieces by |
|---|---|
| All | every piece |
| Source | which Impact Source they come from |
| Collection | membership of a collection (children included) |
| Volume | being inside an object (read at the first frame) |
| Height | a band along Z, as fractions of the selection’s height, with From and To |
| Property | a custom property on the piece objects, tested for equality, existence or a number range |
| Random | a fraction of the pieces, with a Seed (the same seed picks the same pieces) |
Other options: Invert selects the pieces the test does not, and Grow also selects the pieces up to a number of bonds away.
Impact Group Combine joins two groups as a union, an intersection or a difference.
Impact Activation
Section titled “Impact Activation”An Impact Activation node decides when a group of pieces wakes.
| Setting | Default | What it does |
|---|---|---|
| Mode | Start deactivated | How the pieces start: simulated, asleep, held until triggered, or fixed in place. |
| Pin in Place | off | Anchors the pieces. They never move until a Trigger unpins them. |
| Trigger Radius (m) | 0 | Distance at which a collider releases the pieces. 0 is contact. |
| Wake Lookahead (m) | 0.5 | Releases held pieces just before a moving collider arrives, looking this far along its path. 0 is off. |
| Impact Energy Transfer | 1 | Strength of the kick a collider gives these pieces. 1 is a normal bounce. 0 is none. |
| Proximity Delay (s) | 0.2 | How long a held piece waits, once moving pieces are near, before release. |
| Proximity Radius (m) | 0.4 | How close moving pieces must come to start the release. |
| Speed Threshold (m/s) | 0.1 | The slowest speed at which a moving piece releases its neighbours. |
This is what makes a wall collapse progressively: pieces wake as their neighbours start moving.
Impact Trigger
Section titled “Impact Trigger”An Impact Trigger is a volume or a frame range that wakes, releases or weakens the pieces it touches. Animate the trigger object to sweep through a structure, as the Staged Demolition template does to take a tower down floor by floor.
| Setting | What it does |
|---|---|
| Where | Every piece, or the pieces inside the object’s box or sphere. |
| Object | The volume. Animate it to sweep through the pieces. |
| Start, End | The frame range during which the trigger acts. |
| Activate | Releases held pieces. |
| Wake | Wakes sleeping pieces. |
| Unpin | Releases anchored pieces. |
| Break Bonds | Snaps every bond of the pieces inside. |
| Weaken Bonds | Multiplies the strength of their bonds, once, by Factor. 0.5 halves it, 0 removes it, 1 changes nothing. |
| Freeze | Stops the pieces and holds them in place. |
| Add Velocity | Kicks each piece once, with a Velocity in world space and an Outward speed away from the volume’s centre. |
| Once per Piece | Acts on a piece the first time only. Off, it acts every substep it is inside. |
Example: a wall that collapses in order
Section titled “Example: a wall that collapses in order”- Fracture the wall.
- Pin the footing: a Group by height (From 0, To 0.1) into an Activation with Pin in Place.
- Give the rest a Start deactivated Activation with a Proximity Radius and Delay.
- Hit it with a projectile. The pieces it touches wake, and wake their neighbours in turn.
The Mortar Wall template uses groups in this way: a running-bond wall with weak mortar joints, a stronger top course and a pinned footing, hit by a steel cannonball.