Basics of Breaking 3D Objects - Part 02 - Houdini Tutorial

The first column-fracture lesson creates pieces that collapse as soon as the simulation runs. This follow-up adds glue so the pieces remain connected, then moves a colored box through the constraint network to release them from top to bottom. The animation controls where bonds disappear; the rigid-body simulation determines how the freed pieces move.

This is a historical shelf-created Houdini DOP workflow, with an unverified exact release. It includes several corrections to attribute ownership and deletion setup, plus a cache-related workaround whose underlying cause is not established. Those corrections are part of understanding the demonstrated result.

Add glue and distinguish strength from release timing

At 00:01, inspect the existing dynamics network: the static ground and its solver, the fractured column, the merges, and gravity are already present. Use Glue Adjacent from the rigid-body shelf, select the column’s rigid bodies, and confirm in the viewport. Confirming with focus in the wrong pane initially opens a node instead of completing the selection.

The tool creates glue constraints and an associated geometry network. Display that geometry to see connections between the pieces. These are relationships controlling the fractured objects, not extra surfaces of the column.

The first playback remains assembled. Lowering glue strength to about 10 allows collapse; testing about 500 changes the breakup. Neither setting supplies the requested top-to-bottom timing. The instructor also adjusts the column’s vertical placement because a small drop into the ground affects how it starts breaking. Ground contact and glue strength should be checked separately before interpreting the result.

Around 06:01, set strength to −1. The narration informally calls this full strength; more precisely, SideFX documents −1 as an unbreakable glue connection. The workflow can still release pieces by explicitly deleting their constraint geometry. This setting does not anchor the assembled object to the world or replace the ground collision.

Build a moving color region on the constraint geometry

Enter the generated Remove Broken setup and replace the existing deletion stage with the demonstrated custom chain. Keep in mind what is flowing through this network: the glue connection geometry. Deleting its primitives removes bonds; it does not erase the visible column fragments.

From 07:01, create a Box and use the older Point SOP’s Add Color controls to make it red. Initialize the receiving constraint points to black. Attribute Transfer then carries color from the box onto nearby receiving points.

Tune the transfer distance while displaying the colored constraint geometry. The default affects too large a region. A test around 0.5 becomes too restricted in parts of the setup, and a value around 1 is preferred for the example. The instructor notices that this is transferring point data: a coarse box’s point positions and the distance threshold matter, so its apparent solid interior is not automatically a filled influence volume.

Use a Transform on the box to animate the region. Start high above the column and key its position, then move it down to the lower position at frame 48. The source uses Alt-click on the channel to create the keys. At this stage, playback shows the color region moving; it does not yet break bonds. Verify that intermediate result before adding deletion.

Promote the intended attribute before deleting primitives

Around 11:05, insert Attribute Promote to move color from the point level to the primitive level. The subsequent deletion targets constraint primitives, so color must be available at the level where the condition is evaluated.

The first attempts do not delete anything. The instructor experiments with a red-channel condition, then discovers that Attribute Promote was not told which attribute to promote. Set the original attribute name to Cd, with point-to-primitive promotion, and inspect the primitive data afterward.

The recording eventually succeeds with a condition described as color greater than zero. Black initialization gives the unmarked region zero color, while transferred red supplies a positive component in the selected area. The captions are inconsistent about the exact expression spelling and component syntax; they also do not establish the promotion method. Consequently, this guide preserves the data flow and test rather than supplying an invented executable expression.

The practical verification is to display the constraint geometry through the chain: black initialization, localized red transfer, Cd on primitives after promotion, and missing bond primitives after deletion. If the box moves but no constraints disappear, inspect attribute name and ownership before increasing the transfer distance or changing the animation.

This is also why the visible pieces must remain present. The intended effect is loss of a relationship between fragments, followed by physical motion, not disappearance of the fragments themselves.

Re-evaluate the simulation after the constraint test works

At 15:01, the constraint preview shows bonds disappearing as the box moves through the column. The full simulation still does not initially respond as expected. A warning mentions an immediate/baked cache state, and the instructor pauses to investigate.

On return around 18:04, the reported workaround is to change glue strength to a positive value and then back to −1. This clears the observed warning and restores the intended result without another reported network change. Treat that as what worked in this scene; the recording does not establish it as a universal solution to cached DOP problems.

Return to the beginning and play the full simulation. The column now breaks from top to bottom as the moving region deletes its glue bonds. This final check tests the connection between the changing constraint network and the simulated pieces, beyond the earlier isolated geometry preview.

Slow the release by changing the box animation

Near 19:04, the instructor revisits the Transform keys because the two-second release feels fast. The box begins at the top on frame one and reaches the lower position around frame 120. Playback produces a differently timed collapse.

Changing those keys changes the schedule of bond removal; it does not directly prescribe every fragment’s trajectory. The example keeps the same basic color-transfer method while adjusting its animation. Other paths are proposed as ways to vary the release, but only the demonstrated top-to-bottom versions are established here.

The finished lesson adds control to the first tutorial without turning it into a structural-engineering model. Its essential checkpoints are intact glue at the start, a correctly localized color region, primitive-level deletion of constraints, and a freshly evaluated simulation that responds to those deletions. Exact expression syntax and the cache-warning cause remain documented limitations.