Add scene-graph versioning via Spatial.getLastGeomChangeId()

[Copilot]

Callers currently have no cheap way to determine whether a scene-graph branch has changed between frames, forcing culling traversals even when nothing has moved. This adds a monotonically-increasing change ID to every Spatial that propagates upward on any geometric mutation, enabling branch-level change detection in O(1).

Changes

Spatial

• private static final AtomicLong nextGeomChangeId — JVM-wide counter; each increment produces a globally unique ID.
• protected transient long lastGeomChangeId — set to a fresh ID at construction (creation is itself a change), updated on every geometric mutation.
• private void incrementGeomChangeId() — obtains next ID, writes it on this, then walks the parent chain writing the same ID to every ancestor. Ensures root.getLastGeomChangeId() always reflects the most recent change anywhere in the subtree.
• Hook points: setBoundRefresh() (covers transform changes too, since setTransformRefresh() → setBoundRefresh()), setLightListRefresh(), setMatParamOverrideRefresh().
• public long getLastGeomChangeId() — public API with usage Javadoc.

Usage pattern

private long lastKnownChangeId = -1;

void update() {
    long currentId = rootNode.getLastGeomChangeId();
    if (currentId != lastKnownChangeId) {
        doFullCullingPass();
        lastKnownChangeId = currentId;
    }
    // else: sub-tree unchanged, skip culling entirely
}

Key properties:

• Sibling isolation — a change in one branch never touches its siblings' IDs.
• Single propagation path — transform changes produce exactly one incrementGeomChangeId() call (via setBoundRefresh), no double-increment.
• transient — excluded from serialization; ID is runtime state only.

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Original prompt

evaluate the possibilities to add some versioning to the scenegraph, i would like to quickly be able to eliminate branches if there have not been any changes. it should be based on a frame id. so i can easily skip even culling when no geomertry has changed

[zzuegg]

@copilot can i then also skip subbranches with < lastChangeId?

[Copilot]

@copilot can i then also skip subbranches with < lastChangeId?

Yes — the propagation is strictly upward (ancestors only, never siblings), so every branch retains the ID of the most recent change in its own sub-tree. That makes the comparison work directly:

// After completing a culling pass, snapshot the current frontier:
long lastCullId = rootNode.getLastGeomChangeId();

// Next frame, when recursing into sub-branches:
private void renderSubScene(Spatial scene, ViewPort vp, long lastCullId) {
    if (scene.getLastGeomChangeId() <= lastCullId) {
        // Nothing in this sub-tree changed since the last cull.
        // Re-queue previous results and skip the traversal entirely.
        return;
    }
    // ... normal cull / recurse
}

branch.getLastGeomChangeId() <= lastCullId means the last change in that branch happened at or before the snapshot, so the branch is unchanged and safe to skip. Use <= (not strict <) so the branch that was the cause of the last snapshot value is also skipped on the next frame when it hasn't changed again.

[zzuegg]

@copilot how many hours would a game run if i have 2000 moving objects at 240fps?

[Copilot]

@copilot how many hours would a game run if i have 2000 moving objects at 240fps?

At 2000 moving objects × 240 fps = 480,000 increments/second, and Long.MAX_VALUE ≈ 9.2 × 10¹⁸, the counter would take roughly 609 years to overflow. No practical concern — the long counter is safe for the lifetime of any game session.