Camera Patches

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Camera Patches are short-lived, additively-composable overlays that sit above the normal camera evaluation tree. The Director evaluates the active camera's node chain first to produce a base pose; Patches then run in layer order on top of it — each Patch reads the upstream pose, runs its own node graph, and contributes a blended result.

Use a Patch when you need a temporary, time-bounded effect that overlays the running camera without changing its configuration: a hit-reaction shake, a scope-zoom push, a recoil impulse, or any gameplay-triggered effect that should fade in, do its job, and fade out.

Per-frame evaluation flow

The diagram below shows what happens inside one Director::Evaluate call — first without any Patches, then with two active Patches.

flowchart TB subgraph NoPatch["Normal camera (no Patches)"] direction TB A1["Director::Evaluate"] --> B1["EvaluationTree\n(active camera node chain)"] B1 --> C1(["Base pose"]) C1 --> D1["Output → PCM\nUpdateViewTarget"] classDef ev fill:#e8f7ee,stroke:#0e9f6e; classDef pose fill:#fffbea,stroke:#d97706; classDef out fill:#eff6ff,stroke:#3b82f6; class B1 ev; class C1 pose; class D1 out; end subgraph WithPatch["Camera with two active Patches (layer 0 and layer 1)"] direction TB A2["Director::Evaluate"] --> B2["EvaluationTree\n(active camera node chain)"] B2 --> C2(["Base pose"]) C2 --> P0["Patch — layer 0\nnode graph ticks with Base pose\nBlendBy α₀"] P0 --> C3(["Blended pose₀"]) C3 --> P1["Patch — layer 1\nnode graph ticks with Blended pose₀\nBlendBy α₁"] P1 --> C4(["Blended pose₁"]) C4 --> D2["Output → PCM\nUpdateViewTarget"] classDef ev fill:#e8f7ee,stroke:#0e9f6e; classDef pose fill:#fffbea,stroke:#d97706; classDef patch fill:#fdf4ff,stroke:#a855f7; classDef out fill:#eff6ff,stroke:#3b82f6; class B2 ev; class C2,C3,C4 pose; class P0,P1 patch; class D2 out; end

Key points from the diagram:

  • The EvaluationTree runs unconditionally — Patches never replace it, only layer on top.
  • Each Patch receives the previous layer's output as its upstream pose, so Patches compose in a defined chain rather than a random pile.
  • α is the Patch's current envelope alpha (0 → 1 entering, 1 active, 1 → 0 exiting). A Patch at α = 0 contributes nothing; BlendBy makes this a no-op rather than a sudden switch.
  • The PatchManager runs the full loop (advance envelope → check expiration → tick evaluator → BlendBy → sweep expired) inside Director::Evaluate, just after the tree.

The four tools compared

Node Action Modifier Patch
Part of the permanent chain Yes No No No
Runs its own node graph Yes No No Yes
Has enter/exit envelope No No No Yes
Layered by index (multiple at once) No No No Yes
Works in Sequencer timeline No No No Yes

Actions fill the "per-frame logic, no graph" niche. Patches fill the "full node graph, time-bounded, envelope-blended" niche. When the effect is complex enough to need its own graph (e.g. a spring-damped offset that references a pivot actor), a Patch is the right tool.

Lifecycle

Every Patch instance passes through four phases:

flowchart LR EN["Entering\nα: 0 → 1\n(EnterDuration)"] AC["Active\nα = 1\n(expiration channels checked)"] EX["Exiting\nα: 1 → 0\n(ExitDuration)"] EP(["Expired\n(removed)"]) EN -->|"α reaches 1"| AC AC -->|"Duration / Manual /\nCondition / CameraChange"| EX EX -->|"α reaches 0"| EP classDef phase fill:#fdf4ff,stroke:#a855f7; classDef term fill:#f1f5f9,stroke:#94a3b8; class EN,AC,EX phase; class EP term;

The easing curve (EComposableCameraPatchEase: Linear, EaseIn, EaseOut, EaseInOut, Smooth) is applied symmetrically to both ramps. EnterDuration ≤ 0 short-circuits directly to Active on the first frame — no invisible α = 0 frame is wasted.

Expiration channels

A Patch can expire through any combination of:

  • Duration — after a fixed number of seconds in the Active phase.
  • Manual — when ExpirePatch(handle) is called explicitly.
  • Condition — when the asset's CanRemain() override returns false.
  • OnCameraChange — when the Director's running camera changes (per-call flag on FComposableCameraPatchActivateParams).

The first channel to fire flips the Patch to Exiting via the exit envelope ramp. Channels stack additively.

Two surfaces

Patches are designed to work from two independent surfaces:

  • Blueprint / C++ runtime path — AddCameraPatch on the active Director, driven by gameplay events. Suitable for dynamic, event-driven effects whose timing isn't known ahead of time.
  • Sequencer timeline path — a UMovieSceneComposableCameraPatchTrack on a Level Sequence, driving a UComposableCameraLevelSequenceActor. Suitable for authored cinematic overlays whose timing is locked to the timeline.

The two surfaces are orthogonal: a gameplay Patch applies on the PCM Director's running camera, while a Sequencer Patch applies on the LS Actor's internal CineCamera. Both can be active simultaneously without interfering.

For the full authoring guide, see User Guide → Camera Patches.

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