✨ Caustics Generator - Dynamic Underwater Light Patterns

Real-time caustics generation using GPU-accelerated particle grids for physically-accurate underwater light refraction patterns.

Overview

Caustics are the light patterns visible underwater and on submerged surfaces, created by light refraction through the water surface. The Caustics Generator provides a flexible system for creating these effects using Hierarchical Instanced Static Meshes (HISMC).

---

Caustics Generator Actor

AOceanologyCausticsGenerator

UCLASS(Blueprintable)
class AOceanologyCausticsGenerator : public AActor
{
    // Generate water preview mesh grid
    UFUNCTION(BlueprintCallable)
    void SpawnWaterPreviewGrid(
        UHierarchicalInstancedStaticMeshComponent* HISMC,
        float GridSize,
        int GridTiles
    );

    // Generate caustic particle grid
    UFUNCTION(BlueprintCallable)
    void SpawnCausticParticleGrid(
        UHierarchicalInstancedStaticMeshComponent* HISMC,
        float GridSize,
        int GridTiles
    );

    // Editor tick for real-time preview
    UFUNCTION(BlueprintNativeEvent, CallInEditor)
    void EditorTick(float DeltaSeconds);
};

---

Grid Systems

Water Preview Grid

Visualizes the water surface mesh for caustic ray calculation:

Parameter	Description
GridSize	Total area covered in world units
GridTiles	Subdivision count per axis
HISMC	Target instanced mesh component

Caustic Particle Grid

Generates light ray particles that project caustic patterns:

Parameter	Description
GridSize	Coverage area matching water preview
GridTiles	Particle density (tiles × tiles)
HISMC	Particle mesh instances

---

How It Works

Ray Tracing Approach

1. Surface Sampling: Sample wave heights across grid
2. Normal Calculation: Compute surface normals
3. Ray Refraction: Calculate light ray bending
4. Projection: Project caustic intensity to surfaces

Physical Accuracy

The system accounts for:

• Snell's Law refraction
• Wave-based normal variation
• Depth attenuation
• Light source direction

---

Editor Integration

Real-Time Preview

virtual bool ShouldTickIfViewportsOnly() const override;
virtual void Tick(float DeltaSeconds) override;

UFUNCTION(BlueprintCallable)
void SetEditorTickEnabled(bool bEnabled);

Enable EditorTickIsEnabled for live caustic preview in editor viewport.

---

Configuration

Grid Density

Quality	GridTiles	Particles	Performance
Low	32	1,024	Excellent
Medium	64	4,096	Good
High	128	16,384	Moderate
Ultra	256	65,536	Demanding

Optimization Tips

• Reduce grid density for distant water
• Use LOD system for particle meshes
• Limit update frequency for static scenes
• Bake caustics for fixed sun angle

---

Integration with Underwater

Caustics work with the underwater post-process system:

Component	Caustics Role
Underwater Volume	Defines caustic visible zone
Water Material	Receives caustic projection
Sun Direction	Primary light source for rays
Wave Amplitude	Caustic intensity variation

---

Visual Parameters

Caustic Appearance

Parameter	Effect
Intensity	Overall brightness
Scale	Pattern size
Speed	Animation rate
Chromatic Aberration	RGB separation

Depth-Based Falloff

• Near surface: High contrast patterns
• Mid depth: Softer, blended patterns
• Deep water: Minimal visibility

---

Blueprint Integration

Setting Up Caustics

1. Add Generator Actor to level
2. Configure Grid size and density
3. Assign HISMC components
4. Enable Editor Tick for preview
5. Adjust Parameters for desired look

Runtime Control

// Toggle caustics at runtime
CausticsGenerator->SetEditorTickEnabled(bEnableCaustics);

---

Performance Considerations

GPU Instancing

HISMC provides efficient GPU instancing:

• Single draw call per grid
• Hardware instancing support
• Automatic LOD management

Update Frequency

Mode	Update Rate	Use Case
Every Frame	60 Hz	Dynamic scenes
Fixed Interval	10-30 Hz	Performance mode
Baked	Static	Cinematic

---

Future Enhancements

Feature	Status
Ray Marching Caustics	Planned
Screen-Space Caustics	Research
Volumetric Caustics	Planned
Compute Shader Path	Planned

---

Resources

• Example Levels: See Maps/CausticsDemo
• Material Functions: MF_CausticProjection
• Documentation: Underwater rendering guide