📐 Jump Flood SDF - Real-Time Distance Field Computation

The Jump Flood Algorithm (JFA) provides GPU-accelerated Signed Distance Field computation for shore detection, breaking waves, and foam generation.

Overview

Traditional distance field computation requires O(n²) operations. The Jump Flood Algorithm reduces this to O(n log n) by using a series of passes with decreasing step sizes, making real-time SDF computation practical for water simulation.

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Algorithm Fundamentals

How Jump Flood Works

1. Seed Pass: Initialize pixels with shore/edge positions
2. Jump Passes: Each pass propagates distance information with halving step sizes
3. Final Pass: Complete distance field with sub-pixel accuracy

Complexity

Method	Complexity	Real-Time Viable
Brute Force	O(n²)	❌ No
Jump Flood	O(n log n)	✅ Yes
Approximation	O(n)	⚠️ Low Quality

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Implementation

UOceanologyJumpFloodComponent2D

Core component for 2D distance field computation:

UCLASS(Blueprintable, BlueprintType)
class UOceanologyJumpFloodComponent2D : public UActorComponent
{
    // Execute complete Jump Flood algorithm
    UFUNCTION(BlueprintCallable)
    void JumpFlood(
        UTextureRenderTarget2D* SeedRT,
        float SceneCaptureZ,
        FLinearColor Curl,
        bool UseDepth,
        float ZxLocationT
    );

    // Single jump step for custom pipelines
    UFUNCTION(BlueprintCallable)
    UTextureRenderTarget2D* SingleJumpStep();

    // Edge detection from seed texture
    UFUNCTION(BlueprintCallable)
    UTextureRenderTarget2D* FindEdges(
        UTextureRenderTarget2D* SeedRT,
        float CaptureZ,
        FLinearColor Curl,
        bool UseDepth,
        float ZxLocationT
    );
};

Material Pipeline

Material	Purpose
JumpStepMaterial	Single propagation pass
FindEdgesMaterial	Edge detection from seed
BlurEdgesMaterial	Optional smoothing pass

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Render Target Configuration

Ping-Pong Buffers

The algorithm uses two render targets (RTA, RTB) for ping-pong rendering:

void AssignRenderTargets(
    UTextureRenderTarget2D* InRTA,
    UTextureRenderTarget2D* InRTB
);

Pass Calculation

Required passes = ceil(log2(max(width, height)))

For a 1024×1024 texture: 10 passes

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Use Cases

Shore Detection

Generate distance-to-shore for:

• Breaking wave positioning
• Foam intensity gradients
• Shallow water effects
• Shoreline wetness

Wave Breaking Zones

SDF enables accurate surf zone identification:

• Detect approaching shore
• Trigger wave transformation
• Scale breaking intensity
• Position foam generation

Flow Field Generation

Combine SDF with gradient computation:

• Water flow toward shore
• Current direction fields
• Velocity magnitude mapping

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Water Textures Component

UOceanologyWaterTexturesComponent

Integrates Jump Flood with the water rendering pipeline:

UPROPERTY(VisibleAnywhere, Transient)
UTextureRenderTarget2D* RT_JumpFlood = nullptr;

UPROPERTY(VisibleAnywhere, Transient)
UTextureRenderTarget2D* RT_SDF_Gradient = nullptr;

UPROPERTY(EditAnywhere, BlueprintReadOnly)
TEnumAsByte<EOceanologyWaterTexturesMode> WaterTexturesMode;

Capture Modes

Mode	Description
RenderTarget	Real-time updates each frame
Baked	Pre-computed for static shorelines

Auto-Capture

UPROPERTY(EditAnywhere)
bool bAutoCaptureWithRenderTargets = true;

UPROPERTY(EditAnywhere)
float AutoCaptureDelay = 0.1f;

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Optional Blur Pass

Enable blur for smoother distance fields:

UPROPERTY(EditAnywhere)
bool UseBlur = false;

UPROPERTY(EditAnywhere)
int32 BlurPasses = 1;

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Performance

GPU Optimized

• All computation on GPU via material draws
• Minimal CPU overhead
• Supports dynamic resolution scaling

Optimization Tips

Technique	Benefit
Lower Resolution	Faster computation
Baked Mode	Zero runtime cost
Reduced Blur	Fewer passes
Async Capture	Non-blocking updates

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Debug Visualization

UFUNCTION(BlueprintCallable)
void FindEdges_Debug(
    UTextureRenderTarget2D* SeedRT,
    float CaptureZ,
    FLinearColor Curl,
    UTextureRenderTarget2D* DestRT,
    float ZOffset
);

Visualize:

• Seed points
• Distance gradients
• Edge detection results
• Completed SDF

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Integration with Breaking Waves

The SDF provides critical data for breaking wave simulation:

1. Shore Distance → Wave transformation trigger
2. Gradient Direction → Wave approach angle
3. Depth Estimation → Breaking intensity
4. Surf Zone Width → Foam spread area

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Resources

• Console Variables: r.Oceanology.JumpFlood.*
• Debug Mode: Enable visualization in editor
• Documentation: Technical deep-dive in docs