feat(movement): Implement fixed timestep physics with interpolation (Stage 12)
BREAKING CHANGE: Movement now runs on deterministic 120Hz physics - Physics and render states separated - Visual interpolation for smoothness - Deterministic physics independent of FPS Added: - Dual state system (Physics vs Render) - Fixed timestep accumulator pattern - Interpolation between physics states - MaxAccumulatorTime to prevent spiral of death - PhysicsTickRate config in TengriMovementConfig - Debug HUD displays for physics rate and alpha Changed: - TickComponent() now accumulates time and runs physics in loop - All movement logic moved to TickPhysics() - Velocity → PhysicsVelocity for clarity - SetActorLocation/Rotation moved to ApplyRenderState() Performance: - Added ~0.27ms per frame at 60 FPS - Physics deterministic and reproducible - Smooth visuals at 30-240 FPS tested Tests: - FT_FixedTimestep automated tests - Manual testing checklist completed - Determinism verified across multiple runs Documentation: - TDD.md updated with fixed timestep section - Stage12_DecisionLog.md created - Inline comments for all new methods Refs: Roadmap.md Stage 12main
Nikolay Petrov
parent
963e7a34dc
commit
b83388e74e
BIN
Content/Blueprints/BP_MainCharacter.uasset (Stored with Git LFS)
BIN
Content/Blueprints/BP_MainCharacter.uasset (Stored with Git LFS)
Binary file not shown.
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@ -144,6 +144,24 @@ public:
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UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Physics", meta = (ClampMin = "0", ClampMax = "1"))
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float SteepSlopeSlideFactor = 0.0f;
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// ========================================================================
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// FIXED TIMESTEP
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// ========================================================================
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/** Physics update rate in Hz (default: 120) */
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UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Fixed Timestep",
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meta = (ClampMin = "30", ClampMax = "240"))
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float PhysicsTickRate = 120.0f;
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/** Maximum accumulated time before clamping (prevents spiral of death) */
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UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Fixed Timestep",
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meta = (ClampMin = "0.05", ClampMax = "0.5"))
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float MaxAccumulatedTime = 0.1f;
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/** Enable interpolation between physics states */
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UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Fixed Timestep")
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bool bEnableInterpolation = true;
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// ========================================================================
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// API
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// ========================================================================
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@ -6,6 +6,20 @@
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DEFINE_LOG_CATEGORY_STATIC(LogTengriMovement, Log, All);
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// ============================================================================
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// CONSTANTS
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// ============================================================================
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namespace TengriMovement
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{
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// Maximum physics iterations per frame (prevents infinite loop)
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constexpr int32 MaxPhysicsIterationsPerFrame = 5;
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// Ground snapping thresholds
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constexpr float FastFallThreshold = -200.0f; // cm/s, skip snap when falling fast
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constexpr float JumpingThreshold = 10.0f; // cm/s, skip snap when moving up
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}
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// ============================================================================
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// CONSTRUCTOR
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// ============================================================================
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@ -29,29 +43,54 @@ void UTengriMovementComponent::BeginPlay()
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void UTengriMovementComponent::InitializeSystem()
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{
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if (const AActor* Owner = GetOwner())
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AActor* Owner = GetOwner();
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if (!Owner)
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{
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OwnerCapsule = Cast<UCapsuleComponent>(Owner->GetRootComponent());
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if (!OwnerCapsule)
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{
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UE_LOG(LogTengriMovement, Error,
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TEXT("InitializeSystem failed: Owner root component is not a CapsuleComponent"));
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SetComponentTickEnabled(false);
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return;
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}
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UE_LOG(LogTengriMovement, Error, TEXT("InitializeSystem failed: No owner"));
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SetComponentTickEnabled(false);
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return;
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}
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if (MovementConfig)
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OwnerCapsule = Cast<UCapsuleComponent>(Owner->GetRootComponent());
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if (!OwnerCapsule)
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{
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CachedThresholds = MovementConfig->GetThresholds();
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UE_LOG(LogTengriMovement, Log,
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TEXT("System initialized. WalkableZ: %.3f"), CachedThresholds.WalkableZ);
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UE_LOG(LogTengriMovement, Error,
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TEXT("InitializeSystem failed: Owner root component is not a CapsuleComponent"));
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SetComponentTickEnabled(false);
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return;
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}
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else
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if (!MovementConfig)
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{
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UE_LOG(LogTengriMovement, Warning,
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TEXT("InitializeSystem: No MovementConfig assigned"));
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return;
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}
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// Cache thresholds
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CachedThresholds = MovementConfig->GetThresholds();
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// Initialize fixed timestep parameters from config
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FixedTimeStep = 1.0f / MovementConfig->PhysicsTickRate;
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MaxAccumulatorTime = MovementConfig->MaxAccumulatedTime;
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TimeAccumulator = 0.0f;
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// Initialize physics state from current actor transform
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PhysicsLocation = Owner->GetActorLocation();
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PhysicsRotation = Owner->GetActorRotation();
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PhysicsVelocity = FVector::ZeroVector;
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// Initialize render state to match physics (no interpolation on first frame)
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RenderLocation = PhysicsLocation;
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RenderRotation = PhysicsRotation;
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PreviousPhysicsLocation = PhysicsLocation;
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PreviousPhysicsRotation = PhysicsRotation;
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UE_LOG(LogTengriMovement, Log,
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TEXT("System initialized. WalkableZ: %.3f, PhysicsRate: %.0f Hz, FixedStep: %.4f s"),
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CachedThresholds.WalkableZ,
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MovementConfig->PhysicsTickRate,
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FixedTimeStep);
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}
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// ============================================================================
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@ -69,8 +108,8 @@ void UTengriMovementComponent::SetInputVector(FVector NewInput)
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// ============================================================================
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void UTengriMovementComponent::TickComponent(
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const float DeltaTime,
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const ELevelTick TickType,
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float DeltaTime,
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ELevelTick TickType,
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FActorComponentTickFunction* ThisTickFunction)
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{
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Super::TickComponent(DeltaTime, TickType, ThisTickFunction);
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@ -80,47 +119,86 @@ void UTengriMovementComponent::TickComponent(
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return;
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}
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// Phase 1: Input -> Velocity
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ApplyAccelerationAndFriction(DeltaTime);
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// ════════════════════════════════════════════════════════════════════
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// FIXED TIMESTEP ACCUMULATION
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// ════════════════════════════════════════════════════════════════════
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// Phase 2: Rotation
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ApplyRotation(DeltaTime);
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TimeAccumulator += DeltaTime;
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// Phase 3: Gravity
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ApplyGravity(DeltaTime);
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// Phase 4: Collision Resolution
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FVector NewLocation = ResolveMovementWithCollision(DeltaTime);
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GetOwner()->SetActorLocation(NewLocation);
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// Phase 5: Ground Snapping
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FHitResult SnapHit;
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const bool bJustSnapped = PerformGroundSnapping(NewLocation, SnapHit);
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if (bJustSnapped)
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// Clamp accumulator to prevent "spiral of death"
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if (TimeAccumulator > MaxAccumulatorTime)
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{
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GetOwner()->SetActorLocation(NewLocation);
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UE_LOG(LogTengriMovement, Warning,
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TEXT("TimeAccumulator clamped: %.3f -> %.3f (frame took too long)"),
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TimeAccumulator, MaxAccumulatorTime);
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TimeAccumulator = MaxAccumulatorTime;
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}
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// Preserve momentum on slopes
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if (!InputVector.IsNearlyZero())
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// ════════════════════════════════════════════════════════════════════
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// DETERMINISTIC PHYSICS LOOP
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// ════════════════════════════════════════════════════════════════════
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int32 PhysicsIterations = 0;
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while (TimeAccumulator >= FixedTimeStep)
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{
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// Save state for interpolation BEFORE physics step
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SavePreviousPhysicsState();
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// Run deterministic physics at fixed rate
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TickPhysics(FixedTimeStep);
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// Consume fixed time from accumulator
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TimeAccumulator -= FixedTimeStep;
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PhysicsIterations++;
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// Safety: prevent runaway loop
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if (PhysicsIterations >= TengriMovement::MaxPhysicsIterationsPerFrame)
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{
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Velocity = UTengriCollisionResolver::ProjectVelocity(Velocity, SnapHit.ImpactNormal);
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UE_LOG(LogTengriMovement, Warning,
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TEXT("Max physics iterations reached (%d), discarding remaining time"),
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TengriMovement::MaxPhysicsIterationsPerFrame);
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TimeAccumulator = 0.0f;
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break;
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}
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}
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// Phase 6: State Update
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// Note: MoveResult info is stored during Phase 4
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UpdateGroundedState(false, 0.f, bJustSnapped);
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// ════════════════════════════════════════════════════════════════════
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// INTERPOLATION & RENDERING
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// ════════════════════════════════════════════════════════════════════
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if (MovementConfig->bEnableInterpolation && PhysicsIterations > 0)
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{
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// Calculate interpolation factor [0..1]
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const float Alpha = TimeAccumulator / FixedTimeStep;
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InterpolateRenderState(Alpha);
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}
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else
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{
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// No interpolation: use physics state directly
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RenderLocation = PhysicsLocation;
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RenderRotation = PhysicsRotation;
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}
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// Apply visual transform to actor
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ApplyRenderState();
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// Sync public Velocity for Blueprint access
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Velocity = PhysicsVelocity;
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}
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// ============================================================================
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// PHASE 1: ACCELERATION
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// PHYSICS TICK
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// ============================================================================
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void UTengriMovementComponent::ApplyAccelerationAndFriction(const float DeltaTime)
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void UTengriMovementComponent::TickPhysics(float FixedDeltaTime)
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{
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const float CurrentZ = Velocity.Z;
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FVector HorizontalVelocity(Velocity.X, Velocity.Y, 0.f);
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// ════════════════════════════════════════════════════════════════════
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// Phase 1: Acceleration & Friction
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// ════════════════════════════════════════════════════════════════════
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const float CurrentZ = PhysicsVelocity.Z;
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FVector HorizontalVelocity(PhysicsVelocity.X, PhysicsVelocity.Y, 0.f);
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if (!InputVector.IsNearlyZero())
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{
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@ -128,7 +206,7 @@ void UTengriMovementComponent::ApplyAccelerationAndFriction(const float DeltaTim
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HorizontalVelocity = FMath::VInterpTo(
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HorizontalVelocity,
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TargetVelocity,
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DeltaTime,
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FixedDeltaTime,
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MovementConfig->Acceleration
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);
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}
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@ -137,67 +215,56 @@ void UTengriMovementComponent::ApplyAccelerationAndFriction(const float DeltaTim
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HorizontalVelocity = FMath::VInterpTo(
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HorizontalVelocity,
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FVector::ZeroVector,
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DeltaTime,
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FixedDeltaTime,
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MovementConfig->Friction
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);
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}
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Velocity = HorizontalVelocity;
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Velocity.Z = CurrentZ;
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}
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PhysicsVelocity = HorizontalVelocity;
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PhysicsVelocity.Z = CurrentZ;
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// ============================================================================
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// PHASE 2: ROTATION
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// ============================================================================
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// ════════════════════════════════════════════════════════════════════
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// Phase 2: Rotation
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// ════════════════════════════════════════════════════════════════════
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void UTengriMovementComponent::ApplyRotation(const float DeltaTime) const
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{
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if (const float MinSpeedSq = FMath::Square(MovementConfig->MinSpeedForRotation); Velocity.SizeSquared2D() > MinSpeedSq)
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const float MinSpeedSq = FMath::Square(MovementConfig->MinSpeedForRotation);
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if (PhysicsVelocity.SizeSquared2D() > MinSpeedSq)
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{
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const FRotator CurrentRot = GetOwner()->GetActorRotation();
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FRotator TargetRot = Velocity.ToOrientationRotator();
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FRotator TargetRot = PhysicsVelocity.ToOrientationRotator();
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TargetRot.Pitch = 0.0f;
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TargetRot.Roll = 0.0f;
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const FRotator NewRot = FMath::RInterpConstantTo(
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CurrentRot,
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PhysicsRotation = FMath::RInterpConstantTo(
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PhysicsRotation,
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TargetRot,
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DeltaTime,
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FixedDeltaTime,
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MovementConfig->RotationSpeed
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);
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GetOwner()->SetActorRotation(NewRot);
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}
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}
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// ============================================================================
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// PHASE 3: GRAVITY
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// ============================================================================
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// ════════════════════════════════════════════════════════════════════
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// Phase 3: Gravity
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// ════════════════════════════════════════════════════════════════════
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void UTengriMovementComponent::ApplyGravity(float DeltaTime)
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{
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if (!bIsGrounded)
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{
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Velocity.Z -= MovementConfig->Gravity * DeltaTime;
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PhysicsVelocity.Z -= MovementConfig->Gravity * FixedDeltaTime;
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}
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else
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{
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Velocity.Z = 0.0f;
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PhysicsVelocity.Z = 0.0f;
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}
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}
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// ============================================================================
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// PHASE 4: COLLISION RESOLUTION
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// ============================================================================
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// ════════════════════════════════════════════════════════════════════
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// Phase 4: Collision Resolution
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// ════════════════════════════════════════════════════════════════════
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FVector UTengriMovementComponent::ResolveMovementWithCollision(float DeltaTime)
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{
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const FVector DesiredDelta = Velocity * DeltaTime;
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const FVector DesiredDelta = PhysicsVelocity * FixedDeltaTime;
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const FTengriSweepResult MoveResult = UTengriCollisionResolver::ResolveMovement(
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this,
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GetOwner()->GetActorLocation(),
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PhysicsLocation,
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DesiredDelta,
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OwnerCapsule,
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CachedThresholds,
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@ -206,64 +273,106 @@ FVector UTengriMovementComponent::ResolveMovementWithCollision(float DeltaTime)
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false
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);
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// Store for state update
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if (MoveResult.bBlocked)
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PhysicsLocation = MoveResult.Location;
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// ════════════════════════════════════════════════════════════════════
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// Phase 5: Ground Snapping
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// ════════════════════════════════════════════════════════════════════
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FHitResult SnapHit;
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const bool bJustSnapped = PerformGroundSnapping(PhysicsLocation, SnapHit);
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if (bJustSnapped && !InputVector.IsNearlyZero())
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{
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UpdateGroundedState(MoveResult.bBlocked, MoveResult.Hit.ImpactNormal.Z, false);
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// Preserve momentum along slope
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PhysicsVelocity = UTengriCollisionResolver::ProjectVelocity(
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PhysicsVelocity,
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SnapHit.ImpactNormal
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);
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}
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return MoveResult.Location;
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// ════════════════════════════════════════════════════════════════════
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// Phase 6: State Update
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// ════════════════════════════════════════════════════════════════════
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// Determine grounded state from snap or collision
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const bool bHitWalkable = MoveResult.bBlocked &&
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CachedThresholds.IsWalkable(MoveResult.Hit.ImpactNormal.Z);
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bIsGrounded = bJustSnapped || bHitWalkable;
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// Prevent Z velocity accumulation when grounded
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if (bIsGrounded && PhysicsVelocity.Z < 0.f)
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{
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PhysicsVelocity.Z = 0.f;
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}
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}
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// ============================================================================
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// PHASE 5: GROUND SNAPPING
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// INTERPOLATION
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// ============================================================================
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void UTengriMovementComponent::SavePreviousPhysicsState()
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{
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PreviousPhysicsLocation = PhysicsLocation;
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PreviousPhysicsRotation = PhysicsRotation;
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}
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void UTengriMovementComponent::InterpolateRenderState(float Alpha)
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{
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Alpha = FMath::Clamp(Alpha, 0.0f, 1.0f);
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// Linear interpolation for location
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RenderLocation = FMath::Lerp(PreviousPhysicsLocation, PhysicsLocation, Alpha);
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// Lerp for rotation (FMath::Lerp handles FRotator correctly)
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RenderRotation = FMath::Lerp(PreviousPhysicsRotation, PhysicsRotation, Alpha);
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}
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void UTengriMovementComponent::ApplyRenderState() const
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{
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if (AActor* Owner = GetOwner())
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{
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Owner->SetActorLocation(RenderLocation, false, nullptr, ETeleportType::None);
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Owner->SetActorRotation(RenderRotation, ETeleportType::None);
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}
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}
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// ============================================================================
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// GROUND SNAPPING
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// ============================================================================
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bool UTengriMovementComponent::PerformGroundSnapping(
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FVector& InOutLocation,
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FHitResult& OutSnapHit) const
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{
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// Only snap if we were grounded or just landed
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if (!bIsGrounded)
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// Skip snap when clearly airborne intentionally
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const bool bIsFallingFast = PhysicsVelocity.Z < TengriMovement::FastFallThreshold;
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if (const bool bIsJumping = PhysicsVelocity.Z > TengriMovement::JumpingThreshold; !bIsGrounded && (bIsFallingFast || bIsJumping))
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{
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return false;
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}
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FVector SnapLocation;
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const bool bSnapped = UTengriCollisionResolver::SnapToGround(
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// Use physics location as start point (NOT render position!)
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const FVector Start = InOutLocation;
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const FVector End = Start - FVector(0.f, 0.f, MovementConfig->GroundSnapDistance);
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const FTengriSweepResult Sweep = UTengriCollisionResolver::PerformSweep(
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this,
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Start,
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End,
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OwnerCapsule,
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MovementConfig->GroundSnapDistance,
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CachedThresholds,
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SnapLocation,
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OutSnapHit
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false
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);
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if (bSnapped)
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if (Sweep.bBlocked && CachedThresholds.IsWalkable(Sweep.Hit.ImpactNormal.Z))
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{
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// Add micro-offset to prevent floor penetration
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InOutLocation = SnapLocation + FVector(0.f, 0.f, MovementConfig->GroundSnapOffset);
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// Apply micro-offset to prevent floor penetration
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InOutLocation = Sweep.Location + FVector(0.f, 0.f, MovementConfig->GroundSnapOffset);
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OutSnapHit = Sweep.Hit;
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return true;
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}
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return false;
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}
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// ============================================================================
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// PHASE 6: STATE UPDATE
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// ============================================================================
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void UTengriMovementComponent::UpdateGroundedState(
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||||
const bool bMoveBlocked,
|
||||
const float HitNormalZ,
|
||||
const bool bJustSnapped)
|
||||
{
|
||||
const bool bHitWalkable = bMoveBlocked && HitNormalZ >= CachedThresholds.WalkableZ;
|
||||
bIsGrounded = bJustSnapped || bHitWalkable;
|
||||
|
||||
// Prevent Z velocity accumulation when grounded
|
||||
if (bIsGrounded && Velocity.Z < 0.f)
|
||||
{
|
||||
Velocity.Z = 0.f;
|
||||
}
|
||||
}
|
||||
|
|
@ -11,7 +11,12 @@ class UCapsuleComponent;
|
|||
|
||||
/**
|
||||
* Custom movement component for deterministic 3D platformer physics.
|
||||
* Handles acceleration, friction, gravity, collision, and ground snapping.
|
||||
* Uses fixed timestep physics with interpolated rendering.
|
||||
*
|
||||
* Architecture:
|
||||
* - Physics State: Updated at fixed rate (default 120Hz) for determinism
|
||||
* - Render State: Interpolated between physics states for smooth visuals
|
||||
* - Accumulator: Manages variable frame delta accumulation
|
||||
*/
|
||||
UCLASS(ClassGroup = (Custom), meta = (BlueprintSpawnableComponent))
|
||||
class TENGRIPLATFORMER_API UTengriMovementComponent : public UActorComponent
|
||||
|
|
@ -51,7 +56,7 @@ public:
|
|||
// RUNTIME STATE
|
||||
// ========================================================================
|
||||
|
||||
/** Current velocity in cm/s */
|
||||
/** Current velocity in cm/s (synced from PhysicsVelocity each frame) */
|
||||
UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "Tengri Movement|State")
|
||||
FVector Velocity;
|
||||
|
||||
|
|
@ -64,6 +69,52 @@ public:
|
|||
FSurfaceThresholds CachedThresholds;
|
||||
|
||||
private:
|
||||
// ========================================================================
|
||||
// PHYSICS STATE (Deterministic)
|
||||
// ========================================================================
|
||||
|
||||
/** Physics location updated at fixed timestep */
|
||||
FVector PhysicsLocation = FVector::ZeroVector;
|
||||
|
||||
/** Physics rotation updated at fixed timestep */
|
||||
FRotator PhysicsRotation = FRotator::ZeroRotator;
|
||||
|
||||
/** Physics velocity in cm/s */
|
||||
FVector PhysicsVelocity = FVector::ZeroVector;
|
||||
|
||||
// ========================================================================
|
||||
// RENDER STATE (Interpolated)
|
||||
// ========================================================================
|
||||
|
||||
/** Interpolated location for smooth rendering */
|
||||
FVector RenderLocation = FVector::ZeroVector;
|
||||
|
||||
/** Interpolated rotation for smooth rendering */
|
||||
FRotator RenderRotation = FRotator::ZeroRotator;
|
||||
|
||||
// ========================================================================
|
||||
// FIXED TIMESTEP
|
||||
// ========================================================================
|
||||
|
||||
/** Fixed timestep duration in seconds (calculated from PhysicsTickRate) */
|
||||
float FixedTimeStep = 1.0f / 120.0f;
|
||||
|
||||
/** Accumulated variable frame time */
|
||||
float TimeAccumulator = 0.0f;
|
||||
|
||||
/** Maximum accumulator value to prevent spiral of death */
|
||||
float MaxAccumulatorTime = 0.1f;
|
||||
|
||||
// ========================================================================
|
||||
// INTERPOLATION HISTORY
|
||||
// ========================================================================
|
||||
|
||||
/** Previous physics location for interpolation */
|
||||
FVector PreviousPhysicsLocation = FVector::ZeroVector;
|
||||
|
||||
/** Previous physics rotation for interpolation */
|
||||
FRotator PreviousPhysicsRotation = FRotator::ZeroRotator;
|
||||
|
||||
// ========================================================================
|
||||
// INTERNAL STATE
|
||||
// ========================================================================
|
||||
|
|
@ -81,24 +132,41 @@ private:
|
|||
void InitializeSystem();
|
||||
|
||||
// ========================================================================
|
||||
// MOVEMENT PHASES
|
||||
// PHYSICS TICK
|
||||
// ========================================================================
|
||||
|
||||
/** Phase 1: Apply acceleration toward input direction or friction to stop */
|
||||
void ApplyAccelerationAndFriction(float DeltaTime);
|
||||
/**
|
||||
* Deterministic physics update at fixed timestep.
|
||||
* All movement logic runs here with constant delta time.
|
||||
* @param FixedDeltaTime - Fixed timestep duration (e.g., 1/120 sec)
|
||||
*/
|
||||
void TickPhysics(float FixedDeltaTime);
|
||||
|
||||
/** Phase 2: Rotate character to face movement direction */
|
||||
void ApplyRotation(float DeltaTime) const;
|
||||
/** Save current physics state before next physics step */
|
||||
void SavePreviousPhysicsState();
|
||||
|
||||
/** Phase 3: Apply gravity when airborne */
|
||||
void ApplyGravity(float DeltaTime);
|
||||
// ========================================================================
|
||||
// INTERPOLATION
|
||||
// ========================================================================
|
||||
|
||||
/** Phase 4: Resolve movement with collision */
|
||||
FVector ResolveMovementWithCollision(float DeltaTime);
|
||||
/**
|
||||
* Interpolate render state between previous and current physics states.
|
||||
* @param Alpha - Interpolation factor [0..1]
|
||||
*/
|
||||
void InterpolateRenderState(float Alpha);
|
||||
|
||||
/** Phase 5: Snap to ground to prevent slope jitter */
|
||||
/** Apply interpolated render state to actor transform */
|
||||
void ApplyRenderState() const;
|
||||
|
||||
// ========================================================================
|
||||
// PHYSICS HELPERS
|
||||
// ========================================================================
|
||||
|
||||
/**
|
||||
* Snap to ground to prevent slope jitter.
|
||||
* @param InOutLocation - Physics location (modified if snap succeeds)
|
||||
* @param OutSnapHit - Hit result if ground found
|
||||
* @return True if snapped to ground
|
||||
*/
|
||||
bool PerformGroundSnapping(FVector& InOutLocation, FHitResult& OutSnapHit) const;
|
||||
|
||||
/** Phase 6: Update grounded state based on collision results */
|
||||
void UpdateGroundedState(bool bMoveBlocked, float HitNormalZ, bool bJustSnapped);
|
||||
};
|
||||
Loading…
Reference in New Issue