[UE4][Linear Algebra]Lerp calculation formula and usage example

keywords: [UE4][Linear Algebra]Lerp calculation formula and usage example

UnrealMathUnlity.h：

/** Performs a linear interpolation between two values, Alpha ranges from 0-1 */
template< class T, class U > 
static FORCEINLINE_DEBUGGABLE T Lerp( const T& A, const T& B, const U& Alpha )
{
    return (T)(A + Alpha * (B-A));
}

/** Performs a linear interpolation between two values, Alpha ranges from 0-1. Handles full numeric range of T */
template< class T > 
static FORCEINLINE_DEBUGGABLE T LerpStable( const T& A, const T& B, double Alpha )
{
    return (T)((A * (1.0 - Alpha)) + (B * Alpha));
}

/** Performs a linear interpolation between two values, Alpha ranges from 0-1. Handles full numeric range of T */
template< class T >
static FORCEINLINE_DEBUGGABLE T LerpStable(const T& A, const T& B, float Alpha)
{
    return (T)((A * (1.0f - Alpha)) + (B * Alpha));
}

/** Performs a 2D linear interpolation between four values values, FracX, FracY ranges from 0-1 */
template< class T, class U > 
static FORCEINLINE_DEBUGGABLE T BiLerp(const T& P00,const T& P10,const T& P01,const T& P11, const U& FracX, const U& FracY)
{
    return Lerp(
        Lerp(P00,P10,FracX),
        Lerp(P01,P11,FracX),
        FracY
        );
}

/** Interpolate vector from Current to Target. Scaled by distance to Target, so it has a strong start speed and ease out. */
static CORE_API FVector VInterpTo( const FVector& Current, const FVector& Target, float DeltaTime, float InterpSpeed );

/** Interpolate rotator from Current to Target. Scaled by distance to Target, so it has a strong start speed and ease out. */
static CORE_API FRotator RInterpTo( const FRotator& Current, const FRotator& Target, float DeltaTime, float InterpSpeed);

/** Interpolate float from Current to Target. Scaled by distance to Target, so it has a strong start speed and ease out. */
static CORE_API float FInterpTo( float Current, float Target, float DeltaTime, float InterpSpeed );

Usage example: Uniform velocity mode

Define two member variables:

float LerpTime = 0.f;
const float LerpDuration = 2.5f;

Then calculate in Tick：

void AMyActor::Tick(float DeltaSecond)
{
    if(LerpTime < LerpDuration)
    {
        LerpTime += DeltaSecond;
        
        FVector NewLoc = FMath::Lerp<FVector>(SrcLoc, DistLoc, LerpTime / LerpDuration);
    
        MyCharacter->SetActorLocation(NewLoc);
    }
}

Usage example: Damping mode (Cushion mode: strong at first, then soft)

Rotation changing example:

void AMyActor::Tick(float DeltaSecond)
{
    FVector MyLoc = GetActorLocation();
    FVector TargetLoc = PlayerCharacter->GetActorLocation();
    FVector Dir = (TargetLoc - MyLoc);
    Dir.Normalize();

    MyCharacter->SetActorRotation(FMath::Lerp(GetActorRotation(),Dir.Rotation(),0.05f));
}

Reference:
https://answers.unrealengine.com/questions/223469/smoothly-rotate-object.html

Lerp Example：

NewLoc = FMath::Lerp<FVector>(NewLoc, DistLoc, LerpTime / LerpDuration);

VInterpTo Example：

NewLoc = FMath::VInterpTo(NewLoc, DistLoc, DeltaTime, InterpSpeed);