Unreal Engine C++ Fundamentals – LineTraceSingleByChannel and friends ( DrawDebugBox, DrawDebugLine, FHitResult and VRandCone )

By | Development, Tutorial, Unreal | 4 Comments

Hey guys,

Back to fundamentals with LineTraceSingleByChannel and a bunch of other little methods that go, oh so well, with line traces.

First thing’s first, what is LineTraceSingleByChannel ?

Trace a ray against the world using a specific channel and return the first blocking hit … straight out of the documentation.

If that is not clear enough perhaps this picture can illustrate roughly what a line trace does. Let’s break it down:

  • So the line trace in our case is the stick
  • The start point of the line trace in the world is the bottom right hand corner where someone is holding it
  • The end or the destination of the line trace is Ned Flanders’s eye.
  • Cool.

Now what can we do with line traces ?

Turns out a bunch of stuff.

We can use them to determine where objects are in space relative to us.

For example, in the picture above, the game creator is sending a line trace from the GetRightVector and GetForwardVector location and based on the collision of those hits.

He can then determine the next move for the tile based character component.

He is also determining if the line trace is interacting with any static meshes and if that occurs he shoots out another line trace now parallel to the characters forward vector whatever the remainder of the length of the trace was.

So, all those games you enjoy playing, that are turn / tile based, can be very easily created using this technique !


Let’s take a look at the other example.

Here we can shoot a line trace against a component in order to determine if it can be destroyed.

We are going to do something similar to this example but don’t let my lack of imagination stop you from trying other wacky possibilities.

Ok let’s get to drawing some line traces and stop screwing around


A basic line trace needs a few components, let’s take a look at the code below.

float LineTraceDistance = 100.f;

FVector Start;
FVector End;

// get the camera view
FVector CameraLoc = FollowCamera->GetComponentLocation();
FRotator CameraRot = FollowCamera->GetComponentRotation();

Start = CameraLoc;

End = CameraLoc + (CameraRot.Vector() * LineTraceDistance);
// additional trace parameters
FCollisionQueryParams TraceParams(FName(TEXT("InteractTrace")), true, NULL);
TraceParams.bTraceComplex = true;
TraceParams.bReturnPhysicalMaterial = true;

//Re-initialize hit info
FHitResult HitDetails = FHitResult(ForceInit);

bool bIsHit = GetWorld()->LineTraceSingleByChannel(
  HitDetails,			// FHitResult object that will be populated with hit info
  Start,			// starting position
  End,				// end position
  ECC_GameTraceChannel3,	// collision channel - 3rd custom one
  TraceParams			// additional trace settings

  // something was hit
  // we missed

Breaking it down we can see that the major parts are the start and end locations of where we want the line trace to travel to and from. We obtain those details from the players camera so, wherever our mouse is looking we will shoot out a line up to 100 units.

We can also see that we are going to use the third custom collision channel we created so whatever those settings connect with will be hit.

In addition we are passing in a few line trace parameters, specifically the following:

bTraceComplex – which tells us to use complex collision on whatever we interact with to provide better precision.

bReturnPhysicalMaterial – which tells us to provide details about the physical material, if one exists on the thing we hit, to come back in our hit result.

Lastly we can see that the result of the hit interaction goes into the FHitResult struct from which we can examine various bits of information about what we hit.

You holdin’ FHitResult ?

There are various bits of data we can get back from each hit result so let’s take a look at a few basic ones

if (bIsHit)
    Log(ELogLevel::WARNING, "We hit something");

    Log(ELogLevel::WARNING, HitDetails.Actor->GetName());
    Log(ELogLevel::DEBUG, FString::SanitizeFloat(HitDetails.Distance));

In this example we are simply printing out the name of the actor we hit as well as the distance between our point of origin and where the hit connected.

Based on those two details we already can do a bunch of stuff like cast the actor to the correct type and perform operations on him, like causing damage or displaying a menu or playing a sound.

Also based on the distance we can make certain assumptions, if the distance is greater than X perhaps our hit doesn’t generate damage or generates less damage due to distance. Or if the distance is too far we cannot interact with a certain lever in the game.

Lot’s of possibilities.

Great but I can’t see where I am aiming ?

No problem, we can drop in a few debug helpers to get us on our way. Specifically DrawDebugBox and DrawDebugLine which let us visualize in the world which way the line goes as well as where the hit occurs.

Here is the code snippet:

if (bIsHit)
  Log(ELogLevel::WARNING, "We hit something");
  // start to end, green, will lines always stay on, depth priority, thickness of line
  DrawDebugLine(GetWorld(), Start, End, FColor::Green, false, 5.f, ECC_WorldStatic, 1.f);

  Log(ELogLevel::WARNING, HitDetails.Actor->GetName());
  Log(ELogLevel::DEBUG, FString::SanitizeFloat(HitDetails.Distance));
  DrawDebugBox(GetWorld(), HitDetails.ImpactPoint, FVector(2.f, 2.f, 2.f), FColor::Blue, false, 5.f, ECC_WorldStatic, 1.f);
  Log(ELogLevel::WARNING, "Nothing was hit");
  // start to end, purple, will lines always stay on, depth priority, thickness of line
  DrawDebugLine(GetWorld(), Start, End, FColor::Purple, false, 5.f, ECC_WorldStatic, 1.f);

You can see that when we hit something we draw a green debug line and a debug box at the point of impact.

While when we miss we just draw a purple line all the way to the extend of our line trace.

Not bad, but make something interesting !

Ok fine, calm down ! We will make a shotgun spread using Math..s !

For this we are going to dive into the FMath library specifically VRandCone

// convert the degrees to radians
const float Spread = FMath::DegreesToRadians(LineTraceSpread * 0.5f);

FVector Start;
FVector End;

// convert the degrees to radians
const float Spread = FMath::DegreesToRadians(LineTraceSpread * 0.5f);

// get the camera view
FVector CameraLoc = FollowCamera->GetComponentLocation();
FRotator CameraRot = FollowCamera->GetComponentRotation();

Start = CameraLoc;

// handle spread
End = CameraLoc + FMath::VRandCone(CameraRot.Vector(), Spread, Spread) * LineTraceDistance;

Now with a bit of imagination we can take this bit of logic and turn it into a dangerous weapon, for example:

We build a weapon object of type shotgun

That object contains one to many shells in it’s magazine

When each shell is fired we generate 6 pellets

We then take our logic and loop over it 6 times using the direction of the weapon as our starting line traces and the weapons range as the end.

Boom !

Not something we will look at today but in the near future, I promise !

For more details check out the links below:

Unreal + Blender Tutorial – How to add root bones / motion or keep animations in place

By | Development, Tutorial, Unreal | No Comments

Hey guys,

Today we are going to roll into the New Year by adding in root motion to our kick animations as well as convert them to stay in place rather than move the mesh on their own.

This will help clean up the crappy roundhouse kick animation we had in place at the end of our last tutorial.

You can find the GitHub project in the usual location if you want to play with the finished animation sets.

Additionally here are a few links to do some further reading on the items we discussed:

Thanks and see you next time.

Unreal C++ Tutorial – Player Character Series – Punch – Part 6 – Review Lesson using Kicks !

By | Development, Tutorial, Unreal | One Comment

Hey guys,

Today we are finally back with the Player Character series , picking up where we left off, as well as incorporating all the previous tutorials into this lesson.

This time we are going to do a review lesson, where we go back over all the topics we covered earlier, and use them to implement a new set of attacks in the form of kicks.


The GitHub starter project can be found here as well as the final version with all the things.

The major component that is net new to this series is the inclusion of an additional Blend Node which you can read about here.

Take a look and we will see you next time.

Unreal Engine C++ Fundamentals – UAnimInstance

By | Development, Tutorial, Unreal | No Comments

Hey guys,

Yet again we are going to dive into Unreal Engine C++ Fundamentals by exploring UAnimInstance aka Animation Instance.

The project for this tutorial is available on GitHub as always.

What is a Anim Instance ?

An Anim Instance is the animation instance that drives our class.

Great ! So what is an Anim Instance ?

It’s really just a container and / or facade object that interacts with our class and any animations / montages that this class is currently executing.

What it allows us to do, is have fine grain control over the state and playback of our animations.


That’s great for you but what about my needs ?

Well since the UAnimInstanceobject lets you take control of your animations it will be beneficial to a lot of game play mechanics.

Let’s just quickly glance over some of the parts of the Anim Instance to see how they could help by examining the PlayAnimMontage call.

float ACharacter::PlayAnimMontage(class UAnimMontage* AnimMontage, float InPlayRate, FName StartSectionName)
  UAnimInstance * AnimInstance = (Mesh)? Mesh->GetAnimInstance() : nullptr; 
  if( AnimMontage && AnimInstance )
    float const Duration = AnimInstance->Montage_Play(AnimMontage, InPlayRate);

    if (Duration > 0.f)
      // Start at a given Section.
      if( StartSectionName != NAME_None )
        AnimInstance->Montage_JumpToSection(StartSectionName, AnimMontage);

      return Duration;

  return 0.f;

As you guys can see, the guys at Epic have taken a bunch of pieces of the Anim Instance class and wrapped them in a helper method.

So what is this method doing ?

  • Well it brings in the AnimInstance from our Mesh object by calling GetAnimInstance().
  • Then it validates that the montage we want to interact with is available and the Anim Instance is not null.
  • If that passes it then kicks off the Montage_Play method right on the Anim Instance by passing in the montage and the Play Rate.

So far so good.

  • We then look at a few more conditionals that just ensure the data that is required is available to us.
  • And then lastly it calls Montage_JumpToSection as the overall method takes in a StartSectionName value that determines the playback position.


Even Epic likes Anim Instances ? How do I subscribe to their newsletter ?

Well you can do that through their web site or simply by getting a reference to your local neighborhood Anim Instance and start performing some of these operations.

What are these common operations ?

There are a bunch of them but let’s go over the immediately meaningful ones.

Montage_Stop– stops the montage and blends in the next animation using the blend speed parameter. Good for killing montages mid playback.

// takes in a blend speed ( 1.0f ) as well as the montage
AnimInstance->Montage_Stop(1.0f, AttackMontage->Montage);


Montage_Pause– pauses the current animation and freezes parts of the object tied to those animations. In our case we are able to pause a punch mid stream. Great for effects where you need to halt an animation, do some other activities and then resume.

// takes in the montage you want to pause


Montage_Resume– called after you call Montage_Pause as it will resume the playback of the animation from that point in time.

// takes in the montage you want to resume playback for


Montage_Play – triggers the playback of an animation based on various criteria like montage, speed, return type, playback start position and a control variable to stop other montages if they are currently executing.

// kicks off the playback at a steady rate of 1 and starts playback at 0 frames
AnimInstance->Montage_Play(AttackMontage->Montage, 1.0f, EMontagePlayReturnType::Duration, 0.0f, true);


Montage_GetPosition– returns the current position of the playback frames as they are contained within the duration of the animation.

// returns current playback position
float CurrentPosition = AnimInstance->Montage_GetPosition(AttackMontage->Montage)


Montage_IsPlaying– returns a boolean to let the caller know if we have an active animation.

// checks to see if any animations in a montage are active


Montage_JumpToSectionUAnimInstance– moves the animation starting point to the location of the section as it’s setup in the Animation Montage.

// starts the animation at a specific section
AnimInstance->Montage_JumpToSection(StartSectionName, AnimMontage);


Pretty neat eh !

As you can see, we get a lot freedom with access to anim instances as well as the ability to control our animations easily.

Not a huge collection of additional resources but here is the doc to the UAnimInstanceclass:

Unreal Engine C++ Fundamentals – UAnimNotify & UAnimNotifyState

By | Development, Tutorial, Unreal | One Comment

Hey guys,

Today we are going to continue exploring Unreal Engine C++ Fundamentals before we get back to our Player Character series by taking a look at Anim Notify and Anim Notify States.

You can find the GitHub project for this video in the usual location.

What is a Anim Notify ?

An Anim Notification or a UAnimNotifyis a way of assigning events to parts of our animation that will be triggered at specific times of the animation playback.
UAnimNotifiesreally only have one notification event. It either fires or it doesn’t.


Why do I care ?

Well most games have various animations and each animation may trigger different effects depending on it’s life cycle.
For example if we want to play back a noise that a weapon makes as our character slices it through the air we may want to use an UAnimNotifyto trigger this event and then we can tie that event to the playback of our wooooshing sound.
We can also use notification events to trigger special effects, events in the game, UI updates, whatever you can imagine you can tie into this execution.

Ok smart guy what about Anim Notify States ?

Anim Notify States or UAnimNotifyStateare almost identical to our Anim Notifies except for one difference and that is that Anim Notify States come with three events:

  • NotifyBegin
    • This even is fired once when the notification is first triggered.
  • NotifyTick
    • This event fires continuously every tick, during the execution of an UAnimNotifyState.
  • NotifyEnd
    • This even fires once at the end of the UAnimNotifyStateexecution.


Alright so we have these magical events, how do we code them ?

First let’s take a look at a UAnimNotify


class UE4FUNDAMENTALS03_API UPunchThrowAnimNotify : public UAnimNotify

  virtual void Notify(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation) override;



void UPunchThrowAnimNotify::Notify(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation)
    // print method name to screen
    GEngine->AddOnScreenDebugMessage(-1, 4.5f, FColor::Purple, __FUNCTION__);

So, as you can see, by inheriting from UAnimNotifywe are able to overwrite the “Notify” method and then print some stuff to the screen.
Pretty easy.

Now let’s see what a UAnimNotifyStatelooks like


class UE4FUNDAMENTALS03_API UPunchThrowAnimNotifyState : public UAnimNotifyState

  virtual void NotifyBegin(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation, float TotalDuration) override;
  virtual void NotifyTick(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation, float FrameDeltaTime) override;
  virtual void NotifyEnd(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation) override;


void UPunchThrowAnimNotifyState::NotifyBegin(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation, float TotalDuration)
    GEngine->AddOnScreenDebugMessage(-1, 4.5f, FColor::Yellow, __FUNCTION__);

void UPunchThrowAnimNotifyState::NotifyTick(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation, float FrameDeltaTime)
    GEngine->AddOnScreenDebugMessage(-1, 4.5f, FColor::Yellow, __FUNCTION__);

void UPunchThrowAnimNotifyState::NotifyEnd(USkeletalMeshComponent* MeshComp, UAnimSequenceBase* Animation)
    GEngine->AddOnScreenDebugMessage(-1, 4.5f, FColor::Yellow, __FUNCTION__);

Cool ! Now show me how to tie those events into my game.

Well the majority of that is covered in the video but if you are curious this is what the events look like when you add them to the animations.

The way we added these notifications was by going to the bottom part of the animation montage labeled “Notifications”.
Then by Right Clicking we were able to pick our notifications.

This was all done from the Melee animation montage.



UAnimNotifyand UAnimNotifyStatescan be added to single animations as well as animation montages. Which gives you a lot of freedom as to where and how you want these events to be triggered.


So there we go guys, with just a few lines of code and a bit of clicking we have notifications coming out of our animations and into our game.
If you would like to learn a little bit more about these various topics, check out the following links: