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‫Hello and welcome.

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‫In this lecture, we're going to be getting some line tracing set up so that we can shoot at the walls

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‫and see these debug points appearing.

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‫And we're going to be doing this using a new function line trace by channel, and we're going to see

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‫why we're using that over line trace by object type, which is what we've used before.

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‫Let's dive in and see how it's done.

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‫So back in our building escape, we used line traces and we use this function here, the line trace

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‫by object type.

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‫And is this what we want to be using?

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‫Well, what is the difference between line trace by object type and its very close sibling, which is

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‫a line trace by channel.

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‫Well, object type says, I want to search through this line and see if I find an object that matches

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‫this type.

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‫So in I believe what we used in building escape was something like the physics body object type and

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‫that's great, it'll hit physics bodies.

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‫But if you decide suddenly that you want something else to be hit a ball, it doesn't leave much room

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‫for options.

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‫So channel is better for that because it allows us to define our own custom channels and then define

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‫what sorts of objects are transparent to that channel and which ones block it.

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‫So that's great because what we want to be able to do is say, okay, maybe a Bush type of thing is

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‫can't block a bullet, whereas a wall will block a bullet, but it won't be taking any damage but that

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‫we can deal with later on.

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‫So channels are the route we're going to go down and we can find the correct method here by doing control.

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‫P And by the way, there's this little extra cool bit that I only recently just learnt is if you use

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‫the ACT symbol, you can search for symbols within this file and or even other files, but if you have

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‫the file open it's much quicker.

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‫Then you could do something like line trace and you can see there's giving us the list of all the symbols

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‫which are functions or classes or variables, and we can find that line, trace, test line, trace,

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‫single by channel by object type.

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‫We want this first one line trace, single BI channel, which is going to have an out parameter for

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‫what we hit.

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‫The starting point, the ending point, the collision trace channel will come to that in just a little

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‫bit and then some query parameters which have got some defaults so we actually don't have to worry about.

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‫So just those first few things, the Outhit Start, end and Trace Channel are what we want to worry

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‫about.

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‫Let's set up a Trace channel.

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‫We're going to go to settings and project settings for this.

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‫And then we're looking under engine for the collisions area.

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‫And under here you can see we've got object channels and trace channels.

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‫Object channels is actually just another way of saying different types of objects.

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‫So if you wanted to have as well as the default sets of objects like pawns and stuff like that, if

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‫you wanted a new category of object, you could go down the object channel route.

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‫But the traces allow you to define the opposite end of the spectrum, which is like, I've got a bullet,

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‫what objects can it hit?

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‫So we're going to go down this route, add a new trace channel, and we're going to call this bullet.

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‫And the default response is going to be.

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‫BLOCK Which basically means that all object types are going to.

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‫BLOCK Unless we now specify otherwise, we can go ahead and accept this and we can look at our presets

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‫down here.

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‫If we just expand that out, the presets are going to show you that we have a bunch of different things

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‫that we can apply to, things we can say, we want this to be a physics actor, in which case it's object

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‫type, which is going to be what is a predefined object channel will be physics body.

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‫But if we expand this out, you can see what its reaction is to the different types of traces here.

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‫So we've got our trace types camera, our own bullet visibility, it blocks all of these and then it's

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‫got object types.

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‫So that's how it interacts physics wise with other objects such as pawns and vehicles, etc. That's

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‫not how it interacts with line trace by object.

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‫That's slightly confusing and it's confused me before.

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‫So the trace type is what's interesting and we can just go and have a look through here and see no collision

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‫should be set up to not collide.

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‫So let's make our bullets not collide with that as well.

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‫So just overriding some of the presets such that our bullet works as we would expect, overlap all,

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‫for example, should basically overlap with the bullet.

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‫And probably invisible wall and invisible wall dynamic should also be ignored for the purposes of our

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‫bullet.

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‫Same with dynamic here.

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‫You can see that we're basically ending up with something that's quite similar to a visibility trace.

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‫But the reason for keeping it separate is that we might want to make slight distinctions, such as you

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‫can see, you can't see through this bush, but you can shoot through it.

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‫Those kinds of things mean that having our own trace channel is really, really handy.

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‫Now we'd like to be able to use this trace channel in code, and it's a bit funky how we have to do

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‫this because there's an enum here, an E collision channel, and if we go F12 to its definition, you

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‫can see there's a bunch for the predefined ones, world static -- visibility, etc. down here, but

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‫then there's a bunch more engine trace channel one, two and Game Trace Channel one, two, three,

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‫etc. predefined.

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‫Which one of ours is this?

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‫You know, we don't have a bullet in here.

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‫We actually have to use one of these game trace channels one.

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‫So it's really a little bit confusing.

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‫Which one is it?

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‫So the way you do this is by going to your project folder and go into config.

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‫And what we want is the default engine INI and we want to open that in a text editor.

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‫So you can do that in any text that you like.

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‫I like to drag it into Visual Studio code, so that Visual Studio code is the one opening it.

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‫And in here we want to essentially look for our bullet and see what it has been assigned to.

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‫And I think this first line here tells us that the channel it's been given is the game Trace Channel

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‫one.

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‫Fantastic.

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‫So that's what we are going to use when we're doing our line trace.

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‫So let's start to get the structure of this line trace in place.

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‫The first thing we're going to need is a start point.

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‫We already have that.

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‫That's the location of our viewport.

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‫We're also going to need an endpoint for our line trace that's slightly harder to calculate.

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‫So let's create an F vector for our end, and we're going to calculate this by using our stop point,

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‫plus the rotation multiplied by the amount, the distance that we want to go away.

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‫Not the rotation, but a vector that points in the direction of the rotation feel slightly confusing,

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‫but actually if you do rotation vector, this gives you a vector that points in the direction of this

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‫rotation.

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‫It's kind of doing the opposite of what we were doing in the animation blueprint to get a rotation from

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‫a vector.

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‫This goes the other way.

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‫So we're going to do location plus this rotation vector, and then we want essentially a max range.

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‫So I'm going to introduce a new property here, make it edit anywhere and it's going to be a float called

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‫Max Range like so.

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‫And we can set it to a default of something like 10,000 or a thousand max range.

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‫That's going to be about 100 meters, sorry, about ten meters.

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‫And we're going to go over to the rotation vector multiplied by the max range.

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‫So that's going to be the vector that we get from that rotation is going to be a size one.

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‫Multiplying it by the max range will make it a vector of size, whatever the max range is.

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‫And then we add that on top of the location to get us the end point.

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‫Don't worry if you don't follow that math's 100%.

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‫The important thing is that this is how we want to get to our endpoint.

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‫Then I'm just going to put a to do line in here to add in some line tracing and we're going to remove

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‫this draw debug camera because I want to do a different draw debug helper here.

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‫I want to draw a debug point which you can find here.

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‫It takes in the world, takes in a position and a size and a color.

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‫And whether it's persistent or not, we probably do want it to be persistent, to be visible.

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‫So we should be able to do something along these lines.

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‫Let's see, draw debug point and we're going to do a get world right off the bat here and then we'll

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‫start off with the location of our viewport instead, just as a test to see how this draw debug point

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‫works, give it size to give it an f color of red and make its persistence true.

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‫And then we are going to capital for the semicolon and see whether this is working as hit compile over

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‫in the engine and let's hit play.

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‫So now if I shoot and move slightly back, you can see there's a point there.

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‫However, it's really, really tiny and hard to see.

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‫So they're going to increase this to 20 instead.

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‫Let's see if that's better.

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‫Compile and play.

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‫Yep.

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‫Sure enough, now I can see a very, very clear point.

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‫So it's time for your challenge to do a line trace by channel.

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‫We've done it with object types before, so hopefully we can extrapolate that to doing it by channel.

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‫So use the location and end that we have defined already as the start and end.

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‫Check the success of the result making an if statement.

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‫So only getting the out parameter or getting the parameter of the hit result and only drawing a debug

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‫point.

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‫If we successfully hit the point and remember that we're using that channel that we found in the dot

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‫INI file as well.

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‫So pause the video and see if you can sort out all of those parameters and arguments into the line trace

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‫and get that debug point drawing where we hit with our line trace.

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‫Now at this stage we aren't doing anything smart, like ignoring the player actor.

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‫So your line trace may end up hitting the player and that's okay for this challenge.

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‫Pause video.

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‫Have a go.

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‫Okay.

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‫Welcome back.

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‫So let's dive right in.

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‫We're going to get the world, first of all, and do an arrow to that.

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‫We're going to go to do a line trace single by channel.

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‫Then we're going to need a hit result.

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‫So our parameter, we need to first declare our hit result struct to pass in as an out parameter.

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‫It's going to call it hit and are going to pass it in as the first argument here.

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‫Then we need the location.

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‫And we need the end.

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‫And then what?

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‫What comes up next?

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‫The E collision e collision channel.

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‫So that's going to be an E collision channel.

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‫Colon.

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‫And then it should be e c, what did we say?

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‫Let's go and have a look in the default engine.

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‫Any e cc game trace channel one is the one that the default any has configured it as.

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‫Are there any more arguments here that we need?

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‫Let's go along.

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‫We've got the collision parameters which are defaulted out.

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‫So no, we do not need anything in here.

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‫And then what we can do is we need a boolean result for this.

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‫So B success.

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‫And it sounds very helpful.

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‫Doesn't it be success?

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‫Let's say an if be success.

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‫We're going to want to draw that debug point.

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‫So we're going to move this down.

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‫I can do this with Alt and then the arrow moves a line around like this and then I can indent it with

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‫a control and the right square bracket on your keyboard and that gets it to look like so.

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‫And now instead of the location, which is the start point, we actually want the hit location.

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‫So that's going to be hit dot location like so.

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‫And it looks like I'm missing a semicolon on the end of this line here.

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‫Let's build that and give it a go.

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‫Once that's compiled, let's hit play, go into the world and try and shoot at our walls.

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‫There you go.

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‫You can see we've got a point showing up on the wall just where I shot.

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‫I can do this multiple times.

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‫It seems to hit if I go to far from a wall, it doesn't hit at all because of our max range being quite

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‫short.

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‫And you can see once I get within the max range, it does hit and it does draw a point.

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‫So that is a challenge successfully completed.

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‫We're now able to show and line trace to a particular point.

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‫The next step is going to be getting that to actually show some particle effects, which will be much

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‫cooler than just drawing a debug point for that.

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‫I'll see you in the next lecture.