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‫Hello and welcome.

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‫In this lecture, we're going to be looking at how we can set our moving platform using a vector velocity

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‫in its member variables.

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‫And we're going to look at how we can make its movement frame rate independent using something called

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‫Delta time.

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‫Okay, let's dive in.

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‫So up until this point, we have been just updating a single component of our vector at a time.

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‫Now, what if I told you that we don't have to do it this way?

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‫What if we can just update the whole vector by a certain amount in one go?

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‫Well, we can vectors can be added to other vectors if we've seen in the previous section.

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‫So what we could do is go over to our moving platforms H and create something which is our moving platform

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‫velocity vector that tells us both the direction and the speed at which we want to go.

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‫So the way we do this is create a rather familiar by now U property and we're going to make it edit

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‫anywhere so it's visible in the inspector and then it's going to be a type F vector as we've done before,

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‫and we'll call it the platform velocity and we can give it something for now.

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‫So we're going to do an F vector constructor and then we'll just give it a say 100 in the X component

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‫and zero and zero in the others and then a semicolon to finish it off.

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‫And I want to show you just one more trick here is that along with edit, anywhere we can put another

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‫comma separated argument to this U property thing, which looks a little bit like a function.

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‫Actually, you can put one that is called category with a capital C and then you put the equal sign.

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‫And I usually leave a space between these equal signs category equals.

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‫And then you can give a category that you want to show up.

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‫Down here you can see there are categories for transform rendering replication.

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‫These are the kinds of categories we mean I'm going to call mine moving platform so that we have a kind

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‫of moving platform category to put these variables into.

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‫And note that I use a double quotes around that category.

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‫That's the correct syntax.

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‫You do need to include those.

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‫And then what we can do is we can make use of this new member variable over in the C++.

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‫So instead of setting the X component of current location, let me entirely remove that line and rewrite

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‫it as the current location is equal to the current location plus platform velocity like.

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‫So put semicolon on the end of the line and let's go over into Unreal and hit compile and see how this

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‫goes.

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‫So that's compiled successfully.

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‫And you can see I now have this moving platform section on my moving platform with a platform velocity

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‫and its x component is set to 100.

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‫What happens if I go and hit play?

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‫Well, the platform speeds off rapidly into the distance because I'm adding 100 units every single frame.

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‫That is a lot.

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‫But I do want it to move at a velocity of around 100 centimeters per second.

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‫That's just one meter per second.

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‫It's not very fast.

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‫So why is it going so fast?

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‫Well, because of my frame rate.

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‫Well, if I was on a slower system, it would be moving slower.

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‫And if I was on a faster system, it would be moving faster.

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‫That doesn't seem to make sense.

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‫So let's talk about using Delta time.

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‫What is Delta time?

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‫Well, using Delta time in Unreal can tell us how long each frame took to execute.

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‫Where's Delta time?

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‫Well, if you go to Visual Studio code, you can see Delta time is a variable mentioned here at the

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‫top of tick.

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‫This is actually a input parameter.

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‫It's the input to our function.

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‫So we can use it here essentially like we'd use a local variable, we can use it within the body of

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‫the tick function.

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‫So it tells us how long a frame took to execute.

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‫But why is that important?

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‫Because when we multiply something by delta time, it essentially makes our game frame rate independent,

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‫which basically means that the game behaves the same on a fast or a slow computer.

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‫So let's take a look at an illustrative example if I want to each second move something by one unit

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‫to the left.

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‫Here's how it would look on different machines.

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‫I've got a slow computer.

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‫It can only do ten frames per second and a fast computer which can do 100 frames per second.

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‫Now the duration of those frames is different.

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‫So on the slow computer, the duration of the frame is 0.1 of a second.

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‫If I can only fit ten of them into a second and on a fast computer it would be 0.01 of a second, so

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‫much less because it can execute things faster.

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‫It doesn't take as long to compute a frame.

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‫So if I multiply by the duration of the frame, I multiply my one unit by the duration of the frame.

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‫Then you're going to see that in 1/2.

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‫So where ten frames have been executed, I get one unit of movement.

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‫The same is true.

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‫If I've done 100 frames of execution, I still get one unit of movement.

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‫So I'd like to challenge you to multiply by Delta time.

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‫So what we're going to do is remember that we can multiply vectors by a floating point value to scale

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‫their magnitude.

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‫And the way we can do this in C++ is using the star operator.

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‫That is how we do multiplication.

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‫There's no multiplication sign operator.

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‫It would be confused with an X, so we use the star operator to denote multiplication, and I'd like

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‫you to compile and test out your results.

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‫So multiply by delta time, multiply the speed by delta time and see what happens, plus video and have

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‫a go.

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‫Okay.

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‫Welcome back.

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‫So it's the platform velocity that we wanting to multiply here.

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‫Now we could pull this out into a separate variable, but in actual fact, we can just put a multiplied

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‫by delta time at the end of that line, obviously before the semicolon, because it's before the end

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‫of the statement.

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‫And what's interesting here is that there's a bit of a precedence of operators going on here in that

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‫the multiplication between the platform velocity and Delta time is going to happen before the addition

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‫to the current location.

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‫Now this follows normal mathematical rules for a full list of those rules.

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‫I'm not going to go into the details of it right here, but you can have a look at the page on order

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‫of operations on Wikipedia.

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‫There's lots of different rules in different languages, whether it's body mass, bid mass, ped mass,

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‫bed mass.

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‫There's loads of different acronyms.

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‫So whichever one you grew up using, that's the one I would recommend you use.

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‫For me, it was body mass, but in other places it's different.

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‫So it's worth noting that we can have just write it like this, but if we want it to make sure that

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‫something happens in a different order to that, then you can use parentheses to indicate this.

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‫So I could, just to make things clearer, I'm going to put parentheses around this multiplication just

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‫to show you that that's what happens.

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‫First the multiplication and then the addition to the current location.

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‫So let's go over into Unreal and test this out.

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‫We're going to hit, compile and when that's done, let's hit play.

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‫And now we see the platforms moving at a much more normal speed because actually it's moving at one

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‫meter per second, which is okay for a moving platform.

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‫So you can see that this has helped us both reduce the speed, but also now this is a frame rate, independent

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‫speed.

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‫If my computer was chugging along because something else going on in the background, this would still

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‫look the same and play the same in game.

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‫Great stuff.

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‫I will see you in the next lecture.