Simulation

When an object is falling or moved by applying some force, then that object is simulating physics. For physics to simulate, the mesh must have a collision shape first. Because the actual 3D mesh can be complicated, UE4 uses simple proxy shapes, which are called Physics Bodies (also known as BodyInstances), such as a box, sphere, capsule, or custom convex hull to simulate physics. The properties for this physical body are adjusted by selecting the mesh on which you want to apply physics. For example, in Chapter 3, we assigned a Static Mesh Component and created a Blueprint class for our C++ actor.

1. 1.

   Open the Blueprint and select MeshComponent (Inherited) from the Components tab (see Figure 5-1).

    

1. 2.

   In the Details panel, scroll down to the Physics category (see Figure 5-2).

    

Collision

In Unreal, every object with Collision is assigned an object type, which defines how it interacts with other object types. Some object types can block, some can overlap, and others can ignore. An object can also define how it should react to traces using trace channels. By default, there are two types of trace channels: Visibility and Camera.

Collisions are determined by the preset on the component under the Collision category. Collisions generate events with information, including with whom the collision occurred. Figure 5-3 shows the Collision properties.

Let’s now look at defining a new trace type and using it in the game to pick up an item.

Using Trace (Raycast) to Pick up an Item

When you want to pick up an item in a game, you look at the item first and press a specific key. When you press that key, under the hood, the Engine sends an invisible ray from the camera location with a specified length. In between the start location and the end location, if the trace hits an object, it returns a hit result containing hit information. Figure 5-4 illustrates how trace works.

So how does the trace know what to hit? It is done using a trace channel. Unreal Engine comes with two trace channels: Visibility and Camera. In this section, you create our own trace channel, set it to an item, and use trace to detect it.

First and foremost, let’s create a trace channel. Open Project Settings by clicking Edit on the menu bar, and then select Project Settings (see Figure 5-5).

From the Project Settings, click Collision under the Engine section to modify the collision settings (see Figure 5-6).

Click the New Trace Channel button under the Trace Channels category. A new dialog box appears, prompting you to name the trace and a default response. Enter the name as MyItemTrace and set the default response to ignore (see Figure 5-7).

After pressing the Accept button, you see your newly created trace under the Trace Channels category. You can now access this trace channel under every mesh component to enable or disable it.

Before we do anything else, let’s first make sure the mesh you are using (1M_Cube) has collision available. Go to the Content Browser where the mesh is located, and hover your mouse over the mesh. In the tooltip, if the Collision Prims is greater than 0, then the mesh has collision included (see Figure 5-8).

If the mesh is a missing collision, you can generate a collision shape inside the Static Mesh Editor. From the menu bar, click Collision and select an option to generate your desired collision shape. For simple shapes, you can choose box, cylinder, sphere, or so forth, but for complex shapes, select Auto Convex Collision (see Figure 5-9).

To use the trace channel, you first open your previously created Blueprint Actor and select the MeshComponent (inherited) from the Components tab. Assign the Collision that includes Static Mesh. In our case, you use the 1M_Cube static mesh.

Next, go to the Details panel and scroll down to the Collision category. Expand the Collision Presets section and click the BlockAllDynamic drop-down button. From the list of options, select Custom. By selecting Custom, you can choose how the object responds to collisions with other objects. Choose to block the MyItemTrace trace response by enabling Block (see Figure 5-10).

You have now finished the item setup, but what about trace? You do it in the Character class, but you need to have an input to start the trace. To do that, click Edit on the menu bar and select Project Settings. Next, select Input under the Engine section. On the right, click the + button next to Action Mapping. Name the input anything you want, and select a key for it. I named it Trace and chose E for the key (see Figure 5-11).

If you have not added a third-person project, you can do so by clicking the green Add New button in the Content Browser. Select either Add Feature or Content Pack. In the dialog, select Third Person and click +Add to Project.

Next, press Shift+Alt+O to open the Open Asset dialog. Search for ThirdPersonCharacter and open it. Alternatively, you can navigate to Content/ThirdPerson/Blueprints and open ThirdPersonCharacter Blueprint (see Figure 5-12).

In the Event graph, right-click and search for Input Trace (it is under Input → Action Events). You can add the Trace event, which is invoked when you press the E key. Right-click again on the graph and search for Line Trace By Channel. (There are other types of traces as well, like Box, Capsule, and Sphere. Read more about them at https://​docs.​unrealengine.​com/​en-US/​Engine/​Physics/​Tracing/​Overview/​index.​html).

Line Trace By Channel is the primary node that does the tracing from the start location to the end location. Figure 5-4 illustrates where the trace starts from the Camera and shoots to the direction the Camera is facing. To create that setup, right-click the graph and search for GetFollowCamera (no spaces) and select the Get Follow Camera node. This is the getter node for the Camera included with the character in Components. (Alternatively, you can drag the Follow Camera from the Components tab onto the graph to create the node).

From the output pin of this node, drag a wire, search for GetWorldLocation, and select it. This node shows the Camera component’s location in the world space. Connect the GetWorldLocation yellow output to the Start input of Line trace node (see Figure 5-13).

Now you need the trace to end at a specified length where the Camera is looking. To do that, drag a wire from Follow Camera, and search for and select GetWorldRotation, which represents the world rotation of the Camera Component. To find the direction, drag a wire from the GetWorldRotation blue output, and search for and select GetForwardVector.

The GetForwardVector node represents the direction, but it returns a unit vector, which means it is in the 0–1 range and is not useful in our context (https://en.wikipedia.org/wiki/Rotation_matrix). So you multiply it by a higher number to determine how far the trace should go.

Drag a wire from the GetForwardVector node’s yellow output, search for multiply and select the vector * float node. After selecting this node, enter 5000 in the green box of the newly created multiply node. The 5000 is the length of our trace. Connect the multiply node output to the End input of the Line Trace node.

Next, click the Visibility drop-down button on the Line Trace node and change it to MyItemTrace (see Figure 5-14).

You are not completely done, but you did finish the trace setup. If you play the game now and press E (or the key you used for Trace), it might appear that nothing is happening, but you are tracing. To check this, click the None drop-down button at Draw Debug Type and change it to For Duration. Press the Play button again and start the tracing. You now see a red line originating from your Camera that shoots toward where you are looking.

Even though you finished the trace, you are still not using any information from the hit result. Let’s fix that. Drag a pin from the red Return Value of the Line Trace, search, and select Branch node. The return value is a boolean (either true or false). It is true if the hit is successful (if it hits any object with MyItemTrace, trace response is blocking; in our case, it is the Blueprint actor we created). It is false if the trace didn’t hit anything. Drag another wire from the Out Hit node and select Break Hit Result. The resulting node is called a Struct node, which is a container containing other data types.

In the Break Hit Result node, click the downward arrow to expand the node and show all the advanced types. Right-click the event graph, and search for and select Print String, which is a simple node that prints a message to the screen. Drag a wire from Hit Actor and connect it to In String of Print String. This automatically creates a new node between Print String and the Struct node. Finally, connect the Branch node with Print String, and you have a complete setup (see Figure 5-15).

Since you need the trace to react, you must place the Blueprint you created. Drag and drop the Blueprint from the Content Browser to the game world. Now press Play and look at the item placed in the world. Press your Trace input key. You see a red trace line (if you set the Draw Debug Type to For Duration or Persistent) hitting the item and a message printed on the screen (see Figure 5-16).

Destruction Using Physics

In this section, you use the Apex destruction plugin to create a destructible mesh and interact with it in the game. When writing this book, Unreal Engine was making the transition from Physx to Chaos Physics System, rendering this section obsolete in the future. I don’t cover Chaos because it is not production-ready and requires the GitHub source version instead of the launcher version.

To create a destructible mesh, you first need to enable the Apex plugin from the Plugins section. Click the Edit button in the menu bar and select Plugins. Go to the Physics category and enable Apex Destruction Plugin. Restart the Editor (see Figure 5-17).

After the restart, you can right-click any Static Mesh actor (or 1M_Cube in our case) in the Content Browser and select Create Destructible Mesh. A new Destructible Editor opens and a new Destructible Editor actor is available next to the static mesh. Let’s go through the basics of Destructible Mesh Editor in Figure 5-18.

First, set the Cell Site Count to 100 and press the Fracture Mesh button. This breaks the mesh into 100 different chunk pieces (see Figure 5-19).

Go back to the Content Browser and drag the newly created Destructible Mesh actor (1M_Cube_DM) to the game world. If you press the Play button now, nothing happens because the destructible mesh is not simulating physics. To fix it, select the Destructible actor in the viewport, and in the Details panel, enable Simulate Physics (see Figure 5-20).

Now, if you press Play, you see the destructible mesh falling and hitting the ground, but there is no destruction. Why? This is destructible mesh, but there is no destruction, right? Well, this is because you haven’t enabled damage to destruction mesh yet. So how do you do that? Open the Destructible Mesh actor and turn on Enable Impact Damage (see Figure 5-21).

Go back to your game world and press Play. Watch how the mesh falls and breaks into pieces (see Figure 5-22).

Let's spice this up a little bit by applying a particle effect when the mesh breaks. For the particle system, add the Starter Content pack if not added yet. To add starter content, click the Add New button in the Content Browser and select the first Add Feature or Content Pack. In the next dialog window, switch to the Content Packs tab, select Starter Content, and click +Add to Project (see Figure 5-23).

After the Starter Content is added, open the 1M_Cube_DM, and in the Destruction Settings Details panel under the Effects category, expand Fracture Settings. Here you see two numbers: 0 and 1. Expand 1 and assign P_Smoke (which is the particle from Starter Content) (see Figure 5-24).

If you play now, when the object falls and breaks, you see the particle effect playing at the fracture location (see Figure 5-25).