Physics

In this lesson, we’ll experiment with physically enabled Actors in our physics playground. Cannons, rockets, conveyors, balloons, dominoes - they’re all made possible with physics simulation.

Simulation offers incredible out of the box functionality. We can easily control the behavior of physically enabled Actors and components through their public variables. Enabling physics is easy. Getting physics to “feel right” is an art, and effective use requires some care.

Later, we’ll build the Gravity Gun, a tool used to interact with physics objects. The gun is probably the most sophisticated scripting tutorial of the entire series. There are great opportunities to adapt and extend the gun to whatever end best fits your experiences.

• Open the Lessons > Lesson5 > Physics level.

The Physics level includes many physically enabled Actors that you can interact with as BP_PhysicsPlayer. Some Actors are directly interactable by collision with the player or other Actors.

Other Actors like BP_Cannon are interactable via the BPI_Physics_Interact interface. By walking over the multiple BP_TriggerPlatforms in the level, the platform will attempt to call the PhysicsInteract function on its corresponding Actor. You can see how this was implemented in the Level Blueprint.

Take a minute to press Play and explore the level.

Simulate Physics

In order to activate physics and use physics nodes, we have to tell an Actor and/or component to simulate physics.

• To make an Actor simulate physics, check the Simulate Physics box in the corresponding Details panel.

Unless otherwise specified, a force is applied to Actors according to their mass. By default, mass is calculated as a function of mesh volume.

• To adjust an Actor’s mass, check the Mass in Kg option, then edit the value.

Shooting Radial Forces

Let’s give the player a way to explode Actors in the level.

We can start with a Blueprint that fires a radial force impulse and destroys itself. We’ll spawn this Actor from the player Blueprint.

• Create a new Actor Blueprint called BP_RadialImpulse.
• Add a Radial Force component.
  • Set its Radius to 1000.
  • Set its Impulse Strength to 150,000.
  • Set its Force Strength to 0.

Our Radial Force component is rigged to explode. Now we need to fire the impulse and destroy the Actor.

• Open the Event Graph.
• Grab a reference to the Radial Force component.
• Pull a wire from Radial Force and place a Fire Impulse node.
• Event BeginPlayFire Impulse
• Place a Destroy Actor node.
• Fire ImpulseDestroy Actor

Let’s create a new input action mapping for spawning BP_RadialImpulse.

• Open Edit > Project Settings.
• Select the Input submenu.
• Add a new Action Mapping.
• Name it “PrimaryAction”.
• Add Left Mouse Button.
• Add Gamepad Right Trigger.

Now we can use the new InputAction PrimaryAction event in our player Blueprint.

• Open BP_PhysicsPlayer.

Challenge:

Wire the InputAction PrimaryAction event to a line trace from the camera and spawn BP_RadialImpulse at the location of the trace hit.

Head over to the anvil pit and try your new ability. Can you feel the power?!

SloMo

Our explosions would look much cooler in slow motion. Let’s create a new action mapping for a slo-mo super power.

• Open Settings > Project Settings.
• Select the Input submenu.
• Add a new Action Mapping.
• Name it "SloMo".
• Add Left Shift.
• Add Gamepad Left Shoulder.

Enabling slow motion is actually quite easy. We just need to use a node called Set Global Time Dilation.

• Open BP_PhysicsPlayer.
• Place the InputAction SloMo event.
• Place a Set Global Time Dilation with a Time Dilation of 0.2.
• Connect this to the SloMo > Pressed execution output.
• Place a Set Global Time Dilation with a Time Dilation of 1.
• Connect this to the SloMo > Released execution output.

Test it out to feel THE POWER!

Conveyor

Let’s take a quick diversion from player super powers.

You might have noticed that our conveyors don’t actually move objects. We can write a new Blueprint script inside of BP_TutorialConveyor to get physically enabled Actors moving.

• Open BP_TutorialConveyor’s Event Graph.

We need some way to determine which objects should be moved by BP_TutorialConveyor. A collision box will tell us when objects are within a reasonable distance of the conveyor belt.

• Add a new Box Collision component.
• Open the viewport.
• Set Scale to 1.6, 3.0, 0.25.
• Set Location to 0.0, 0.0, 41.0.

Let’s add OnComponentBeginOverlap and OnComponentEndOverlap events for our collision box.

• Open the Event Graph.
• Right-click Box in Components and select Add Event > Add OnComponentBeginOverlap.
• Right-click Box in Components and select Add Event > Add OnComponentEndOverlap.

We need to store all components that overlap our box in a new array variable.

• Add a new variable and name it “Components”.
• In the Details panel, set Variable Type to PrimitiveComponent > Reference.
• Select the array button.

You may be wondering why we’re storing components instead of Actors. This is because we apply forces to components and not Actors.

• Place a reference to Components in the graph.
• Pull a wire from Components and place an Add node.
• OnComponentBeginOverlapAdd

• OnComponentBeginOverlap > Other CompAdd

• Pull a wire from Components and add a Remove Item node.
• OnComponentEndOverlapRemove
• OnComponentEndOverlap > Other CompRemove

Great! This is everything we need to add and remove components from the array.

Now we need to add a force to each component on Event Tick.

• Place Event Tick in the graph.
• Place a new reference to Components in the graph.
• Pull a wire from Components and add a ForEachLoop node.
• Event TickForEachLoop

We need to cast Array Element to a static mech component.

• Pull a wire from Array Element and add a Cast To StaticMeshComponent node.
• ForEachLoopCast To StaticMeshComponent

When the cast is successful, we should add a force to the component in the direction of conveyor belt movement. We can specify this direction with an Arrow component.

• Add a new Arrow component.
• Open the viewport.
• Set Rotation to 0.0, 0.0, -90.0.
• Set Location to 0.0, 0.0, 70.0.

The Arrow component is a useful tool for development. It will not show up in game unless otherwise specified.

We’re nearly done! Let’s add our last function now.

• Pull a wire from Cast To StaticMeshComponent > As Static Mesh Component and add an Add Force node.
• Set Accel Change to True by checking the box.

Targeted components respond according to their mass when Accel Change is not set to true. Experiment with this variable later to see what effct this has on different components.

Challenge:

Set Force by multiplying Arrow’s forward vector with a scalar power value.

When you play the level, physically enabled objects should respond to BP_TutorialConveyor. Choo-choo! All aboard the physics train!

Gravity Gun

Let’s pick up and launch physically enabled Actors with the Gravity Gun. When complete, the gun will pick up Actors when selected and fire them away from the player.

First create a new action mapping. This method will handle picking up and launching Actors.

• Open Settings > Project Settings.
• Select the Input submenu.
• Add a new Action Mapping.
• Name it “SecondaryAction”.
• Add Right Mouse Button.
• Add Gamepad Left Trigger.