Overview of Physics Bodies and Physics Materials

Devin Sherry

September 2015

In this article by Katax Emperor and Devin Sherry, author of the book Unreal Engine Physics Essentials, we will take a deeper look at Physics Bodies in Unreal Engine 4. We will also look at some of the detailed properties available to these assets. In addition, we will discuss the following topics:

  • Physical materials – an overview

For the purposes of this article, we will continue to work with Unreal Engine 4 and the Unreal_PhyProject. Let's begin by discussing Physics Bodies in Unreal Engine 4.

(For more resources related to this topic, see here.)

Physics Bodies – an overview

When it comes to creating Physics Bodies, there are multiple ways to go about it (most of which we have covered up to this point), so we will not go into much detail about the creation of Physics Bodies. We can have Static Meshes react as Physics Bodies by checking the Simulate Physics property of the asset when it is placed in our level:

We can also create Physics Bodies by creating Physics Assets and Skeletal Meshes, which automatically have the properties of physics by default. Lastly, Shape Components in blueprints, such as spheres, boxes, and capsules will automatically gain the properties of a Physics Body if they are set for any sort of collision, overlap, or other physics simulation events. As always, remember to ensure that our asset has a collision applied to it before attempting to simulate physics or establish Physics Bodies, otherwise the simulation will not work.

When you work with the properties of Physics on Static Meshes or any other assets that we will attempt to simulate physics with, we will see a handful of different parameters that we can change in order to produce the desired effect under the Details panel.

Let's break down these properties:

  • Simulate Physics: This parameter allows you to enable or simulate physics with the asset you have selected. When this option is unchecked, the asset will remain static, and once enabled, we can edit the Physics Body properties for additional customization.
  • Auto Weld: When this property is set to True, and when the asset is attached to a parent object, such as in a blueprint, the two bodies are merged into a single rigid body. Physics settings, such as collision profiles and body settings, are determined by Root Component.
  • Start Awake: This parameter determines whether the selected asset will Simulate Physics at the start once it is spawned or whether it will Simulate Physics at a later time. We can change this parameter with the level and actor blueprints.
  • Override Mass: When this property is checked and set to True, we can then freely change the Mass of our asset using kilograms (kg). Otherwise, the Mass in Kg parameter will be set to a default value that is based on a computation between the physical material applied and the mass scale value.
  • Mass in Kg: This parameter determines the Mass of the selected asset using kilograms. This is important when you work with different sized physics objects and want them to react to forces appropriately.
  • Locked Axis: This parameter allows you to lock the physical movement of our object along a specified axis. We have the choice to lock the default axes as specified in Project Settings. We also have the choice to lock physical movement along the individual X, Y, and Z axes. We can have none of the axes either locked in translation or rotation, or we can customize each axis individually with the Custom option.
  • Enable Gravity: This parameter determines whether the object should have the force of gravity applied to it. The force of gravity can be altered in the World Settings properties of the level or in the Physics section of the Engine properties in Project Settings.
  • Use Async Scene: This property allows you to enable the use of Asynchronous Physics for the specified object. By default, we cannot edit this property. In order to do so, we must navigate to Project Settings and then to the Physics section. Under the advanced Simulation tab, we will find the Enable Async Scene parameter. In an asynchronous scene, objects (such as Destructible actors) are simulated, and a Synchronous scene is where classic physics tasks, such as a falling crate, take place.
  • Override Walkable Slope on Instance: This parameter determines whether or not we can customize an object's walkable slope. In general, we would use this parameter for our player character, but this property enables the customization of how steep a slope is that an object can walk on. This can be controlled specifically by the Walkable Slope Angle parameter and the Walkable Slope Behavior parameter.
  • Override Max Depenetration Velocity: This parameter allows you to customize Max Depenetration Velocity of the selected physics body.
  • Center of Mass Offset: This property allows you to specify a specific vector offset for the selected objects' center of mass from the calculated location. Being able to know and even modify the center of the mass for our objects can be very useful when you work with sensitive physics simulations (such as flight).
  • Sleep Family: This parameter allows you to control the set of functions that the physics object uses when in a sleep mode or when the object is moving and slowly coming to a stop. The SF Sensitive option contains values with a lower sleep threshold. This is best used for objects that can move very slowly or for improved physics simulations (such as billiards). The SF Normal option contains values with a higher sleep threshold, and objects will come to a stop in a more abrupt manner once in motion as compared to the SF Sensitive option.
  • Mass Scale: This parameter allows you to scale the mass of our object by multiplying a scalar value. The lower the number, the lower the mass of the object will become, whereas the larger the number, the larger the mass of the object will become. This property can be used in conjunction with the Mass in Kg parameter to add more customization to the mass of the object.
  • Angular Damping: This property is a modifier of the drag force that is applied to the object in order to reduce angular movement, which means to reduce the rotation of the object. We will go into more detail regarding Angular Damping.
  • Linear Damping: This property is used to simulate the different types of friction that can assist in the game world. This modifier adds a drag force to reduce linear movement, reducing the translation of the object. We will go into more detail regarding Linear Damping.
  • Max Angular Velocity: This parameter limits Max Angular Velocity of the selected object in order to prevent the object from rotating at high rates. By increasing this value, the object will spin at very high speeds once it is impacted by an outside force that is strong enough to reach the Max Angular Velocity value. By decreasing this value, the object will not rotate as fast, and it will come to a halt much faster depending on the angular damping applied.
  • Position Solver Iteration Count: This parameter reflects the physics body's solver iteration count for its position; the solver iteration count is responsible for periodically checking the physics body's position. Increasing this value will be more CPU intensive, but better stabilized.
  • Velocity Solver Iteration Count: This parameter reflects the physics body's solver iteration count for its velocity; the solver iteration count is responsible for periodically checking the physics body's velocity. Increasing this value will be more CPU intensive, but better stabilized.

Now that we have discussed all the different parameters available to Physics Bodies in Unreal Engine 4, feel free to play around with these values in order to obtain a stronger grasp of what each property controls and how it affects the physical properties of the object. As there are a handful of properties, we will not go into detailed examples of each, but the best way to learn more is to experiment with these values. However, we will work with how to create various examples of physics bodies in order to explore Physics Damping and Friction.

Physical Materials – an overview

Physical Materials are assets that are used to define the response of a physics body when you dynamically interact with the game world. When you first create Physical Material, you are presented with a set of default values that are identical to the default Physical Material that is applied to all physics objects.

To create Physical Material, let's navigate to Content Browser and select the Content folder so that it is highlighted. From here, we can right-click on the Content folder and select the New Folder option to create a new folder for our Physical Material; name this new folder PhysicalMaterials. Now, in the PhysicalMaterials folder, right-click on the empty area of Content Browser and navigate to the Physics section and select Physical Material. Make sure to name this new asset PM_Test.

Double-click on the new Physical Material asset to open Generic Asset Editor and we should see the following values that we can edit in order to make our physics objects behave in certain ways:

Let's take a few minutes to break down each of these properties:

  • Friction: This parameter controls how easily objects can slide on this surface. The lower the friction value, the more slippery the surface. The higher the friction value, the less slippery the surface. For example, ice would have a Friction surface value of .05, whereas a Friction surface value of 1 would cause the object not to slip as much once moved.
  • Friction Combine Mode: This parameter controls how friction is computed for multiple materials. This property is important when it comes to interactions between multiple physical materials and how we want these calculations to be made. Our choices are Average, Minimum, Maximum, and Multiply.
  • Override Friction Combine Mode: This parameter allows you to set the Friction Combine Mode parameter instead of using Friction Combine Mode, found in the Project Settings | Engine | Physics section.
  • Restitution: This parameter controls how bouncy the surface is. The higher the value, the more bouncy the surface will become.
  • Density: This parameter is used in conjunction with the shape of the object to calculate its mass properties. The higher the number, the heavier the object becomes (in grams per cubic centimeter).
  • Raise Mass to Power: This parameter is used to adjust the way in which the mass increases as the object gets larger. This is applied to the mass that is calculated based on a solid object. In actuality, larger objects do not tend to be solid and become more like shells (such as a vehicle). The values are clamped to 1 or less.
  • Destructible Damage Threshold Scale: This parameter is used to scale the damage threshold for the destructible objects that this physical material is applied to.
  • Surface Type: This parameter is used to describe what type of real-world surface we are trying to imitate for our project. We can edit these values by navigating to the Project Settings | Physics | Physical Surface section.
  • Tire Friction Scale: This parameter is used as the overall tire friction scalar for every type of tire and is multiplied by the parent values of the tire.
  • Tire Friction Scales: This parameter is almost identical to the Tire Friction Scale parameter, but it looks for a Tire Type data asset to associate it to. Tire Types can be created through the use of Data Assets by right-clicking on the Content Browser | Miscellaneous | Data Asset | Tire Type section.

Now that we have briefly discussed how to create Physical Materials and what their properties are, let's take a look at how to apply Physical Materials to our physics bodies. In FirstPersonExampleMap, we can select any of the physics body cubes throughout the level and in the Details panel under Collision, we will find the Phys Material Override parameter. It is here that we can apply our Physical Material to the cube and view how it reacts to our game world.

For the sake of an example, let's return to the Physical Material, PM_Test, that we created earlier, change the Friction property from 0.7 to 0.2, and save it. With this change in place, let's select a physics body cube in FirstPersonExampleMap and apply the Physical Material, PM_Test, to the Phys Material Override parameter of the object. Now, if we play the game, we will see that the cube we applied the Physical Material, PM_Test, to will start to slide more once shot by the player than it did when it had a Friction value of 0.7. We can also apply this Physical Material to the floor mesh in FirstPersonExampleMap to see how it affects the other physics bodies in our game world. From here, feel free to play around with the Physical Material parameters to see how we can affect the physics bodies in our game world.

Lastly, let's briefly discuss how to apply Physical Materials to normal Materials, Material Instances, and Skeletal Meshes.

To apply Physical Material to a normal material, we first need to either create or open an already created material in Content Browser. To create a material, just right-click on an empty area of Content Browser and select Material from the drop-down menu.Double-click on Material to open Material Editor, and we will see the parameter for Phys Material under the Physical Material section of Details panel in the bottom-left of Material Editor:

To apply Physical Material to Material Instance, we first need to create Material Instance by navigating to Content Browser and right-clicking on an empty area to bring up the context drop-down menu. Under the Materials & Textures section, we will find an option for Material Instance. Double-click on this option to open Material Instance Editor. Under the Details panel in the top-left corner of this editor, we will find an option to apply Phys Material under the General section:

Lastly, to apply Physical Material to Skeletal Mesh, we need to either create or open an already created Physics Asset that contains Skeletal Mesh. In the First Person Shooter Project template, we can find TutorialTPP_PhysicsAsset under the Engine Content folder. If the Engine Content folder is not visible by default in Content Browser, we need to simply navigate to View Options in the bottom-right corner of Content Browser and check the Show Engine Content parameter. Under the Engine Content folder, we can navigate to the Tutorial folder and then to the TutorialAssets folder to find the TutorialTPP_PhysicsAsset asset. Double-click on this asset to open Physical Asset Tool. Now, we can click on any of the body parts found on Skeletal Mesh to highlight it. Once this is highlighted, we can view the option for Simple Collision Physical Material in the Details panel under the Physics section. Here, we can apply any of our Physical Materials to this body part.

Summary

In this article, we discussed what Physics Bodies are and how they function in Unreal Engine 4. Moreover, we looked at the properties that are involved in Physics Bodies and how these properties can affect the behavior of these bodies in the game.

Additionally, we briefly discussed Physical Materials, how to create them, and what their properties entail when it comes to affecting its behavior in the game. We then reviewed how to apply Physical Materials to static meshes, materials, material instances, and skeletal meshes.

Now that we have a stronger understanding of how Physics Bodies work in the context of angular and linear velocities, momentum, and the application of damping, we can move on and explore in detail how Physical Materials work and how they are implemented.

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