One of the games that came out recently that surprised me in terms of clever design and depth of its gameplay systems was Legend of Zelda: Breath of the Wild.

After watching the GDC 2017 talk given by the three lead developers of BotW, in particular the section given by the lead programmer:

This talk blew my mind when I first saw it.

The simplicity of the system, and the potential that it possessed, made this method seem like something every game going forward that wants to have an fully interactive world should have, in my opinion.

I started thinking about how you might implement something like this in Unreal, and moreover how you might implement it in a way that allows for ease of use and scaling as you added more materials and elements.

This also coincided with the Unreal Summer Game Jam, and I decided to take the opportunity to create a small game/proof of concept for this system:

This is what I came up with:



The rules of this system are stated quite simply and don’t leave much room for error:

1. Materials can interact with Elements

2. Elements can interact with other Elements

3. Materials cannot interact with other Materials

The system as I implemented it in Unreal has a few basic pieces:

1) Material components – Registered as a C++ base class, this is subclassed into various different Material types like WoodMaterial, MetalMaterial etc in Blueprints.



2) Element components – Registered as a C++ base class, this is subclassed into various different Element types like FireElement, WaterElement etc in Blueprints.

3) Material/Element states – Every component has a state enumeration variable, which by default is “Active” and represents that material/element after being exposed to some element, eg. “Burning”, “Wet”, “Electrocuted” etc. (these are Material state examples, I haven’t quite found a use for Element states yet beyond “Active” and “Disabled” but I’m sure they’ll come in handy at some point)

4) Material and Element types – Another enumeration variable used to identify various types of elements and materials. This variable is chosen at design time.



5) Material Class/Element Class – Another variable that is chosen at design time, this is used to know what subclass of material/element to actually use/spawn. This is also why there is a ‘CreatedMaterial’ or ‘CreatedElement’ variable, as the component in the details panel is only a base Material/Element type, and the actual component that you need is created at runtime and stored in ‘CreatedMaterial’ or ‘CreatedElement’ respectively.

I’m fairly certain there’s better ways to do this that aren’t as hacky, but it worked for me.

It’s also a bit unintuitive to have to select both a ‘Wood’ type and a ‘WoodMaterial’ material class, you’d usually expect to do one or the other, but not both, however those values are needed by the system for proper functioning, in different places.

It’s probably something you can abstract away with some boilerplate if checks, I just didn’t find it necessary to implement for a prototype.

An unexpected benefit of doing it in this somewhat unintuitive way is that you’d never need to change your Material or Element component classes when you add new types of Materials/Elements. If you were to do if-checks based on the ‘Type’ being selected, say, then you’d need to add a new if-check for every type of material/element you add, which can quickly get out of hand. The same would apply vice-versa if you were using the Class instead.

This way introduces greater chance of user error, having one of those two properties not selected will cause silent errors that are hard to find, but if that’s a tradeoff you can live with, more power to you.

6) Blueprint subclasses of Material/Element – Having the actual Material/Element behaviors be defined in BPs, allows for designers to utilize some handy flow control systems that exist in BP and allow for intuitive and scalable scripting of the interaction between different Materials and Elements.

A picture is worth a thousand words, so here’s what the BP of the wood Material component looks like:

What’s happening in the gif is that the Player is tagged as a wood Material, and walks into a volume tagged as a fire Element.

This causes them to catch on fire (as wood in fire typically does), and when that’s done it sets the player to a ‘burning/on fire’ state.

This allows for different objects to own Material or Element components, have a type (Material example would be wood, Element example would be fire), and then allow for designers to implement different types of response behavior for when an Element and Material interact, in Blueprints.

Having state values associated with both Materials and Elements, also means you can script additional or altogether different interactions based on the state of the Material/Element in question, for eg:

A wood material in its normal active state on contact with water does nothing, but a wood material in a ‘burning’ state on contact with water, has the fire removed.

It also uses the neat visual flow control of the ‘Switch-On-Enum’ nodes to keep things simple and compartmentalized for designers.

How the actual functions of the Material/Element components are triggered is done using the BeginOverlap events from colliders, which looks something like this:

Something I did for ease of use was to create a BP class which has a collider, Static/Skeletal mesh/Particle system and Material/Element component, and then designers can easily create new interactive objects by inheriting from this BP and switching around the mesh/particle system and selecting different Material/Element types.

This works okay for demo purposes but in a production environment you’d probably make those as C++ base classes too, as there’s nothing happening in the interactive item base classes that you couldn’t easily transcribe to code.

So essentially, any class that wants to use the Material/Element components, just needs to have a collider, and hook up the BeginOverlap events of that colllider to the Material/Element ‘OnStateChange’ events and provide the appropriate arguments from the object that caused the overlap.

Here’s what that looks like for a typical interactive object:



This is an Element item, which is why it checks for both Element and Material state changes from the other object. A Material item would only check for Element state changes. (remember the three rules!)

A final tidbit, though it doesn’t have anything to do with the interactive Material/Element system persay, is for how to get particles to spawn all over a skeletal mesh, which is an invaluable part of the Cascade particle system and is necessary to sell the effect of things interacting in my opinion.



The circled out Cascade emitter is where all the magic is, the details panel in the lower left is where you choose the parameter name that will point to the target actor of this particle effect.

When you want to actually create the effect, it’s just a matter of spawning the appropriate particle system, and setting that parameter using the name you assigned within the particle system earlier, to the actor that you want to be covered in fire/water/etc.

That’s it for this post, hope that this provided some insight into how to make objects feel more interactive and get more of that lovely ‘game juice’ feel into your games that Nintendo manages to do time and again.

Hope this is helpful!

I’m on Twitter and you can reach me @nightmask3 if you need any help or have a clarification.