Implementing an orbital camera in Unity

Currently I am working at a company on a project where it is very important that the player is able to easily inspect and interact with certain objects from various angles by focusing on, zoom and rotating around them. This also required constraints to limit the camera to predefined areas, to guide the player and to prevent clipping. Nothing I could find really satisfied me, so I wrote my own variant.

Orbital camera systems by themselves are quite simple to implement, but things can get a bit tricky when you start to add constraints as a requirement. I noticed many people having problems with this sort of thing, so I finally took the time to explain the way I decided to approach this problem. It’s also to help me remember this stuff if I need something similar at a later date. Included is a simplified piece of example code.

Defining the problem

• We want our camera (or any object really) to orbit around another object, responding to user actions and keeping at a certain distance from the target object. In other words, we want the camera to be driven by some type of a Spherical Coordinate System.
• We want the ability to control both Pitch (up/down or the local x-axis) and Yaw. (left/right or the local y-axis) We’ll ignore the Roll axis.
• For each point that the camera will focus on, we want the ability to define constraints that determine where the camera is allowed to go.

Implementation

Let’s start with defining our focus points and constraints. When people think about rotations, many of them think in degrees or radians. That’s why it might seem natural to define the constraints simply as a set of angles like MinPitch, MaxPitch, etc.

There is a different way to do it, which may seem a bit of a weird approach at first, but it makes the entire constraint problem much easier to solve. Instead of setting explicit boundary numbers on each side, we define the constraint as a maximum deviation from a center rotation. The constraint is then defined by the angles YawLimit, PitchLimit and the center rotation. (the center rotation in this case being simply the rotation of the focus point object)

Doing it this way means we can avoid the mess of trying to figure out where we are in relation to the constraints and whether we need to rotate left or right to satisfy them. We can simply calculate the difference between the angles and then use linear interpolation to move the required amount back towards the target rotation without needing to know or care which direction that is. We also don’t have to deal with issues such as wrapping around from weird angles like at the 0-360 degrees boundary.

For that reason, instead of storing most of our angles as floats, we store them as Quaternions. Quaternions seem complicated, but they really make some calculations much easier. And if we want to work in degree angles, we simply convert between them.

Finally, we multiply our current pitch and yaw rotations together to get our desired rotation. We could stop at this point, and then we would simply have a camera that rotates on the spot. Useful for e.g. a security camera. To turn it into an orbit camera, we calculate our offset by taking our forward vector and multiplying it with our target rotation, then adding that to the position of our target object. Lastly, we can add some damping for smoother movement.

Example

Note that this implementation may not be suitable for everyone, as it largely depends on your use case, but if not then at least it may give you some ideas on how to write your own, suited for your purposes. The example code is also very basic and not optimized so you may want to add a few improvements if you decide to use it.

	using UnityEngine;
	using System.Collections;
	/// <summary>
	/// Defines angle limits as the maximum deviation away from the rotation of this object.
	/// (in other words: if the yawlimit is 45, then you can only move up to 45 degrees away from this rotation in both directions.
	/// This means the total angle available would be an angle of 90 degrees)
	/// An angle of 180 allows complete freedom of movement on that axis.
	/// </summary>
	public class FocusPoint : MonoBehaviour
	{
	[SerializeField]
	private float _yawLimit = 45f;
	[SerializeField]
	private float _pitchLimit = 45;
	public float YawLimit { get { return _yawLimit; } }
	public float PitchLimit { get { return _pitchLimit; } }
	}
	/// <summary>
	/// A basic orbital camera.
	/// </summary>
	public class OrbitCamera : MonoBehaviour
	{
	// This is the target we'll orbit around
	[SerializeField]
	private FocusPoint _target;
	// Our desired distance from the target object.
	[SerializeField]
	private float _distance = 5;
	[SerializeField]
	private float _damping = 2;
	// These will store our currently desired angles
	private Quaternion _pitch;
	private Quaternion _yaw;
	// this is where we want to go.
	private Quaternion _targetRotation;
	private Vector3 _targetPosition;
	public FocusPoint Target
	{
	get { return _target; }
	set { _target = value; }
	}
	public float Yaw
	{
	get { return _yaw.eulerAngles.y; }
	private set { _yaw = Quaternion.Euler(0, value, 0); }
	}
	public float Pitch
	{
	get { return _pitch.eulerAngles.x; }
	private set { _pitch = Quaternion.Euler(value, 0, 0); }
	}
	public void Move(float yawDelta, float pitchDelta)
	{
	_yaw = _yaw * Quaternion.Euler(0, yawDelta, 0);
	_pitch = _pitch * Quaternion.Euler(pitchDelta, 0, 0);
	ApplyConstraints();
	}
	private void ApplyConstraints()
	{
	Quaternion targetYaw = Quaternion.Euler(0, _target.transform.rotation.eulerAngles.y, 0);
	Quaternion targetPitch = Quaternion.Euler(_target.transform.rotation.eulerAngles.x, 0, 0);
	float yawDifference = Quaternion.Angle(_yaw, targetYaw);
	float pitchDifference = Quaternion.Angle(_pitch, targetPitch);
	float yawOverflow = yawDifference – _target.YawLimit;
	float pitchOverflow = pitchDifference – _target.PitchLimit;
	// We'll simply use lerp to move a bit towards the focus target's orientation. Just enough to get back within the constraints.
	// This way we don't need to worry about wether we need to move left or right, up or down.
	if (yawOverflow > 0) { _yaw = Quaternion.Slerp(_yaw, targetYaw, yawOverflow / yawDifference); }
	if (pitchOverflow > 0) { _pitch = Quaternion.Slerp(_pitch, targetPitch, pitchOverflow / pitchDifference); }
	}
	void Awake()
	{
	// initialise our pitch and yaw settings to our current orientation.
	_pitch = Quaternion.Euler(this.transform.rotation.eulerAngles.x, 0, 0);
	_yaw = Quaternion.Euler(0, this.transform.rotation.eulerAngles.y, 0);
	}
	void Update()
	{
	// calculate target positions
	_targetRotation = _yaw * _pitch;
	_targetPosition = _target.transform.position + _targetRotation * (-Vector3.forward * _distance);
	// apply movement damping
	// (Yeah I know this is not a mathematically correct use of Lerp. We'll never reach destination. Sue me!)
	// (It doesn't matter because we are damping. We Do Not Need to arrive at our exact destination, we just want to move smoothly and get really, really close to it.)
	this.transform.rotation = Quaternion.Lerp(this.transform.rotation, _targetRotation, Mathf.Clamp01(Time.smoothDeltaTime * _damping));
	// offset the camera at distance from the target position.
	Vector3 offset = this.transform.rotation * (-Vector3.forward * _distance);
	this.transform.position = _target.transform.position + offset;
	// alternatively, if we desire a slightly different behaviour, we could also add damping to the target position. But this can lead to awkward behaviour if the user rotates quickly or the damping is low.
	//this.transform.position = Vector3.Lerp(this.transform.position, _targetPosition, Mathf.Clamp01(Time.smoothDeltaTime * _damping));
	}
	}
	public class CameraMouseInput : MonoBehaviour
	{
	[SerializeField]
	private OrbitCamera _cam;
	private Vector3 _prevMousePos;
	void Update()
	{
	const int LeftButton = 0;
	if (Input.GetMouseButton(LeftButton))
	{
	// mouse movement in pixels this frame
	Vector3 mouseDelta = Input.mousePosition – _prevMousePos;
	// adjust to screen size
	Vector3 moveDelta = mouseDelta * (360f / Screen.height);
	_cam.Move(moveDelta.x, -moveDelta.y);
	}
	_prevMousePos = Input.mousePosition;
	}
	}

view raw

OrbitCamera.cs

hosted with ❤ by GitHub

There’s many ways this example can be improved upon. The following suggestions are left as an exercise for the reader:

• Zooming. (hint: you’ll want to look at logarithmic or exponential functions instead of just linearly increasing/decreasing the distance)
• A fancy editor that allows you to see the constraint angles in the scene view. (hint: Handles.DrawWireArc)
• Can you spot the minor bug? (Well. It may or may not be considered a bug depending on your perspective.)
• Using local space instead of world space allows different camera angles, but also makes your camera more dependent on the transform hierarchy. Useful e.g. if your target object is a spaceship and you want the camera to keep a certain orientation in relation to the ship instead of the world.
• Smooth switching from one target object to another.

Creating a spline level editor in Unity

As I am writing this, me and my team are almost done developing and polishing our game Size Matters from the GGJ 2012 for commercial release. The only things left to do are some art and gui work, and organizing and perfecting our collection of levels.

I’ve solved quite a few problems during the making of this game, and today I will be writing about one of them. First some background story.

Introduction

The first prototype of the game was made during the Global Game Jam 2012 in Hilversum. We had a small team and thus we decided to rely heavily on Unity’s strengths namely built-in physics and ease-of-use. (In an ad-hoc team not everyone will be familiar with any chosen game engine) So we quickly made a fun game just by scaling and duplicating the standard cube, and using that in our levels. Anyone who’s seen the original trailer or played the game will realize it looks pretty blocky. This was the result of building everything out of cubes and together with our chosen art style, we did not need any artists.

After the jam was over and realizing we were going to develop this thing for real, I took the time to create a proper editor for level building to allow for smoother lines and an easier workflow than messing about with tons of cubes. Essentially, we wanted some kind of vector drawing editor in Unity. Goals of this system:

• Allow for both straight line segments and curves
• Easy to create and edit lines
• Separation between visual and collision meshes

How it’s done

Data

The first step is determining what data you need and how you want to organize it. For our purposes, it was logical to create a Line class inheriting from MonoBehaviour to stick on a gameObject. This meant we could make prefabs of lines or edit them directly in a scene.

This Line component then contains a list of serializable control points, which determine where it goes and how it curves. A ControlPoint has the following properties:

• Vector3 Position
• Vector3 Tangent1
• Vector3 Tangent2
• bool Curved

A ControlPoint also has the Handle1 and Handle2 properties, which are useful shortcuts to allow editing of the tangents in an intuitive way. While a tangent represents a direction and magnitude, a handle represents the position of the control point added to the tangent.

The boolean property Curved is not strictly necessary as it is also possible to set both tangents to zero and achieve a similar result, but it helps to simplify certain calculations in a later stage when the meshes are created.

There are many other ways to do curves, such as Catmull-rom and NURBS. We chose to use Bezier curves here because of the balance between complexity and flexibility. It’s also familiar to anyone who has used vector drawing programs such as Inkscape or Illustrator.

We’ll also store some extra settings in the Line to define visual line width, collider width, and curve resolution.

Editor

The second step is creating an editor to make this data accessible without having to dive into an obscure list to change numerical values at random. Here we  see another strength of Unity in it’s extendability. We can simply replace the default Line inspector with our own by creating a new class, inheriting from Editor and adding the CustomInspector attribute.

We can implement OnInspectorGUI to change the contents of the inspector itself, and we can implement OnSceneGUI to draw all our lines and edit handles in the scene view using the Handles class.

Every editor event, the OnSceneGUI method will iterate over all control points in the line and draw lines connecting them. For every control point, it will also draw handles for position and tangents so the level designer can simply drag them around the scene.

This is probably also the time where you start wondering about how to draw curves. Conveniently there is already a Handles.DrawBezier method available for us courtesy of Unity. But in the next stage, we will have to familiarize ourselves with the bezier function.

Of course sometimes special operations are needed so having functionality to select control points is also a good idea. This, for example, allows the user select a point and delete it, or select an endpoint and create new points from there onwards. These special operations can be triggered from the inspector, or in response to a shortcut, or both.

Another feature that turned out to be very useful is snapping. Unity editor handles give you raw data, so if you want any sort of snapping, you’ll have to do it yourself in the editor script.

Processing

Now that we have our data representation of lines and the ability to edit them, it is time to make them actually useful by generating meshes from them. Again we’ll divide this process up into several steps.

The first step is to take our collection of control points which define our line and convert it into something sensible to work with. We iterate over all control points and use them to create a list of straight line segments. For the normal control points this is simply copying their position. For the curved control points, we need to create some intermediate points to follow the curve’s shape at a predefined resolution. We iterate over these points while increasing a progress variable t from 0 to 1. We feed this to our cubic bezier function to get the required position at each point on the curve.

public static Vector3 Bezier3(Vector3 s, Vector3 st, Vector3 et, Vector3 e, float t) {
return (((-s + 3 * (st - et) + e) * t + (3 * (s + et) - 6 * st)) * t + 3 * (st - s)) * t + s;
}


Once we have this segmented line, the next step is to convert it into a Mesh. We can iterate over all the points and construct the mesh in parallel. Every line segment will correspond to two triangles, the size of which is dependent on the width of the line. Another approach is to simply create an outline and feed that to a polygon triangulation library.

Do this two times and you will have a render mesh and a collision mesh, both with different sizes. It is advisable to modify the collision mesh into something with depth, as Unity physics does not really like flat meshes.

And that’s how you make a 2D vector line editor in a 3d engine like Unity. Hopefully, some of you will find this post useful, or maybe you got some new ideas of your own.

Analysing the second prototype

It’s been a while since the last update. The last month I have been working more on the main project, which successfully passed an important deadline. The past weeks I have been working on my own research and assignment. The first prototype has been well received and with this second prototype, the project is slowly nearing completion.

The past few weeks I have been expanding on this to create the second prototype, or iteration, for my Unity persistence layer. I detailed the major weaknesses of the first prototype in my previous post, along with some other considerations. The most important problems were solved in the second prototype, but there were some difficulties as well and some problems turned out to be impossible to solve reliably.

Chosen solutions

Dynamic Entities Instantiation

The biggest weakness of the original prototype was the fact that it was only able to fill existing entities with persistent data from storage and not able to instantiate new entities in the scene if this was required for the game. Initially, a work around was used to mitigate this problem.

In the new prototype, this problem has been solved by the addition of the Type property and the IEntityFactory interface. Developers using this persistence layer can implement an entity factory and plug it into the system to add enable instantiating dynamic entities.

This solution was chosen because, depending on the game and the design and implementation of game entities, instantiating an entity is likely to be quite complex. In Unity, game entities are likely to be inherited from the MonoBehaviour class, which means that they can not be instantiated normally. Instead, they usually need to be instantiated from a prefab, together with meshes, colliders and other components they are composed of.

The type property allows the EntityFactory to select the right prefab, even though the entity classes of different prefabs might be the same. For example, a red car and a blue car might each have their own prefab, but both are a Car entity. Note that entities do not HAVE to be inherited from MonoBehaviour and can also be plain old c# objects. In that case the factory can just ‘new’ an object like normal.

Handling References between Entities

With dynamic entity instantiation, there occurs a related problem, namely the handling of references between entities. Since entities are instantiated elsewhere, we would need to override the serialization behavior to return references to these already existing entities. This is a very useful feature to have when dealing with collections of entities.

An appropriate example would be a Track entity which maintains a list of all TrackPiece entities.

This presented a bit of a problem because these references to entities might be embedded quite deep in the persistent data fields of an entity. This means that the serialization system needs to deal with this somehow. Whatever serialization mechanism is used needs a way to detect the presence of a reference to a game entity, and instead of trying to serialize it, must write the id of this entity, because the entity itself will be handled manually by the rest of the persistence layer. Upon loading, the serialization mechanism will again need to recognize that it should not attempt to deserialize references to entities in the normal way, but instead call back up to the persistence layer to obtain a reference to the entity with that id.

This was accomplished two of the three implemented encoders. In the JsonEncoder the JsonConverter class could be overridden to change the behaviour of the JsonSerializer.

Likewise, in the BinaryEncoder, the ISerializationSurrogate interface was used in a similiar way.

Unfortunately the XmlSerializer used by XmlEncoder did not support any feature like this and the more modern XmlObjectSerializer is not included in the version of Mono that ships with Unity, so this encoder does not support serialization of references between entities.

Restoring the hierarchy

With dynamic entity instantiation, another problem that crops up is the scene hierarchy. In an variety of situations, it might be needed to restore an entity to its proper location in the hierarchy.

This feature was attempted, but abandoned due to several reasons.

Writing the hierarchy data did not prove to be a significant problem. Recursion was used to filter the transforms of all entities and their parents. These were written to the save file and loaded again correctly.

Unfortunately restoring the hierarchy in the scene properly proved impossible to do because of limitations in Unity. This stems from the fact that individual transforms in the scene do not have any unique identifiers. Different transforms can have the same names which leads to unpredictable behavior when loading since different transforms can not be distinguished from each other.

The only way to implement this would be to assign a unique identifier to each transform upon saving and upon loading to load every single one back into the scene. Obviously this does not work in cases where some entities are already defined in the scene editor as they would be duplicated instead of merely filled with the right data. This solution would make saving and loading an all-or-nothing approach and does not fit with Unity’s design philosophy in my opinion.

Another, less important reason to scrap this feature was that saving the entire hierarchy proved to save quite a bit of redundant data that might not be needed. Instead, having each entity manage it’s own location in the hierarchy if relevant was considered a better option. This option is less friendly to the developer but offers a lot more flexibility to the developer, who can make his own judgment on how to handle this issue on a case by case basis.

Integration

To enable easier integration with other systems, a change in the design was necessary to make the method of storage more accessible, so that parameters can more easily be changed and data can be exchanged. The new design is further described in the following section.

Design

The design of the system was changed quite a bit compared to the initial design.

The IPersister and IPersister manager and implementing classes have been merged. This makes it easier to access the storage options or data from outside.

In addition, there is now an abstract base class StreamEncoder, which handles extracting the data from entities and instantiating them. Subclasses inheriting from StreamEncoder override abstract methods to implement serialization details and writing and reading all data to and from the stream. This separates the actual extraction and restoration work from the serialization method chosen, even though they are still tightly related.

The additional feature of handling references between objects required a change in the general algorithm for loading entities and restoring the scene. This now happens in two passes, first instantiating all entities, and then filling all entities with data and reconnecting references between entities.

Evaluation

Since the original Workshop project is pretty much complete right now, testing these changes is better done somewhere else. Having a smaller project also helps with testing only the functionality of this system.

To test and evaluate this prototype, a new project was created containing a test scene with a collection of entities implemented to cover all functionality during testing. The following features were tested in the following ways:

• Persist static entities. Saving and restoring entities defined in the scene in the editor. Three moving Ball entities were placed into the scene and given persistent properties for position, orientation, scale and velocity.
• Persist dynamic entities. Saving and restoring entities instantiated at run-time. A BallSpawner entity was added to the scene which instantiates smaller ball entities on the press of a button.
• References. Saving and restoring persistent references between entities. The BallSpawner was given a list with references to all balls it spawned. Balls were given a reference to the last Ball they collided with. All of these references were rendered by drawing lines in the scene for easy visual confirmation.

Together this test scene and it’s contents covered all features I designed into this system. Static Balls are correctly restored to the right position on load. Spawned balls are instantiated correctly and restored to the the right position as well. All references among entities are restored correctly.

Strengths

All strengths of the first prototype, plus the following:

• Support for dynamic entities
• Support for persistent references between entities
• A more flexible design with less code duplication

Weaknesses

• The game object hierarchy is not restored automatically. In situations where this is needed, the developer will need to take care of that himself, usually by implementing entities’ OnAfterLoad methods.
  Note that transform members like position can still be easily persisted by wrapping them into a property. Restoring the hierarchy could be as simple as attaching the entity’s transform to a parent entity.
• Some serialization mechanisms can not support references between entities and other features like polymorphism. At the moment this is the case with the XmlEncoder.
• This will need to be considered. Special care still needs to be taken to ensure the value types of persistent fields and properties are serializable in the chosen serialization mechanism. Every serializer demands it’s own rules and constraints for serializable types. This really can not be avoided.

Other Comments

• Recently another use case was discovered which, in hindsight, should have been considered. That is the case where game entities are defined in the scene and removed during run-time. An example of this would be pickups that vanish when the player touches them. This should be handled in the final version but is not a difficult problem.

Conclusion

The current system is usable for most of the use cases it is likely going to deal with. One exception is the use case for removed entities, which was not initially considered, but handling this should not be difficult so it will likely work in the final version. Overall the project seems on track, including my thesis which is slowly accumulating a nice amount of content.

Analysing the first prototype

The past weeks I have been working on working on some general gameplay functionality for the main project, in addition to further integration of the first prototype of my persistence layer. It is now possible to store the game state in a save slot on a server. I have also analyzed the current strengths and weaknesses of the system. The following diagram shows how it is designed:

The most important parts are the IEntity interface which is implemented by game entities and enables them to mark fields to be persisted and to receive signals after loading and other persistence events. The bulk of the work is handled in the implementations of IStreamEncoder, of which currently only XmlEncoder is implemented and uses reflection and the XmlWriter and XmlSerializer classes to serialize the entities and their fields to an xml document. Which can be saved to a file, or somewhere else, by the IPersister.

I described the relevant problems and my chosen solutions in my previous post so I will not go into those here and now. I was mostly on the right track, and the overall system is working fine, though there are still quite a few points of improvement.

Strengths

• Seperation of responsibility: Serialization and storage details are abstracted away from the rest of the game logic.
• Simple and Intuitive: Adding extra persistent game entities is easily done by implementing IEntity and marking the relevant fields and properties with the PersistField attribute.
• Convenient: It is possible to persist fields defined in an object related or attach to the entity by wrapping them in a persistent property.
• Flexible: different implementations of IPersister can easily be switched out to change the overall behaviour of the system.

Weaknesses

• Dynamic entities: The current system collects all entities in the scene during saving, and extracts their data. Upon loading, all entities in the scene are again collected, and filled with the data that was saved earlier. This works for scenarios where the state of entities changes, but it does not work in scenarios where entities have to be created, or removed to reach the saved state. We used a workaround in the form of a manager entity to handle the state of a variable number of other game elements. Recreating entities at the right place in the scene and hierarchy would be desirable. This also will require a way to instantiate game entities without needing references to prefabs.
• Dependencies: Some entities turned out to have dependencies with others and expected those to have been initialized already. I quickly solved this by adding a priority to IEntity to govern the order in which OnAfterLoad is called on them. This is perhaps not the best solution. Priority based on location in the scene hierarchy might be an intuitive way to handle this, but is less flexible. Food for thought.
• Integration: Integrating with existing systems turned out to be a bit trickier than expected. The save methods do not take parameters so options like save names needed to be set in the current IPersister while ideally, only the IPersistenceManager should be accessed once the system is configured. This part of the design could be reworked.

Other Comments

• IEntity: This interface now contains four methods implemented by entities to receive signals when needed on persistence events. These should be optional but the interface requires an implementation, which can be irritating. This could be changed.
• Design: As of this moment the bulk of the work happens in the XmlEncoder, which will require duplicating functionality when different encoders are desired. The extraction of data and serialization of data should be separated.
• IEntity: My colleague offered another suggestion to handle the extraction problem. Instead of code annotations, this would involve an object with a list of names of fields to be persisted, which could be attached to a game entity. In Unity this could be a component, granting the user the ability to set values to save in the inspector. It is an interesting idea which merits further investigation.

Conclusion

The current prototype is quite successful. It does the job and does it reliably, and has kept the persistence implementation of most game entities relatively simple. It has proven that the chosen solutions work for this problem. There are still aspects where it falls short, which should be addressed in the following iterations of new prototypes. Handling dynamic game entities and dependencies should have the highest priority right now.

The First Prototype

As planned, I have been working on my first prototype for my persistence layer. This prototype will be integrated into a current project with a deadline so it needs to be simple.

I have chosen the most promising methods for dealing with each sub-problem which I have identified in my earlier preliminary research.

Extraction

By extraction I mean the method by which the relevant data is collected and extracted from the current state of the game. I found three candidate solutions to solving this:

1. Domain model: Have all game objects contain references to separate data objects.
2. Reflection: Have all game objects use annotations to mark data they want serialized, and extract using reflection.
3. Manifest: Keep all data separate in a single object graph, commonly called a manifest

I decided that reflection is the most promising candidate for the first prototype, since it would enable a lot of flexibility and not require a major restructuring of the already existing code base. It results in a slightly more complicated persistence layer but is offset by the ease of development for the user. It was inspired by the way object relational mapping is commonly used in Java and to a lesser extend .NET systems.

The domain model and manifest solutions would result in a simpler persistence layer, but would require a lot of changes in the existing code base to supply the relevant data.

Serialization

I am working in Unity, which means I have access to most functionality in .NET implemented by Mono, including serialization.

1. XmlWriter/XmlSerializer: serialize to XML
2. BinaryFormatter: serialize to binary stream

The choice of serialization method to use is sometimes very dependent on the situation, but usually comes down to preference. In this case I was given to understand that the data would be sent to a server later, so encoding to text was recommended. Xml was the logical solution to use in this case.

Storage

During testing, storing data on the filesystem makes analyzing it and debugging easier, but later the data should be sent to a web server connected to a database. I decided to make the system flexible by creating interchangeable modules to change the behavior when needed. I made one module to save to the filesystem using a FileStream and another to interface with a different system handling the remote server.

Reconstruction

To prepare for reconstruction, I made sure to structure the serialized data in such a way as to be able to keep it apart later and still know where everything belongs. This is why I gave every entity a unique identifier. This allows the system to restore all the data to where it belongs. The actual restoring of the scene can by a combination of the following methods:

1. Automatically save and restore entire scene hierarchy related to the entity
2. Save entity and use custom methods per entity for restoration after loading the data.
3. For dynamic entities that should be instantiated, the factory pattern is a good approach.

I decided to go for the second option for two reasons. The most important reason is that option one would make the persistence layer very complex, and I do not have the time right now to implement such a solution, while the sacrifice in usability is very small. Each entity can easily handle its own initialization after being loaded with data by implementing OnAfterLoad and other methods. The other reason is that the first option might lead to a lot of redundant data, which might create storage issues and would introduce a lot of places where things could go horribly wrong.

To summarize

I have chosen the best solutions for the sub-problems identified and started implementing the first prototype. Initial results seem very promising. The prototype has already been integrated into the main project to have actual data to save and it is performing nicely. There are still a few kinks to work out, and a few glaring shortcomings, but it will serve for the current purposes.

Research on Persistence

The past week I have been doing preliminary research on persistence in the context of game development in preparation to developing a standard system to handle this.

I have scoured the web and found quite a few sources on this topic, though most of them have not been very useful. Most questions and responses in the Unity community focus on the smaller problems like serialization. There is very little deeper discussion on the design of systems that should handle this. The same goes for most game development forums. It is commonly said that every game has different needs and thus persistence should be custom-built for every game. I am not satisfied with that, because I feel that most of them deal with fundamentally the same problems, just in different situations. In my opinion, a lot of this could be generalized.

The problem of persistence has been largely solved in enterprise software. There are advanced databases and other data stores available, with abstraction layers to ease development. Application functionality is divided into domain models for persistence and other parts for the rest of the functionality. This makes persistence a trivial problem for most software engineers.

This is usually not the case in game development. In my experience a lot of games are developed more in an ad-hock fashion than most  applications. For reasons of performance and ease of use, the data we want to save is often embedded in the scene in ways that make it difficult to get it out if persistence was an afterthought. Any Unity developer who has ever tried to save and restore entire game object hierarchies at run-time will know the difficulties I am talking about. Whether these designs are wise is another matter I will not go into here. In addition to this, there is the fact that different games use different storage methods. Some might use a save file on a local hard disk while others will save state to a web service or database. All these circumstances make persistence for games a touchy subject.

I took another close look at all the resources I had gathered and noted that the entire problem of persistence in game design can be summarized in a few sub-problems:

1. Extraction: Separating the data you want to persist from the rest of the game
2. Serialization: Encoding the data into a sequential format suitable for storage and back
3. Storage: Actually storing and retrieving the serialized data
4. Reconstruction: Reconstructing the scene or game state from loaded data

In a lot of situations, these problems might overlap or even blur as to be almost indistinguishable from each other. At all times every one of these problems is present in some way. Most resources I have found focused on serialization and storage. The more difficult problems are actually extraction and reconstruction, but often overlooked, which often causes problems in serialization.

For each of these problems I have found a number of ways to deal with the problem. I will choose the most promising solutions I have found and integrate these into my first prototype.