Physics interface and Graphics are Complete

Posted by Isaac Lichter on July 01, 2020

Two weeks ago, I left off with two goals for the core structure. The first that I worked on was creating a wrapper for the Body class. As I discussed two weeks ago, there was a difficulty with the way that that class, which is the bridge between the physics and the game, interacts with components.

The Gestalt library requires that the information in components be “serializable”, which means that the state of a component can be converted into a byte stream, for storage or for sending over the network. The Body class doesn’t allow for serialization, so it can’t be a field in a component.

In addition, putting a Body into a component violates an ECS design principle. Components should only contain information and/or indicate ways that an entity will behave. The actual logic should be handled by systems. Body is a mix of data and functionality, which doesn’t fit neatly into ECS design. More detail about that can be found here.

One of my mentors, Adrijaned, came up with a creative solution. He suggested creating a system to manage all of the bodies, using a hashmap to link each entity with its respective Body. Terasology, which is an inspiration for many aspects of this project, uses a similar structure. The entities have a BodyLinked component to indicate that they have an assciated Body. The components and events that have to do with the Body - position, angle, velocity, and force - interact with the Body through the new system. The code for that can be found here.

The other project that I worked on was creating an ECS-based way to interact with the graphics. In the previous code, the DrawableManager would maintain a list of all of Drawable objects and a list of the visible ones. When it was time to draw the game, the DrawableManager would go through all of the Drawable objects and see if they were visible, and if so, draw them.

In general, ECS architecture removes the need for maintaining lists. (The Body problem was a rare exception.) The ‘EntityManager’ can directly get every entity that has one or more specific components. Therefore, to render the entities, the EntityManager is queried for each entity with a Graphics component, a Position component, and an Angle (i.e. rotation) component. Then, the system renders that entity’s graphics at that position and angle.

The Graphics component contains a list of GraphicsElement objects. Each GraphicsElement contains four pieces of data: the Texture, which is the actual sprite to be drawn; the DrawableLevel, which represents how deep in the foreground or background the sprite should be drawn; the relativePosition, which is the spatial offset that the sprite should have from the entity’s center, and the relativeAngle, which is the rotational offset of the sprite from the entity. Each GraphicsElement represents a discrete part of the overall graphics for an entity. For example, a ship could have one GraphicsElement for its hull, a second element for its first gun, and a third element for its second gun. The code can be found here.

There are still a few bugs that need to be addressed before my graphics PR can be tested and merged. After that, I’ll do a prefab for Asteroid and Transcendent, and then get started on ship design.