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@@ -24,9 +24,94 @@ workflows.
 
 \section{Overview}
 
-\subsection{Core}
+As described above, the cr\^epe game engine's goal is to offer a Unity-like
+experience tailored for developing 2D games similar to Jetpack Joyride. That is why
+Jetpack Joyride and Unity provided the main inputs for this game engine design.
+Firstly, a quick overview will be given of the Unity game engine, in particular the
+\gls{ecs}. Secondly, this Overview will quickly talk you through some of the most
+important parts of the game engine, and why these parts are needed to create the
+Jetpack Joyride game.
+
+\subsection{ECS}
+
+The Unity game engine is structured using the Entity Component System (\gls{ecs}) (as
+shown in \cref{fig:ecs-block-diagram}). The \gls{ecs} is made out of three main
+subsystems, namely entities, components and systems. Entities are just IDs. An entity
+is also called a GameObject in Unity and it is made out of one (or more) components.
+Components are the classes that hold the data. The components determine what kind of
+entity it is (e.g. an enemy, audio, and so on). Systems take care of the behavior of
+the entities. Systems mainly read and write the enity's components data. The
+\gls{ecs} clearly distinguishes the data (components) from the functionality
+(systems).
 
-\subsection{Patterns}
+\begin{figure}
+	\centering
+	\includegraphics[width=0.5\textwidth]{img/ECSBlockDiagram.png}
+	\caption{ECS design pattern}
+	Source: \autocite{img:ecs-block-diag}
+	\label{fig:ecs-block-diagram}
+\end{figure}
+
+The \gls{ecs} will also be used at the cr\^epe game engine. Everything (from the
+protagonist and bullets to the walls and enemies) in the cr\^epe game engine will be
+a GameObject (i.e.~entity). The game programmer must program his game by creating all
+kind of GameObjects and placing them in one (or multiple) scenes, just like Unity.
+
+\subsection{Jetpack Joyride}
+
+Firstly, some background information about Jetpack Joyride. Jetpack Joyride is a
+side-scrolling endless runner action video game created by Halfbrick Studios. The
+protagonist is called Barry Steakfries, who the player controls as he steals a
+bullet-powered jet pack from a top-secret laboratory
+\autocite{wikipedia:jetpack-joyride}. A screenshot from the game can be seen in
+\cref{fig:jetpack-joyride} (pleae be aware that the goal of this project is not to
+create an exact replica of Jetpack Joyride, it is only used as a source of
+inspiration).
+
+\begin{figure}
+	\centering
+	\includegraphics[width=0.5\textwidth]{img/JetpackJoyride.jpg}
+	\caption{Jetpack Joyride}
+	Source: \autocite{img:jetpack-joyride}
+	\label{fig:jetpack-joyride}
+\end{figure}
+
+The protagonist wears a jetpack with which he can float in the air. The player must
+avoid obstacles (such as lasers, missiles and zappers) by floating at the right
+height. The player can control the protagonist's jetpack, thereby also controlling
+the protagonist's height. The protagonist experiences gravity and other forces (like
+the force from his jetpack pushing him upwards). These forces should be easily
+programmable by the game programmer. That is why a physics system is needed in the
+cr\^epe game engine. Only very limited/easy physics are needed for Jetpack Joyride,
+that is why this is only supported by the cr\^epe game engine.
+
+The protagonist must avoid obstacles. That is why the cr\^epe game engine should also
+support a collision system. Again, only very limited/easy collision is needed for
+Jetpack Joyride, that is why only very limited/easy collision is supported by the
+cr\^epe game engine.
+
+The game must, of course, also be visible to and playable by the user. A rendering
+system will take care of rendering (displaying) the game and its GameObjects. An
+input system will take care of all the inputs (mouse and keyboard).
+
+Jetpack Joyride also offers audio. A system will take care of the audio in the
+cr\^epe game engine.
+
+Particles are very common in Jetpack Joyride, e.g. underneath the jetpack and behind
+the rockets. Particles will be supported by the particle system.
+
+The start of a scene is described in a scene. However, the game programmer might also
+want to write game logic code which is running during the game (e.g. to switch to a
+new scene or to perform a custom action at a collision). For these purposes, Unity
+uses scripts. These scripts will also be supported by the cr\^epe game engine.
+
+Finally, as an extra, replay functionality will be supported by the cr\^epe game
+engine. A dedicated replay system will be used to support replay.
+
+It turns out that a physics, collision, rendering, input, audio, particle, script,
+and replay system are needed to create the Jetpack Joyride game. These systems form
+the main part of the \gls{ecs}. The design of these eight systems in combination with
+\gls{ecs}, will be briefly discussed in the next parts of this design document.
 
 \section{Design}