Skyscraper Internals and Development

Working with the Skyscraper source code can be intimidating at first, but if you're familiar with C++, the code has been formatted to be easy to read and hopefully understand.
The following diagram shows a birds-eye view of the simulator, when three sim engines are running.
Skyscraper's source code is divided class-wise in order to create layers of abstraction in the code. This makes it relatively easy to create object-related code in C++. In the image above, each major component is in a subfolder in the "src" directory. The SBS engine, as an example, is in the "src/sbs" folder. Skyscraper's frontend is relatively small, and is mainly used as a wxWidgets frontend to the simulator, the ability to add other frontends is possible.
Frontend
Skyscraper's frontend is is both a small wxWidgets application and separately (if wxWidgets is not used), a native Ogre frontend for Skyscraper. It is used for the startup screen, initializing the simulator, it's initial objects, and the initial wxWidgets windowing framework.
VM (Virtual Manager)
VM, or Virtual Manager, is a simulator management frontend, designed to manage simulator engine instances. It operates in a similar way to a nanokernel or hypervisor, in that it is a monitor and environment for the simulator engines to run on. VM is responsible for creating simulator engine instances, shutting down engines, running sim engines in parallel, and moving between sim engines. It also manages the client components, such as the GUI and HAL. VM manages Engine Contexts, which are the basic environments that host the sim engines and script processors.
GUI
The GUI component is the majority of the wxWidgets windowing code, used for the simulator's windows and dialogs. This component is a client of the VM (Virtual Manager) component.
HAL
The HAL (Hardware Abstraction Layer) component is responsible for all hardware library-related functions, such as graphics and sound, currently for the frontend. In the future this will also handle all related functions for SBS, currently SBS does most of the same functionality separately. This component is a client of the VM (Virtual Manager) component.
Script Processor
The Script Processor component is not only the script interpreter, but also demonstrates how to perform the C++ equivalents of script code. The script processor is responsible for translating script commands into C++ functions. The language is loosely based on the TrueVision3D graphics engine syntax (Visual Basic).
OpenXR
The OpenXR component is an experimental Virtual Reality feature. Skyscraper currently supports VR headsets, but doesn't yet support hand controls and some other things like on-screen displays.
Scalable Building Simulator (SBS)
The Scalable Building Simulator, or SBS, is a simulation engine designed with vertical simulation in mind. SBS was designed to simulate a single building in it's entirety, multiple SBS instances can be run together in parallel to create cities. SBS is a client of the VM (Virtual Manager) component. It runs within an environment called an Engine Context, which is created and managed by the VM system. An engine context is the combination of an instance of SBS and a Script Processor, together in a context.
PolyMesh
PolyMesh, Skyscraper's geometry processor, is a subsystem of SBS. It was developed starting in 2010 to replace the Crystal Space game engine's ThingMesh system, which Skyscraper Alpha 7 and earlier used. ThingMesh was removed from Crystal Space, prompting the creation of a new geometry processor, followed by Skyscraper moving over to the Ogre graphics engine in late 2010, with the PolyMesh switch having made the Ogre transition possible. PolyMesh, via the DynamicMesh system, generates 3D geometry and render buffers, that are submitted to Ogre for rendering, and are uploaded to the graphics card in batches.