/OpenRCT2

An open source clone of Roller Coaster Tycoon 2 built by decompiling the original game one bit at a time.

Primary LanguageCGNU General Public License v3.0GPL-3.0

OpenRCT2

An open source clone of Roller Coaster Tycoon 2 built by decompiling the original game one bit at a time.

Contents

1 Introduction

1.1 Background

OpenRCT2 is an attempt to decompile RollerCoaster Tycoon 2 into C. RollerCoaster Tycoon 2 was originally written in MASM and Visual C++ where functions related to interfacing with the operating system were written in C (supposedly 1%) and the rest of the game in pure x86 assembly. OpenTTD went through the same treatment where Transport Tycoon Deluxe was decompiled into C and then have thousands of more features added to the game. RollerCoaster Tycoon 2 uses the third version of Chris Sawyer's engine. It still shares some code seen in Transport Tycoon and this is reflected in OpenTTD 0.1 such as the window system and graphics rendering. Chris Sawyer's Locomotion is a more up to date version, however OpenRCT2 will only use RollerCoaster Tycoon 2's engine code for now.

1.2 Decompiling the game

In order to decompile the game gradually and successfully, each procedure in RollerCoaster Tycoon 2 is to be re-written in C one by one. To help test the accuracy of the re-written procedures, the decompiled C procedures are compiled into a DLL (openrct2.dll) which exports an entry procedure mimicking the WinMain function in RollerCoaster Tycoon 2. The original executable rct2.exe has been patched so that openrct2.dll and WinMain are in the DLL import table and the WinMain export procedure in openrct2.dll is called at the start of the WinMain procedure in rct2.exe before returning. This way, starting rct2.exe now simply calls the new DLL which can then run all the decompiled code whilst still able to read / write to the rct2.exe memory model and run rct2.exe procedures.

The project therefore acts as a patch to RollerCoaster Tycoon 2 which can gradually implement each procedure whilst also adding new features where possible. Until all procedures of the original game are re-written in C, the project must remain a DLL which is called from the patched rct2.exe.

1.3 Progress

Currently the window system, graphics rendering and basic game loop are gradually being decompiled. Decompiling the all the game windows is a convenient way of identifying the game's memory structure. SDL2 has been used to replace the operating system calls so that game is cross-platform after the original game is no longer required.

1.4 Aim

The aim is to decompile RollerCoaster Tycoon 2 fully into C so that it can remain a cross platform game which runs on the latest operating systems with new features introduced and game play experience to be improved just like OpenTTD. Allowing the game to run in a resizeable window has already been accomplished using SDL2. Once the game has been fully decompiled, game logic such as peep path-finding, window / ride / object / map / construction limits increased, more sandbox friendly, editing available objects during a game, improved title sequence, mechanics only found in RCT1 (e.g. Shuttle Loop compatibility, pay for rides and park, have fun objective, finish five coasters objective, mountain tool during game) and lots more.

2 Building the source code

2.1 Prerequisites

2.2 Compiling and running

  1. Checkout the repository. This can be done using GitHub Windows or other tools.
  2. Download SDL2 development library for Visual C++ and copy it to a new directory called "sdl" in the repository. This directory should contain "include". The path should be something like \GitHub\OpenRCT2\sdl\include\.
  3. Open the solution in the projects directory (openrct2.vcxproj) with Visual C++.
  4. In rct2.c, GAME_PATH can be edited to reflect your RollerCoaster Tycoon 2 installation. Each slash needs to be marked twice, like this: C:\\Program Files (x86)\\GOGcom\\RollerCoaster Tycoon 2\\RollerCoaster Tycoon 2 Triple Thrill Pack. Existing registry keys and the original RCT2 executable are not required.
  5. Select the 'Release' configuration and click Build -> Rebuild Solution. The dropdown menu to enable the 'release' configuration is towards the top of the VS Express window, near the "TEST" menu.
  6. Start debugging. Press the "Local Windows Debugger" button with a green "play" icon next to it. If warned about openrct2.exe not having debug information, just continue.
  7. If the game crashes, you may need to press the red, square button along the top of VS Express (for "stop") to stop the program.

3 Contributing

3.1 Decompiling

Experience with reverse engineering and x86 assembly is neccessary to decompile the original game. The game is currently being decompiled using IDA. Feel free to to take a procedure that hasn't currently been decompiled and decompile it. Contact IntelOrca for more information and for the lastest IDA database.

3.2 Naming of procedures and variables

Many variables and procedures are referenced in OpenRCT2 only by address. This may be because their function has not yet been identified. These can often be identified by removing their call and checking how they affect the game or reverse engineering the original game assembly to see where it is called / used.

3.3 Cleaning and documenting the source code

A lot of the source code is undocumented and messy. Whilst the structure of the code should be kept the same so that it closely resembles the original game. Various blocks of code can be moved into smaller functions and macros can be created for common operations.

3.4 Implementing new features / fixing bugs

If enough of the game has been decompiled to implement a certain feature or fix a certain bug. This can be written. Comments should be added to clearly identify where code has been changed on purpose causing it to differ from the original game assembly.

4 License

OpenRCT2 is licensed under the GNU General Public License version 3.