Adventure Game Interpreter for Sierra's classic 80's adventure games written in TypeScript.
This project aims to recreate one of the most beautiful game engines ever created. Sierra On-Line's Adventure Game Interpreter was created in the mid 80's by Sierra employees Jeff Stephenson and Chris Iden and quickly became the cornerstone on which many of Sierra's early titles would be based. AGI was built around the concept of a virtual machine that made it fairly easy for Sierra to port their games to multiple platforms.
The virtual machine was based around an in-house developed C-like programming language called "Logic". When the programmer compiled a Logic file, the compiler produced an 8-bit "opcode" for each statement. In the interpreter, each opcode was defined in a jump table that pointed to an assembler subroutine. This made the system very fast and ridiculously elegant because the logic itself never needed to be recompiled, no matter what the target platform was. It was also reasonably easy to extend the system, which can be witnessed by Sierra's numerous versions of AGI interpreters.
By the time these games were created, storage was limited and expensive. To reduce asset size, the game's background images were defined as vector graphics instead of full raster images. When the game needed to display a new background image, it was drawn into a buffer and finally blitted to the screen. This is interesting because this theoretically allows us to rescale the images to any resolution. It's also interesting from an artistic perspective as it allows us to trace how the artist originally drew the pictures, command by command.
Another interesting feature of the engine was its ability to render a room with a pseudo-perspective. The engine divided the screen into 15 sections, called priority bands. This was a kind of low-resolution Z-buffer. All background drawing commands were drawn both with a visual color and a priority value, so every time the engine had to draw an animated object, each of its pixels was compared against the background pixel's priority value to decide if the pixel was to be drawn or not. All control lines and boundaries where the player could not go were also drawn into this priority buffer.
Most of the code has been "clean room" implemented from the specs defined by Lance Ewing, Peter Kelly, Claudio Matsuoka, Stu George, and David Symonds at http://wiki.scummvm.org/index.php/AGI/Specifications. Some parts of the PIC rendering code were borrowed from Lance Ewing's "SHOWPIC" program.
This TypeScript implementation parses the original asset files without any precompilation of any sort. The system is divided into a renderer, a logic parser, an interpreter, and a resource manager. The resource manager is mostly complete, save for a few assets that haven't been taken into consideration yet (dictionary, etc.).
The interpreter works in cycles, best described in the documentation above. Every cycle, the interpreter executes logic script 0 and any referenced logic scripts. The script itself may execture commands on the interpreter that update game objects and draw them to both a visual frame buffer and a priority buffer. The visual buffer is at the end of the cycle, blitted to a canvas.
The logic parser is working well, according to my tests. The system mainly needs work on the interpreter; the main issue is that some details are missing from the specs defined above and need slight reverse engineering.
Please fork and contribute! If you make anything useful, please provide a pull request so I can integrate your work into the main repo.
Licensed under the GPL version 3.
This is in no way affiliated with or endorsed by Sierra Entertainment or any other company.
To build the project, use yarn tsc
AGI.js in CrafterCMS: Play and Edit games using AGI.js and CrafterCMS. https://github.com/russdanner/craftercms-sierra-agijs