Porting Terraria and Celeste to the Browser with WebAssembly

Summary

Porting Terraria and Celeste to WebAssembly (tldr: terraria in the browser here, celeste in the browser here. terraria git repository, celeste git repository) One of my favorite genres of weird project is "thing running in the browser that should absolutely not be running in the browser". Some of my favorites are the Half Life 1 port that uses a reimplementation of goldsrc, the direct recompilation of Minecraft 1.12 from java bytecode to WebAssembly, and even an emulated Pentium 4 capable of running modern linux. In early 2024 I came across an old post of someone running a half working copy of the game Celeste entirely in the browser. When I saw that they had never posted their work publicly, I became about as obsessed with the idea as you would expect, leading to a year long journey of bytecode hacks, runtime bugs, patch files, and horrible build systems all to create something that really should have never existed in the first place. Strawberry Jam mod running in celeste-wasm Credits to r58 for figuring most of this stuff out with me and bomberfish for making the neat UI. Terraria I knew that both Celeste and Terraria were written in C# using the FNA engine, so we should have been able to port Terraria in the same way they did for Celeste, so we set that as a goal. The original post didn't have too many details to go off of, but we figured a good place to start was setting up a development environment for modding. In theory, all we needed to do was decompile the game, change the target to webassembly, and then recompile it. It turns out that we were very lucky with the game being C#— since the bytecode format (referred to as MSIL or just IL) maps very closely to the original code, and the game was shipped with the .pdb symbol database for mapping function names (and local variables!), we could get decompilation output that was more or less identical to the original code. Setting up a project Running ilspycmd on Terraria.exe, decompilation failed because of a missi...