Coding Projects WASM

Converting an old ray tracer to Wasm

Back around the time the first iPhone was released (feeling old! 😬) I was taking a course where we built a CPU-based ray tracer in C.

If you’re not familiar with the ray tracing technique, check it out here.

What I mostly remember from the course was math, pointers, and segfaults. Oh the segfaults. By the end of the course, however, I had a decent grasp on C and that’s been a valuable skill so many times in my career and hobbies.

How the original project functioned

  1. You wrote an input file that describes various shapes, light sources, and attributes of the environment.
  2. You fed the file into the C program.
  3. You waited a bit (remember, we’re doing everything on the CPU in ~2007).
  4. You got a fancy PPM file. (PPM was a great image format due to its simplicity – we were dealing with enough!)

Wasm motivation

Recently I decided that I wanted to learn more about the inner workings of WebAssembly (Wasm) and figured this would be a great candidate project. It’s fully contained without any external dependencies, *I wrote all the code so it shouldn’t be too mysterious, and if I got it to work there would be a visual payoff.

*Feel free to judge some of the rough spots in the code – it was a long time ago!


The first thing I made sure of was that I could compile the project locally the non-Wasm way. There were no hiccups there – it worked on the first try using the Makefile. ✅

I then started reading this tutorial on converting a C project to Wasm. After installing emscripten on macOS (I used Homebrew) I decided to add a new C source file to the project and added a function that looked something like:

#include "emscripten.h"

int version() {
  return 42;

If I could get this to work I could at least get information from C -> JS for starters. All that it took to make this work was:

  1. Substituting gcc with emcc in the Makefile
  2. Making sure I added the EXPORTED_RUNTIME_METHODS='["cwrap"]' compiler flag
  3. Calling Module.cwrap from JS to use the function

That was pretty much it. I’m not going to go super in-depth with this blog post because I think most of it can be figured out from the source.

Next challenges

I had a bit more to go but was surprised at how easy it was to send a value from C to JS. The next items to figure out were:

  1. The ray tracer expected an input file and an output file, how would this work with a browser?
  2. We can pass integers easily, but what about the big array of pixel data when we ultimately generate an image?
  3. Where would our C calls to fprintf and its siblings go when trying to debug to stdout and stderr?
  4. What about main – does it run?

I’ll go ahead and spoil these really quickly in their respective order:

  1. fmemopen saved the day by taking the input string (which is a char *) and providing a FILE type in return which is an in-memory buffer stream. In other words, no massive overhauling needed although we aren’t using “real” files anymore. In addition a slight refactoring was done to the project to return an array of pixels rather than write out an image file.
  2. From what I understand, Wasm and C can share a heap and both just need to know where to find the data via pointers. Here’s an example of sending a pointer to C, and here’s an example of how JS grabs a pointer from C. In the latter, C sets a global (gross, I know, but they used it in their examples as well) and JS calls a function to get that int. It then is able to initialize an array of UInt8s.
  3. They automagically show up in the browser console! This is a really nice feature, and stderr calls are even properly displayed as errors.
  4. Yes! In my case I got rid of it because it was prompting for the CLI input, but it was interesting to see that it automatically ran. There may be a compiler setting to disable this.

A summary of what it took to convert the ray tracer to Wasm

  1. Tweaked the Makefile to use emcc.
  2. Removed main() because I didn’t need it.
  3. Used fmemopen to substitute a real file with a char array.
  4. Refactored the project to not try to write to a file, but instead return a big array of pixels that ultimately get passed to JS to write to a Canvas.
  5. Expanded the pixel struct to include an alpha channel for the expected RGBA format. Yay for properly using sizeof throughout the code.
  6. Wrote a C source file with everything we needed to interface with JS.
  7. Created an HTML page that calls our compiled JS and gives us access to the exposed functions.
  8. Created a big string using a JS template literal for our input.

That’s mostly it! Check out the GitHub repo here. 🚀