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Why WebAssembly?

Performance.

How?

By reading a lot of documentation, then replacing the core loop of my program with an optimized loop written in C++.

/* BEGIN WEBASSEMBLY CODE */

let pixelBuffer = Module._malloc(
    pixels.length * Uint8Array.BYTES_PER_ELEMENT
);
Module.HEAPU8.set(pixels, pixelBuffer);
Module.ccall(
    "SwapColors",
    null,
    ["string", "array", "number", "number", "number"],
    [ColorMatchingMethod, palette, palette.length, pixelBuffer, pixels.length]
);

let convertedImage = new Uint8ClampedArray(
    Module.HEAPU8.buffer,
    pixelBuffer,
    pixels.length
);

Module._free(pixelBuffer);

/* END WEBASSEMBLY CODE */

The preceding code snippet is a great example of how to call an exported C++ method with a string, static array, integer, and dynamic array as parameters.

Gotcha's

When trying to hack this together, I encountered a couple of areas of confusion: scope, synchronization, and dynamic memory.

  • Scope

    • When I say scope, I'm referring to where my exported functions live and where I can call them.
    • At a minimum, you need a .wasm file to run your compiled code in the browser.
    • The easiest way to access and use this .wasm file is by importing the JavaScript glue file that's generated when compiling with emscripten.
    • The glue file will give you access to the "Module" object, which you can use to call functions exported from your C/C++ code such as malloc, free, and, in my case, the SwapColors method.

  • Synchronization

    • This is a small detail that I spent a lot longer on than I should have.
    • Basically, I was trying to initialize the page by making a call to an exported method, but was getting undefined reference errors: Uncaught TypeError: Cannot read properties of undefined (reading 'malloc') webassembly.
    • This error was occurring because the WebAssembly module hadn't fully loaded in, which confused me because it's the first script I import in "index.html."
    • To fix the error, I simply wait until the Module has fully loaded before calling any exported methods (including malloc): Module.onRuntimeInitialized = () => initializePreviewImages();.

  • Dynamic Memory

    • Code examples helped me out tremendously with this one.
    • To call methods that require dynamically allocated memory, you must use the HEAP found on the WebAssembly Module.
    • You can write to the HEAP using the "set" method and read from it by accessing the buffer (i.e. Module.HEAP.buffer) [1].

A Note On Compilation

emscripten offers many compiler settings. I would recommend sticking to what's necessary while making sure to include the optimization flag (use -O3 for the best performance). My final compiler command came out to:

emcc src/Main.cpp src/Color.cpp -o build/pixel-swap.js -O3 -sALLOW_MEMORY_GROWTH=1 -sEXPORTED_RUNTIME_METHODS=ccall -sEXPORTED_FUNCTIONS=_main,_SwapColors,_free
  • At first, I was using -sLINKABLE=1 and -sEXPORT_ALL=1 to resolve undefined symbol errors, but this resulted in a bloated output file and Module object.
  • -sALLOW_MEMORY_GROWTH=1 was necessary to store larger images (exceeding a few megabytes) on the heap.
  • I didn't include "cwrap" in EXPORTED_RUNTIME_METHODS because I didn't use it, but in other examples, you will see them include both.
  • The .js on the end of -o build/pixel-swap.js tells emscripten to build only the .wasm and .js glue file. You can also build a .html file, but I didn't because I wasn't going to use it.
  • -O3 is the highest level of optimization you can specify for the compiler and I would recommend including this in your production code otherwise you aren't fully taking advantage of the power of WebAssembly.

References

  1. Marco Selvatici. "Webassembly Tutorial - 10. Memory," https://marcoselvatici.github.io/WASM_tutorial/.
  2. emscripten. "Interacting with code," https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html.
  3. WebAssembly. "Developer's Guide," https://webassembly.org/getting-started/developers-guide/.
  4. MDN Web Docs. "Compiling an Existing C Module to WebAssembly," https://developer.mozilla.org/en-US/docs/WebAssembly/existing_C_to_wasm.