Trying C++, OpenGL with WASM

Written by Lenvin Gonsalves.
on April 2, 2021

Disclaimer: This post is meant to put out my experience of using OpenGL with C++ & WASM. I know there is WebGL that could be used instead of what I've done, but I was really curious about WASM and wanted to learn more about it. So I decided to give life to my university's Computer Graphics project by porting it to WASM.

I hope my experience of learning how to use C++, OpenGL & WASM might be of some use to you.

What we are building

Setting up

The first most step is to download emscripten and set it up, You can find the instructions here.

After that, you need to be sure that you have appropriate files installed for C++ & OpenGL, in most Linux distros all the required packages are already present.

Making the C++ program ready for WASM

The OpenGL project I had done for my university was a train that translates (moves) along the x-axis, and this was supposed to be controlled by the user.

I directly compiled the program to WASM and included it in my project, it worked fine on computers (I could control the train using the keyboard), but the problem was on mobile devices.

So I had to expose two functions from the C++ program to the javascript, it was done as follows -

extern "C" void EMSCRIPTEN_KEEPALIVE move_train_forward() { 
    angle -= 0.01f;
    glutPostRedisplay();
 }

 extern "C" void EMSCRIPTEN_KEEPALIVE move_train_backward() { 
    angle += 0.01f;
    glutPostRedisplay();
 }

where angle is the position of the train, and glutPostRedisplay() causes the program to re-render. The two functions would move the train forward and backward.

Also, you'd want to make sure you are importing emscripten.h into your C++ program.

#include <emscripten.h>

After this, we are ready to compile the program to WASM.

emcc main.cpp -lGL -lGLU -lglut  -s WASM=1 -s LEGACY_GL_EMULATION=1 -O3 -o index.js

We are setting LEGACY_GL_EMULATION=1 because I am using an old version of OpenGL, feel free to skip it if you are using a newer version. The output is a js and wasm file, you'll need to use the js file in your HTML.

you can also compile the C++ program to a complete HTML, JS & WASM project by changing the command to

emcc main.cpp -lGL -lGLU -lglut  -s WASM=1 -s LEGACY_GL_EMULATION=1 -O3 -o index.html

Emscripten generates everything for you, you just need to run the project on a local server to see the result. But since we are planning to add much more functionality, we are just going to choose the js option.

Writing the HTML & JS

We need to create a canvas element, the output from the C++ program will be drawn on this canvas. Right after that, we are calling the js generated by Emscripten.

    <canvas id="canvas"></canvas>
    <script type="text/javascript">
      var canvas = document.getElementById("canvas");
      var Module = {
        canvas
      };
    </script>
    <script src="index.js"></script>

To control the train on mobile devices, I thought It'd be fun to control it using the accelerometer. The following code does it.

      const accelerometer = new Accelerometer({ frequency: 30 });
      accelerometer.addEventListener("reading", () => {
        const { x } = accelerometer;
        if (x > 0) {
          _move_train_forward();
        } else if (x < 0) {
          _move_train_backward();
        }
      });
      accelerometer.start();

Here _move_train_forward and _move_train_backward are the functions that we wrote in the C++ program (Emscripten adds the _ in the beginning). I've used the same methods to control the train via buttons as well.

The rest of the code is available in the Codeandbox above. I'm going to stop here, let me know if you have any doubts! Thanks for taking the time to read through this article.