I’ve had an emscripten build of my gameboy emulator binjgb since September of last year. At first, I just got enough working to render the display. If you just use emscripten’s SDL implementation, it does most of this for you automatically, but I wanted to write it myself to see how slim I could make it. Take a look at it running:

This was enough to satisfy me for a while, but I didn’t like that it was so janky. Here’s a little snippet of the implementation:

function start() {
  ...
  update();
  requestAnimationFrame(render);
}

function update() {
  ...
  _run_emulator_until_event(...);
  ...
  // Synchronize the host with the emulated gameboy.
  if (real_ms < delta_ms) {
    setTimeout(update, delta_ms);
  } else {
    setTimeout(update, 0);
  }
}

function render() {
  ctx.putImageData(imageData, 0, 0);
  requestAnimationFrame(render);
}

You can see the full commit here.

Using the Chrome devtools, you can see some of the issues causing jankiness:

initial jank

You can read about the timeline view here. The important parts for us here are the red marks (“long frames”). It looks like Chrome calls a frame “long” if it takes longer than 22ms. Since 60fps is 16.6ms per frame, this gives considerable leeway before telling you that you screwed up somewhere.

Skipping down, you can see a frame that took 32.0ms. Not good. In this case, there isn’t much we could have done because the update just took a long time that frame (27.08ms).

Looking at the frame times after that, there’s definitely room for improvement. We’re trying to get as close to 16.6ms as possible. Notice that the frame time jumps around a lot: 15.8ms, 16.8ms, 20.9ms, 14.3ms, etc.

This happens because the timer is firing to try and synchonize the speed of the emulator with the speed of a gameboy. The timer callback runs for a while (typically between 6-8ms). This blocks the requestAnimationFrame callback from running. It will fire as soon as it can, but this will often be later than we want for a solid framerate.

The simplest way to fix this is to use requestAnimationFrame the way it is meant be used: do all the work for updating the frame in the requestAnimationFrame callback. See the MDN docs.

I didn’t get around to improving this until I added audio earlier this month; read all about it in the next post!