OffscreenCanvas update

Hold up, a blog post before a year’s up? I’d best slow down, don’t want to over-strain myself 🙂 So, a year ago, OffscreenCanvas was starting to become usable but was missing some key features, such as asynchronous updates and text-related functions. I’m pleased to say that, at least for Linux, it’s been complete for quite a while now! It’s still going to be a while, I think, before this is a truly usable feature in every browser. Gecko support is still forthcoming, support for non-Linux WebKit is still off by default and I find it can be a little unstable in Chrome… But the potential is huge, and there are now double the number of independent, mostly-complete implementations that prove it’s a workable concept.

Something I find I’m guilty of, and I think that a lot of systems programmers tend to be guilty of, is working on a feature but not using that feature. With that in mind, I’ve been spending some time in the last couple of weeks to try and bring together demos and information on the various features that the WebKit team at Igalia has been working on. With that in mind, I’ve written a little OffscreenCanvas demo. It should work in any browser, but is a bit pointless if you don’t have OffscreenCanvas, so maybe spin up Chrome or a canary build of Epiphany.

OffscreenCanvas fractal renderer demo, running in GNOME Web Canary

Those of us old-skool computer types probably remember running fractal renderers back on their old home computers, whatever they may have been (PC for me, but I’ve seen similar demos on Amigas, C64s, Amstrad CPCs, etc.) They would take minutes to render a whole screen. Of course, with today’s computing power, they are much faster to render, but they still aren’t cheap by any stretch of the imagination. We’re talking 100s of millions of operations to render a full-HD frame. Running on the CPU on a single thread, this is still something that isn’t really real-time, at least implemented naively in JavaScript. This makes it a nice demonstration of what OffscreenCanvas, and really, Worker threads allow you to do without too much fuss.

The demo, for which you can look at my awful code, splits that rendering into 64 tiles and gives each tile to the first available Worker in a pool of rendering threads (different parts of the fractal are much more expensive to render than others, so it makes sense to use a work queue, rather than just shoot them all off distributed evenly amongst however many Workers you’re using). Toggle one of the animation options (palette cycling looks nice) and you’ll get a frame-rate counter in the top-right, where you can see the impact on performance that adding Workers can have. In Chrome, I can hit 60fps on this 40-core Xeon machine, rendering at 1080p. Just using a single worker, I barely reach 1fps (my frame-rates aren’t quite as good in WebKit, I expect because of some extra copying – there are some low-hanging fruit around OffscreenCanvas/ImageBitmap and serialisation when it comes to optimisation). If you don’t have an OffscreenCanvas-capable browser (or a monster PC), I’ve recorded a little demonstration too.

The important thing in this demo is not so much that we can render fractals fast (this is probably much, much faster to do using WebGL and shaders), but how easy it is to massively speed up a naive implementation with relatively little thought. Google Maps is great, but even on this machine I can get it to occasionally chug and hitch – OffscreenCanvas would allow this to be entirely fluid with no hitches. This becomes even more important on less powerful machines. It’s a neat technology and one I’m pleased to have had the opportunity to work on. I look forward to seeing it used in the wild in the future.

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