> This guide will be relevant until OS compositors fundamentally change from just dealing with bitmaps. Which is unlikely to happen anytime soon.
There is absolutely no reason why browsers have to use the native compositor for CSS. It's a bad fit, and browsers should stop doing it.
> And doing UI layout on background threads breaks the basic design of pretty much every UI framework, web or native, which are usually single-threaded.
That's why you have a render tree on a separate thread from the DOM. Look at how Servo does it (disclaimer: I work on Servo). We've proven that it works.
> And doing UI layout on background threads breaks the basic design of pretty much every UI framework, web or native, which are usually single-threaded.
By "single-threaded" I meant business logic and non-fixed layout being done on the same thread. "Rendering" (drawing to a bitmap or translating to the OS compositor's object model or direct OpenGL/DirectX/etc.) is done on a separate thread in most frameworks I'm aware of.
> There is absolutely no reason why browsers have to use the native compositor for CSS. It's a bad fit, and browsers should stop doing it.
Here are a few:
- On some platforms, animations on native layers are applied every frame, even if the application that owns the layer is busy. This means fewer opportunities to drop frames.
- Sometimes web rendering engines are embedded in apps. Your app may want to draw web content that filters some other content that's behind it. This content may not be available to the browser engine (it may be in a separate process, for example). A native layer can apply this filter in the compositor process, where the content is available.
- Using the native compositor makes it easier to embed components (like video) that are provided by the system as native layers.
> - On some platforms, animations on native layers are applied every frame, even if the application that owns the layer is busy. This means fewer opportunities to drop frames.
The browser can and should do the same with its CSS compositor. There's no reason why the CSS compositor should run on the main thread (and, in fact, no modern browser works this way).
> - Sometimes web rendering engines are embedded in apps. Your app may want to draw web content that filters some other content that's behind it. This content may not be available to the browser engine (it may be in a separate process, for example). A native layer can apply this filter in the compositor process, where the content is available.
For transparent regions, a browser can do this by simply exporting its entire composited tree as a single transparent layer, where it can be composited over other content. In the case of a single-layer page, this is what the browser is doing anyway.
If you're talking about CSS filters, there's no way that I know of in CSS to say "filter the stuff behind me" in the first place. You can only filter elements' contents.
> - Using the native compositor makes it easier to embed components (like video) that are provided by the system as native layers.
I grant that you have to use the native compositor to get accelerated video on some platforms. But that doesn't mean that a browser should do everything this way. In fact, no browser even tries to export all of CSS to the compositor: this is why you have the various "layerization" hacks which give rise to the sadness in this article. Reducing what needs to be layerized to just video would actually decrease complexity a lot over the status quo. (If you don't believe me, try to read FrameLayerBuilder.cpp in Gecko. It would be way simpler if video were the only thing that generated layers.)
> The browser can and should do the same with its CSS compositor. There's no reason why the CSS compositor should run on the main thread (and, in fact, no modern browser works this way).
You still have to swap buffers from your background thread and then composite the buffer instead of compositing the animating layer directly. It's a small advantage, but it is an advantage.
> If you're talking about CSS filters, there's no way that I know of in CSS to say "filter the stuff behind me" in the first place. You can only filter elements' contents.
> You still have to swap buffers from your background thread and then composite the buffer instead of compositing the animating layer directly. It's a small advantage, but it is an advantage.
By this I assume you mean that when you have two compositors, you have an extra blit. This is mostly true (though it's not necessarily true if the OS compositor is using the GPU's scanout compositing), but it's by no means worth the enormous downsides of current layerization hacks. Right now, when you as a Web developer fall off the narrow path of stuff that the OS compositor can do, your performance craters. The current status quo is not working: only about 50% of CSS animations in the wild are performed off the main thread.
There's another enormous downside to using the OS compositor: losing all Z-buffer optimizations. Right now, browsers usually throw away 2x or more of their painting performance painting pixels that are occluded. When using the OS compositor, the browser painting engine doesn't know which pixels are occluded, because that's something only the OS compositor knows, so it has to paint the contents of every buffer just in case. But with a smart CSS compositor, the browser can early Z-reject pixels covered up by other elements.
> yes, it's an experimental property, but it's the one I was thinking about
Ah, OK, I wasn't aware of that because it's only implemented in Safari right now. Well, using the OS compositor would make it easier to apply backdrop filters, as long as the OS compositor supports everything in the SVG filter spec (a big assumption—I suspect this is only the case on macOS and iOS!) But even with that, I think it results in less complexity to just use the OS compositor for this specific case and fall back on the browser compositor for most everything else, just as with video. CSS really does not map onto OS compositors very well.
There's a good reason to, in this case: that the OS compositor is not designed to render the full generality of CSS.
The status quo is not working: only about half of CSS animations in the wild run on the compositor. As browser vendors, we need to admit that the attempt to carve out a limited, fast subset of CSS has failed, and we should do the hard work to make all of CSS run fast.
There is absolutely no reason why browsers have to use the native compositor for CSS. It's a bad fit, and browsers should stop doing it.
> And doing UI layout on background threads breaks the basic design of pretty much every UI framework, web or native, which are usually single-threaded.
That's why you have a render tree on a separate thread from the DOM. Look at how Servo does it (disclaimer: I work on Servo). We've proven that it works.