On WebKit, Layers and Accelerated Compositing

This post is a brief and quick introduction to Accelerated Compositing and the role this plays in WebKit.

The point of accelerated compositing in WebKit is to take advantage of the GPU to accelerate the rendering of web content. To understand how this works one needs to know how a web page is actually rendered by WebKit.

Rendering in WebKit

WebKit groups DOM elements in layers that are rendered separately and then composited together to create the final page. This enables proper handling of transparent objects and overlapping contents for example. These layers, just like the DOM, conform a tree.

WebKit defines some rules to decide when a new layer is needed and which DOM elements should be included in it. For example, if you have a WebGL canvas or a video element, these will go to separate layers, if you use explicit CSS position properties on an object you get another layer for it, etc.

When it is time to paint the web page on the browser, the work consists of traversing the layer tree and composite the layers together to create the final view of the web page. For example, if you have a transparent layer sitting on top of some other layer in the page, the composition will take the two individual layers that have been rendered independently and blend them together in the final page, in the right order, at the right position and with the right
transparency.

Accelerated Compositing

Before we had accelerated compositing, the compositing process happend in software, that is, it was the CPU that would do all the layer compositing work, which is expensive and can hog the CPU, making for a worse user experience. Accelerated compositing however, involves offloading the compositing of the layers to the GPU hardware. It turns out that GPUs can do the compositing very fast and doing so would also free the CPU, delivering a better, more responsive, user experience too.

Conclusions

Accelerated compositing is all about making a better use of the available graphics hardware, offloading the work required to composite the final view of the webpage from the various layers it contains, which results in a better user experience and better overall rendering performance.

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