Chris Harrelson, writing on Google's Chromium Blog:


In order to display content at 60fps, Chrome has only 16ms to render each frame, including JavaScript execution, style, layout, painting, and pushing the resulting pixels to the user's screen. Of that 16ms time budget, the less Chrome uses, the more time web developers have to run scripts, load content, and delight their users. Many of our recent improvements to the rendering pipeline focus on reducing the per-frame workload, rather than simply improving Chrome's raw speed.

For example, when Chrome is preparing to paint pixels to the screen, it must first identify which elements on the page need to be redrawn and which can be copied from the previous frame's cache. On highly dynamic pages with frequent DOM changes, this performance cost can add up quickly. To simplify this task, Chrome now tracks the draw commands generated for each element and can identify visually non-overlapping subsets. If one of these subsets hasn't been modified, the entire block can be copied directly from the cache without any additional work. This optimization reduces the time it takes to paint a new frame to the screen by up to 35%.

Chrome also groups the pixels into tiles to enable smaller and faster updates to the screen. Previously, Chrome would redraw any of these tiles that had been modified by a DOM update, but this is only optimal if the majority of a tile's area needs to be redrawn. If only a few pixels have changed, it's faster to copy the entire tile from the previous frame and then update the new pixels. Chrome can now intelligently choose the redraw pipeline that will be faster, reducing tile redraw time by up to 40%.

While great news from Chrome in general, this is incredible news for anyone wondering when Google is going to get Chrome into VR. The focus on 60fps as a default animation target is critical for VR, and the more focused redraw spaces are a great deal more important when the search bar is being observed as though it were a 3D image. The WebVR API is still very much an experimental thing, but with the increased focus on Daydream and competing products making a lot more noise about being ready soon these performance details mean a lot.

It's the difference between slapping together a proof of concept so people can use it right away and delivering a polished product that everyone will want in every VR headset. If you're using Chrome in your VR headset, you're far more likely to continue using Chrome everywhere else.

Google Daydream



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