Most people now know that modern web browsers use the GPU to render parts of web pages, especially ones with animation. For example, a CSS animation using the
transformproperty looks much smoother than one using the
topproperties. But if you ask, “How do I get smooth animation from the GPU?” in most cases, you’ll hear something like, “Use
These properties have become something like how we used
zoom: 1for Internet Explorer 6 (if you catch my drift) in terms of preparing animation for the GPU — or compositing, as browser vendors like to call it.
But sometimes animation that is nice and smooth in a simple demo runs very slowly on a real website, introduces visual artefacts or even crashes the browser. Why does this happen? How do we fix it?
- Watch out for the number and size of composite layers from the very beginning — especially ones created by implicit compositing. The “Layers” panel in your browser’s development tools is your best friend.
- Modern browsers make heavy use of compositing not just for animation but to optimize the painting of page elements. For example,
position: fixedand the
videoelements use compositing.
- The size of compositing layers is likely be more important than the number of layers. In some cases, the browser will try to reduce the number of composite layers (see the “Layer Squashing” section of “GPU Accelerated Compositing in Chrome“); this prevents so-called “layer explosion” and reduces memory consumption, especially when layers have large intersections. But sometimes, such optimization has a negative impact, such as when a very large texture consumes much more memory than a few small layers. To bypass this optimization, I add a small, unique
translateZ()value to each element, such as
translateZ(0.0002px), etc. The browser will determine that the elements lie on different planes in the 3D space and, thus, skip optimization.
- You can’t just add
will-change: transformto any random element to virtually improve animation performance or to get rid of visual artifacts. GPU compositing has many drawbacks and tradeoffs to be considered. When not used sparingly, compositing will decrease overall performance at best, and crash browsers at worst.
Allow me to remind you of the big disclaimer: There is no official specification for GPU compositing, and each browser solves the same problems differently. Some sections of this article may become obsolete in a few months.