Facebook announced that it is open sourcing Pop, the animation engine behind its recently launched Paper app.

Facebook released Paper in January. It’s a social news reader app for iOS. If you’re unfamiliar with it, watch this:

Pop is responsible for the smooth animations and transitions that take place within the app, including scrolling, bouncing, and unfolding effects.

“Since the original iPhone, iOS has excelled at supporting static animations. The Apple-provided Core Animation framework makes linear, ease-in, ease-out, and ease-in-ease-out animations simple to leverage,” says Kimon Tsinteris, an engineer at Facebook. “The innovation of touch interfaces has ushered in a new wave of software design. Direct manipulation of on-screen elements has removed one level of indirection, which in turn has raised our expectations for the screen as a medium. If objects respond to our touches, they should also respond to the velocity of our flick.”

“When I co-founded Push Pop Press in 2010, our goal was to create a realistic, physics-everywhere experience,” he adds. “We wanted a solution that would allow us to evoke the same delightful experience of UIScrollView throughout the whole application. Pop is the latest manifestation of that vision, allowing us to keep the familiar and powerful programming model of Core Animation while also capturing a gesture’s velocity and better reflecting user intent. Paper has given us the opportunity to further refine both the vision and the animation engine behind it.”

Facebook acquired Push Pop Press back in 2011. Here’s what it looked like at the time:

Tsinteris says they want to make commonly needed animations convenient, and make it easier to create unique effects in apps. They also want to create a developer-friendly, but powerful programming model.

Pop joins other open source iOS releases from Facebook including KVOController, Shimmer, and Tweaks.

You can find Pop on GitHub here.

Image via Facebook