The Animation Compression Library in Unreal Engine 5.3

17 Sep 2023

The very first release of the Animation Compression Library (ACL) Unreal Engine plugin came out in July of 2018 and required custom engine changes. It was a bit rough around the edges. Still, in the releases that followed, despite the integration hurdles, it slowly grew in popularity in games large and small. Two years later, version 1.0 came out with official support of UE 4.25 through the Unreal Engine Marketplace. Offering backwards compatibility with each new release and a solid alternative to the stock engine codecs, the plugin continued to grow in popularity. Today, many of the most demanding and popular console and mobile games that use Unreal already use ACL (Fortnite itself has been using it for many years). It is thus with great pleasure that I can announce that as of UE 5.3, the ACL plugin comes out of the box with batteries included as the default animation compression codec!

This represents the culmination of many years of hard work and slow but steady progress. This new chapter is very exciting for a few reasons:

• Better quality, compression and decompression performance for all UE users out of the box.
• A dramatically smaller memory footprint: ACL saves over 30% compared with the defaults codecs in UE 5.2 and more savings will come with the next ACL 2.1 release later this year (up to 46% smaller than UE 5.2).
• Maintenance churn to keep up with each UE release was a significant burden. Epic will now take ownership of the plugin by continuing its development in their fork as part of the main Unreal Engine development branch.
• With official UE distribution comes better documentation, localization, and support through the Unreal Developer Network (UDN). This means a reduced burden for me as I now can support the plugin as part of my day job at Epic as opposed to doing so on my personal time (burning that midnight oil).

Best of all, by no longer having to maintain the plugin in my spare time, it will allow me to better focus on the core of ACL that benefits everyone. Many proprietary game engines leverage standalone ACL and in fact quite a few code contributions (and ideas) over the years came from them.

Unfortunately, it does mean that some tradeoffs had to be made:

• The GitHub version of the plugin will remain largely in read only mode, taking in only critical bug fixes. It will retain its current MIT License. Contributions should instead be directed to the Unreal Engine version of the plugin.
• The Unreal Engine Marketplace version of the plugin will no longer be updated to keep up with each UE release (since it comes packaged with the engine already).
• Any changes to the plugin will not be backwards compatible as they will be made in the Unreal Engine main development branch. To get the latest improvements, you’ll have to update your engine version (like any other built in feature).
• Each plugin release will no longer publish statistics comparing ACL to the other UE codecs.
• The ACL plugin within UE will take on the engine code license (it already had a dual license like all code plugins do on the Unreal Engine Marketplace).

That being said, once ACL 2.1 comes out later this year, one last release of the plugin will be published on GitHub and the Unreal Engine Marketplace (for UE versions before 5.3). It brings many improvements to the memory footprint and visual fidelity. A future version of UE will include it (TBD).

To further clarify, the core ACL project will remain with me and continue to evolve on GitHub with the MIT License as it always has. I still have a long list of improvements and research topics to explore!