Unity3d Asset Bundle Data Sharing

This article is part of a series. If you’d like to skip ahead, you can go to the master post that links to them all.
Last week on our asset bundle journey we got started converting our old asset bundle system to the new Unity 5 one. It was actually really simple. But there is still a big problem that needs to be solved.

When you build bundles, you mark each asset that the game is going to ask for when you run the game. When you flag a single texture, it works just fine. But what about shared assets? For example a prefab holding a sprite that uses a texture atlas. Unity will gladly pick up the texture for the atlas and add it to the bundle for you. No problem there. But what happens when you add a different prefab in another bundle that wants the same atlas?

That’s when things get confusing. Will unity add the texture to both bundles? Yes. Uh-oh. Then you have wasted some memory on a duplicate texture. Worse, Unity seems to get confused by loading the same texture twice! No, Unity does not figure out that it is the same texture for you, loading it only once. Bad times. But I thought Unity handles dependencies for you in the new system? It does, but only considers each bundle separately. It does not figure out sharing data to other bundles.

But you can do something about it. All you have to do is mark the “shared” atlas as another asset bundle. In our example, the first prefab goes in bundle A, the second prefab in B. Even though the game does not ask directly for it, we set the atlas to be in new bundle C. Unity will then properly take over, making sure to load only one copy of the atlas, and only unload it after both prefab bundles are unloaded.

The more I think about it, the more it feels like there should be some automatic way of dealing with this resource sharing. Both in new projects and legacy ones that we are upgrading. However, this is something Unity does not provide. Why not? It’s easy enough to determine what assets are shared and used by what other assets. The catch is, what’s the most efficient way to assign which asset to which bundle? That will be different for every project.

That said, you can still write some tools that help in the process. For example, something that examines your bundle assignments and works out what should be shared. It could even go so far as to suggest what should share to what.

So there you go! That is how to deal with shared asset bundle data. Join us next week as we explore a useful ability that asset bundles make super simple: automatic texture scaling.