Blender Slot Link¶
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Slot Link helps you manage Blender projects with multiple separate animations.
Requires Blender 4.5 or higher. Not compatible with legacy Actions.
The Issue¶
In games-development, you often need to create multiple separate animations, that animate the same properties.
(I.e. a run-cycle and a walk-cycle for the same character.)
This is unfortunately impossible to create in Blender without workarounds.
(As of Blender version 5.2)
Blender supports only one animation per .blend file.
Actions are modular pieces from which the one animation is composed of.
The Solution¶
Slot Link redefines Actions to be full standalone animations.
To achieve that, you have to set the animation target for each Slot of an Action.
To do so, open the Dopesheets Action Editor, and expand the Action panel on the right. For each Slot on the Action, select a Target.
Press the Link Slots button to play and edit the animation.
Easily switch between animations with only one additional button press.
Whenever you change the active Action, simply press Link Slots again to ensure only the newly selected Action is linked throughout the Scene.
Switching between Actions becomes a breeze, and it is no longer possible to accidentally change the active Action by just selecting another Object, Mesh etc…
Hint
Slot Link purposely allows only selecting Objects as targets.
If you animated a Mesh’s Shape Keys, simply select the Object on which that Mesh is instantiated.
This has the added advantage of being able to animate multiple instances of the same Mesh separately.
Reset Animation¶
Slot Link additionally allows you to specify whether an animation is intended to reset the Scene into a consistent state.
The reset animation should consist of just one frame that animates all desired properties to their default value.
Further animations can select this animation as their reset.
If so, when the Link Slots button is pressed, the reset animation will be applied right before the selected animation.
Import Export¶
With Slot Link, it is possible to deterministically import & export animations in Blender.
At the time of writing, the only importer/exporter that supports Slot-Link is STF. STF is not a production ready format yet.
Until STF matures, or an importer/exporter for another format implements support, Slot-Link still aids you in creating and managing animations.
For now, you can export animations one at a time. The Link Slots feature speeds that process up significantly.
Technical Details¶
In Blender, the animation is composed of all the Actions and Slots linked throughout the Scene.
Each animatable data-block (Object, Mesh, Armature, etc…) links to one Action and one of its Slots.
When you hit the spacebar, which ever Actions and Slots are currently linked on all the data-blocks, will play.
Unfortunately, a data-block can link to only one Action.
In order to create a second Action, targeting the same data-block, you have to remove the previous Action first.
After an Action has been unlinked, Blender no longer knows what it was animating.
(When unlinked, the Action knows which Bone and shape-key it animates, but not on which Armature-instance or Mesh!)
If you need to edit the previous Action, you have to remember yourself where it and its Slots were linked, and restore that manually.
This is a critical design flaw in Blender’s data-model!
While it doesn’t inhibit film and VFX use-cases much, since all they need is the one animation, it severely limits the ability to create assets for video-games.
Animation Export¶
Exporters, like those for FBX or glTF 2.0, do not have the knowledge of the artist.
In order to export Actions, the targets of Actions and Slots have to be guessed.
Depending on the circumstances, that may work well or fail completely.
Error Prone UX¶
The Action displayed in the Action-Editor is linked to the animation-data of the currently selected data-block.
This means, if you switch to another Action in the editor, the data-block’s linked Action is what actually changes, and vice versa.
If you select another Object/Mesh/etc, whichever Action was linked there, will become the active Action in the Action-Editor. Keyframes will be inserted there instead. It is incredibly easy to accidentally and unknowingly animate the wrong Action.