Additive Keyer is a Blender addon for interactive keying of an additive strip (called additive animation layer in other softwares)
Additive animation layers are mainly used for motion capture edition, to keep the main layer untouched while tweaking on an additive layer, or special animators needs, such as animating motions over a walkcycle.
It solves the double transformation issue when keying transforms the standard way.
Supports one additive strip for now, armatures only. Always make sure Auto-key is disabled when setting up an additive layer, it would create bad transforms keys.
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For motion capture retargetting, check the Auto-Rig Pro addon.