Here is the first release of Additive Keyer, an addon to interactively key an additive strip, known as additive animation layer in other software.
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 on an additive strip the usual way. Supports one additive strip for now, armatures only.
You can download it for free by entering $0, or give more to support the effort.