Using AnimationUtils.makeClipAdditive() is mandatory if you define AdditiveAnimationBlendMode. As mentioned in the inline docs, the method makes each track’s values relative to the values at the reference frame which is a prerequisite for additive blending.
It’s about processing the keyframe tracks of the clip. Like mentioned above, the call makes each keyframe track’s values relative to the values at the reference frame. It’s best if you study to source code of the method to see what’s going on.
“the call makes each keyframe track’s values relative to the values at the reference frame.” – since the reference frame is itself, how does that work? Sorry I’m still in confusion after a quick scan of the method code. Thanks for your patience.
The reference frame is 0 by default but you can pass a different value as the second parameter of makeClipAdditive(). Based on this frame the function determines a keyframe and its values that represents the reference. This reference is subtracted from all other values in the keyframe track.