I'm trying to do a pose blending effect in 3dsmax similar to FaceFX so that I can export curve keyframes directly to 3dsmax and modify them manually, but I'm affected by the rotation order and the different blending order of multiple pose will lead to different results.
(Even if I use quaternions, I am still affected by the stacking order)
I would like to ask how the pose blending in FaceFX is implemented.
I'm not sure I understand the question. FaceFX is doing additive pose blending. It doesn't matter the order in which the poses are blended. Everything is blended in "parent space" -- maybe that's the key you're missing?
Still can't do additive mixing, but I found a tutorial for mixing quaternions that solves the problem to some extent, just a bit slowly.