I guess you can use this, it assumes packed tangent quaternions as you described, but you can twist a thing here and there, and by this get a combination of at least 2048 different configurations... I am myself on the hunt for the correct formula. Then there are also optimization possible: for mirrowed meshes you have to fix UV coords where the two parts meet, because a triangle there may face towards you, but the triangle in texturespace is faced away from you. There you have to shift the U value up by 1 to get better tangent calculation. Then epic uses another optimization on top of that, which I havent understood yet, so any input is VERY welcome

my import code and tangent calculationgreetz WV