The reversing sometimes happens because there is no standard in normal maps as to whether more green is up or down. They all agree that green is up-down (Y component) but half of the 3D apps do the opposite of the other half.

I have posted how to flip the Y component in a Poser shader so it can use the other form, using Color_Math nodes. Need a link or can you figure it out?

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)