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Normal Maps


Unwrapping and issues

Well first off, I plan my hard edges as i'm uving, this is pretty simple and straight forward when you start doing it, and it makes a lot of logical sense, because the areas you're likely to want hard edges are also the areas you're likely to want uv seams. I try not to split up my UVs too excessively, however I also try not to stitch them together to excessively, at the expense of distortion or ability to use hard edges.

So generally speaking, areas that I know i'm going to have a uv split, i'll get by with a hard edge, areas where I know its a steep angle, but I want to keep a seamless uv chunk, i'll add a little more geometry to soften out the mesh normals. In addition to all of that, modeling your high and low to avoid lots of excessively steep angles in the end result is important, its all a big process that works together, high, low, uvs. 

Channels

Just a reference image to remember some application need the green channel (that represents up/down) inverted.
3Ds max and CrazyBump have DOWN by default. Then based on whatever game engine you use you have to see it.