In this case I don't think even that's needed, it's just assumed the blue channel is white (set the blue channel to white in the top image and you get much the same output as the second image) with tangent space normal maps there's never really a lot of detail in the blue channel. All the normal perturbation is in the red…
not that i know of. i think that it always has to be the up axis which is derived from the two other axis but somebody more tech saavy than me could say otherwise. if you are viewing the textures like ripped from some game the naming convention might provide a clue. edit: didn't see earthquakes response above. thats the…
there is different coordinate systems. you can google "left hand versus right hand coordinate system" and search the wiki here it has a good explanation. the way it is packed might also be a factor. it is possible to pack normal to two channels only and use a math function to rebuild the missing channel. That way an…