I totally agree with you. I’ve also read everywhere that the vertex order determines wether UVs are inverted or not. Maya even has a tool for that where it is also explained: https://knowledge.autodesk.com/support/maya/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/Maya/files/Editing-UVs-Display-UV-winding-order-htm.html
I wonder why Blender does not provide a tool or an addon. And all because you still cannot bake procedural created Normal-Maps from Cycles … Well anyways … Here is what i want to do in general: I created a texture bake-tool that bakes all neccessary textures for PBR at once and combines multi-material objects into one texture. So if you join multiple objects with different procedural Cylces-textures into one object you can easily bake them down to textures. It even bakes subsurface-, refraction- and alpha masks without the need to change the material-node in cycles. Works pretty well except for procedural generated Normalmaps AND faces with mirrored UVs. I can fix this problem if i flip the faces with mirrored UV coords before baking and flip them back when this process finished. Now why is this even happening? I think (and this is just guesswork but some tests seemed to proof it - correct me if iam wrong):
1: Say you have a mirrored highpoly object. Then you retopo and UV-unwrap it. AFTER THAT you mirror it (e.g. with mirror-modifier und don’t flip U or V coords) -> The mirrored version will get inverted UV coords (because you flipped your mesh, but not it’s according UVs i guess). Now you just drag them to their unique space on the UV Map.
2: Now you bake your normals to a texture and then there is something strange in my oppinion: The baked normals of both islands (the one which is clockwise and the counter-clockwise) look totally the same - no flipped green channel what i would estiamte. That leads to the assumption that blender internally (not only in shading modes) does this flipping internally during bake like gameengines do too.
3: And now in my final step i take this normals and bake them using this node constellation: https://blenderartists.org/forum/showthread.php?348799-Baking-Cycles-Procedural-Normals-to-Texture . It calculate from object-space to tangent-space and uses the UV-tangents and Face-Normals to do so. In this process there is no automatic flipping of the green channel (like in step 3) - and that results into wrong normal information of faces with mirrored UVs.
The solution is to flip faces with mirrored UVs before the baking and flip them bake when baking is done but the actual problem is to find out which faces have mirrored uvs and must be flipped and which does not.
Sorry for that essay but that might help you to understand the whole need for it and maybe you see a completely different way to solve this. Thanks for your time dude!