Hi! Sorry for the delay in answering that, but it came the update and I ended up forgetting about it. I was playing with Maya now, though, and remembered this post.
Yeah... I know all about 3D texturing and how bump and normal maps works, but the problem with Maya is that skin texture from DT Yolanda has a displacement map (see image below), and that's the problem. While bumps and normals only affect the lighting and create an illusion of 3-dimensionality, displacement maps really distorts the mesh during rendering. Though it sounds good in theory, actually, this is a bad practice -- proven by the fact no one eles does that on skin mats and even DT stopped doing this on newer models (didn't check 'em all, but see DT Melina for instance). Unless DPC wants to make Maya looks seek as her mother, he could solve almost all of her skin problems just getting rid of those displacement maps (and even save rendering time).
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On creating bump (and normal) maps, in fact, the best practice is to make a hi-poly model on some 3D software (ZBrush, Character Creator, or even Blender) and bake the bump and normal maps from it (check
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on this topic), but the truth is that, just by checking the textures, one can see that, like, 99% of the model creators on Daz Store, Renderosity, and other stores just make a grayscale from the diffuse (base color) maps (and probably create the normal maps from it). One tell is that the areolae and the nipples should be lighter but they're darker (and, when they're not, it's just because they inverted the maps, but, then, the navel that should be darker -- since it's a hole -- is also lighter as the areolae and nipples). But the fact is that, most of the time -- unless you're doing close-ups AND striving for realism --, it doesn't matter.