Putting it into practice
Since this is not a character modeling tutorial I will not go into the authoring process of modeling a high poly character, one tip I can give you however and that is to work in small parts when working on the high poly mesh. This way you can limit the amount of polys per object you are working on, making it all a lot less painfull when your total polycount is reaching the 500.000 mark. For normal map generating purposes you don't even have to attach and weld the different sub components together, as long as they fit seamlessly you should be fine.
I started out with a rough version of the low poly in game model (low is rather relative since it's aimed at next gen platforms and these can easily deal with 6000-8000 poly character models, in any way it's lower than the source mesh, so low poly is still apt.).
Making a rough low poly version helps you to get started and will give you an idea of where your detail should go and how you are going to capture it in the baking process.
This is of course not a rule and every method is valid as long as the end result is good.
Then I subdivided the organic components such as arms and legs and started working into them, I used a poly proxy object/mesh smoothing on the trousers using zig zag patterns to create the creases. The arms where imported into Z-Brush 2 for some quick soft wrinkling. Z-Brush is great for organic stuff, it can however be a bit tricky to get used to it.
The head I modeled entirely in Z-Brush2 using Z-spheres. I later remodeled the low poly version in Maya to make sure I get the right edgeloops for facial animations.
The hard surface detail I kept fairly low poly compared to the rest, there's no point in making your objects unnessecarily heavy, it will only slow you down and it makes it harder to reshape them when you feel like it. And you will want to do that, since you will find yourself going back and forth between your source and target meshes.
Like I said earlier, cut your model up in smaller manageable parts, this way you can keep focused and organized.
When the high poly version was finally ready I remodeled the entire low poly model from scratch snapping it's vertices directly on the high poly model, of course you need to make sure you place them strategically, this can be a bit challenging when you also need to worry about your target mesh's layout for deformation later. Another good reason to rough it out first.
Finished low poly model, 6611 polys
When your low poly model is finally finished you need to properly unwrap it, assessing each component and giving it the right amount of texture space. I generally give the face a bit more resolution since it is the main focal point on any character, I also decided to tile the ammo belt boxes since the detail was so fine it needed extra space on the map to capture all the tiny details. I did this by only baking a few boxes and then re-attaching the remaining boxes and
map them to the normal map. Of course duplicating the boxes is also possible but was in this case not the
Then it's time to start baking, interestingly there aren't that many normal map baking plugins out there. For Maya there's Turtle's Surface Transfer Editor (which I used and it's great) and Maya's own proprietary tool (which is next to useless) then there's also Microwave which ,allegedly, is also pretty good. Max 7 comes with it's own pretty accomplished normal mapping tools and then there's a bunch stand alone normal mapping tools such as Cry Tek's Polybump and ATI's normal mapping tool.
No doubt all the other big packages out there have their own solutions for this but since I have no experience with them I can't recommend anything.
Anyway, back to baking: this is when the errors start to appear, look out for extreme and hard edged color differences, these can be fixed by painting them away in Photoshop (use the "normalize only" option in Nvidia's Photoshop plugin before saving) or pushing the target mesh in or outward a bit, globally or locally. Don't worry too much about ruining your low poly objects shape, you can always fix this later when the normal map is finished, it may not be entirely accurate but no one's gonna notice. Besides you are an Artiste right? Whatever you made was intentional, even the unintentional.
Final tangent space normal map for the character
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Low poly character (left) and high poly character models