But before we go rushing off exporting this model there's two things we need to do, we need, get rid of half the model (thus making it easier to re-topologise the mesh), and paint a texture for our model. No I haven't gone stark staring mad; the texture we're going to paint will have our edge loops on it. This is going to make things a whole lot easier once we take our high level model into Maya (image 2).
But first a word about how we are going to re-topologise. The way I work personally isn't to get to anal about every polygon, but more use my ‘poly layout' texture as a guide. You must remember to keep the detail you wish to lay out your edges correctly (image 3).
image 2: Click to enlarge
image 3: Click to enlarge
If you have a crease on your model (say a skin fold etc...) you will need to put in at least 3 loops. One for the lowest part of the ‘crease' and the others either side so that the subdivision algorithm is going to see these 3 loops (with one of them lower) and produce the crease of fold that you are after. The same thing goes for sharp ‘mounds', you will need to make sure that you have painted enough supporting loops. It is the distance between these loops that defines how sharp or rounded looking your fold will be as a Subdivision surface. You really want to first draw your main loops and then ones for the detail. DO NOT try and start drawing a fine loop texture when you start, but rather add loops as you go for what it needed.
I'm afraid it is a must to know about subdivision surfaces to use any re-topologising technique effectively. There are no short cuts if you wish to make the very most of of this technique. Any artist must know the tools and theory he uses and Subdivision surfaces are just another tool for an artist. Also remember that although you want your loop texture to be tidy it isn't going to be ever shown to anyone, so there's no point making it ‘ultra neat'.
Be prepared to revise your loops layout once you get it into the app that you are using. We are all human and sometimes you may wish to add an extra loop where you missed one out. Don't be a slave to your loop texture, it's a guide (all be it a pretty good one) and not the be all and end all of it. The more you correct your mistakes, the more you will learn ‘why' the loops work. (People often learn much faster if they work out a problem they have their selves rather than being just shown it). (image 4)
Now let's have a quick word about UV's for this. There is no point on earth spending any length of time doing a full unwrap as we're going to basically junk this version and use it only as a base to draw our new topology on. As a rule I find it easier at this stage just to use a GUV or AUV UV set from within Zbrush. Normally I keep clear of them as I like to do a lot of my texture work these days in Photoshop and the AUV and GUV UV's in Zbrush although evenly spaced are just about impossible to work on in Photoshop meaningfully. For this job they are perfect as the even spacing makes having to worry about having enough UV space to paint on our model a non event (image 5 - Click for a larger image)