For a quick afro hairstyle the texture will look something like this…
…and the geo something like this.
All you have to do is pay more attention to the outline of the hair so that you can create more volume with few geometry. The geometry for the hairstyle above has around 300 triangles.
In the image above there are some angles where you can see the planes ... if you increase the numbers of triangles and add more variations to the planes the effect will be less visible, this was a quick test to show you the workflow.
You can make the dreadlock2 hairdo in the same manner as the one above.
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3.4. Using normal maps:
I will start by showing you the difference between the hair with and without normal map
(left with normal map, right without).
Both versions have diffuse, transparency and specular map but the one in the right looks too flat. By using a proper normal map you will add a lot of variation in the surface direction of the hair and it will catch more light through specularity resulting in richer volumes.
Obviously the version with normal map looks better; the normal map adds more volume and realism to the hair. I have seen a lot of people doing normal map for hair by just using Nvidia normal map filter on the diffuse pass or actually sculpting the individual strands of hair in ZBrush/Mudbox; that will not help too much (the first example in the image below).