Head Texturing Tutorial
When your bump map is more or less ready, save a flattened copy of it in your project's sourceimage folder in tiff or other lossles format (always keep layered Photoshop file for future modifications and other projects). Assign blinn shader to your head, sett color to some skin like shade and apply your bumpmap - you can do a first testrender and see how it works. If it's ok use your flattened bump image for color texture and specular texture.
Color map I usualy make by creating layer filled with general skin color, then overlaying on top bump map with reduced opacity - this way I get darker skin color where the wrinkles and skin pores are.
I also paint around the eyes much darker and red'er on the forehead and nose "suntaned" and adding some minordetails like spots, birth marks etc to liven the skin.
The specularmap is very important to achieve a realistic look of any rendering, but it's especialy true for the human face. Specularity in Maya has two channels - specular rollof and specular color. I use maps for both chanels. Specular rolloff sets the sharpness and intensity of specular color, so I use the grayscale bump map image for that, which reduces specularity in the wrinkles and other impressed detail and highlights positive bumps, just I paint on a new layer brighter areas on the forehead, nose, upper lip and cheekbones and then use multiply mode for the blending- this way small dark details remains intact.
Specular color map is somewhat more complicated. It depends on the key light color in your scene, surrounding environment, physical properties of the skin of your model (depending on age and ethnic group) and lots of other factors. It's a good way to start with loading your specular rolloff map in photoshop, duplicating it as a new layer and applying hue/saturation ajustment with colorize option switched on. Choose hue close to the color of your key light, or main environmant color. Now in a new layer you can paint more reddish specular color for the lips and arround eyes (it gives litle tired appearence, but it makes your model look real) and desaturate areas on the forehead, nose cheekbones and chin. you can as well desaturate areas on the eyebrows and along the hearline, and slightly darken under the lower lip.
Building up materials in maya is always fun. Most appealing is the fact, that you can extend Maya's rendering engine with freeware plugins and mel-scripts available from Maya's users community. Ray Diffuse node was created by Mark Davies as a part of solution of Global Illumination for Maya. So far it simulates just shadow distribution of Global illumination rendering, but even that alone boosts realism of your renderings, and simplyfies light setup. The RayDiffuse node makes shadow distribution very realistic, it helps to accumulate shadows in narrow spaces between tightly spaced geometry elements.
Building up materials
With RayDiffuse plugin installed, create a new Blinn material for your skin. Assign bump map, specular rolloff and specular color textures to your material. Now create RayDiffuse node (right click in Hypershade window Create > Utilities > Color > RayDiffuse) and conect it's outColor node to color of your Blinn material. Double click on raydiffuse node to get it's attribute editor and assign you color texture to a Base Color of RayDiffuse node. Set diffuse density to 1.5 - 1.6 and for startes leave the rest on default setings. Don't forget to enable raytracing in render globals, RayDiffuse node uses raytracing, it increases rendering times considerably, but is well worse it.
Setting lights it's a huge art on it's own, and in a short tutorial it would be difficult to cover even the basics of it. I would recomend to use typical studio setup for the face, based on which setup you can play arround to get the best result. First lets make a key light , I use directional light, with intensity set to 0.7 and slightly blueish color. Place this light a little to the left of your camera and slightly higher. Now at the right side of the camera place a fill light, it could be a spot light or ambient light, or even a real light (it increases rendering times, though results are good) and set intensity to 0.25/0.3). Backlight shoud have intensive blue color, intensity of 0.5/0.6 and be placed behind your model, little to the right and higher. Last light is a sky light placed on top of the model, low intensity 0.15/0.2 and neutral grey blue color.
There are some small tricks and a bunch of settings to tweak to get the final result realy good. First of all it's recommended to increase your object shading sample settings in attribute editor, also it increases rendering time, it will improve the rendering quality. You shoud also change texture filtering in your shader attribute editor (Effects > Filter) for a bump map it shoud do ta 0.75, color and both specular maps at 0.5 or less.
RayDiffuse node does a good job creating diffuse shadows, but you still have to use at least two other lights to get aditional shadows - I used key light and sky light, both with Dmap shadow, with high shadow filtering 4 and 5 respectivly (which gives very soft shadows).
You shoud disable "Use Dmap Autofocus" and experiment with Dmap width focus as well as Dmap Bias setings to get best results.
In final turn it's a lot of experimentation to get a photorealistic human rendering. RayDiffuse node helps tremendously and you should experiment with it, The results will be worth it.
For the rest you have to keep in mind that realism is created by detail - so you shoud make as much of detail as possible - create eyebrows and eyelashes using fur, make hair using Paint Effects (there is a good tutorial with very good brush presets on Alias/Wavefront's web site). Pay lots of attention to the eyes of your model, especialy to reflectivity and refraction of the eye shaders. Try to experiment with adding detail like necklaces, hats or other things, which only increase the realism.
Always use depth of fiel with your camera, it concentrates attention to fine detail in front of the face and simulates real world camera. Use a bacground image to outline your model and give him a real environment.
I hope it helps you to break through the difficult task of recreating a human in CG, but besides the lot's of hours of work it's a realy fun thing to do.