Here is a kind of mini tutorial for something I came up with while making my current piece. Ill be showing you how to get these ghost hairs that cover you face and usually only show up in strong lighting. Also I only tested this for a 3D still. I am not sure how it would work for animations.
See here for the WIP : http://blenderartists.org/forum/showthread.php?265295-Blood-Right-Character-WIP
For this you will need to have a pre rendered image with out the hairs so you can see your lighting to use as a guide.
1. Create a new texture with no alpha. Maybe only half the resolution of your project textures or lower depending on your computer. Enter TexturePaint Mode.
Now assuming that you have scene built and have set yourself with a final camera angle and modelling is completed press 0 for you camera view. Turn project paint OFF. This can cause problems.
Using the guide of your pre rendered image, paint on the edges of your model where you would like you ghost hairs to appear. Fine tune your areas in the eyelid creases and shadows to bring down the chances of them showing up.
When finished, save out your texture.
2. Setting up the particles.
Add a particle system to your object with advanced settings checked.
Change random option to "random" instead of Jittered.
Depending on the scale of your model you need to adjust the normal value. Mine is 0.009.
Under physics set the "brownian" to 0.03.
- when you're not using vertex groups and using texture maps the particles will distribute over the entire head instead of being concentrated. So we need to really ramp up the value. I used 500,000. (again depending on your model) Set the display to 5 so it doesn't clog up your view port.
Add Interpolated children, display, 0, and Render, 35
Too vary the length of the strands adjust the length and threshold settings under Children/effects.
Length, 0.314, Threshold,0.301
3. Setting up Meterial
Add a new material to your object, Don't worry about the defuse colour.
These hairs will pretty much be all specularity.
Set the specular defuse to a 0.64, 0.628, 0.626 RGB. Set intensity to 0.279 and hardness to 144
Turn on Z-transparency , Alpha down to 0 (completely transparent)
In the material Under strand rendering, Check - Blender units.
And the values for my hairs are Root,0.005, Tip,0.002
4. Adding Texture map
Too add the map you painted to the particles. First click on your particle system. then click on the texture tab ( this will ensure that the texture you add will effect the particle system.
Add a new texture slot. Set type to Image. Open the map you saved out earlier.
Under Mapping, set it to UV. And under Influence make sure the only thing check is Density, set to 1.
Okiedoke. So go back to your particle system and under Render make sure you set the Materiel number to the material that you're using on the hairs.
When testing on a normal defuse texture the light will blow it out completely Like in the image below but when you add your SSS skin it will balance it self out nicely.
Keep in mind you may need to tweak a little bit, The most I had to tweak in this was my texture map, refining the areas it showed the hairs.
Hope some of you find this useful and if anyone knows a better way to do this please share :)