| Diffuse | The base Diffuse color of the hair. Diffuse shading actually plays a very minor role, most of the action happens with the three highlights. You can use this for setting a base tone for your hair, and it also can play a more important role for short hair (fur) shading. |
| Ambient | Standard ambient, tuned down to be very dark by default. |
Speculars |
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I'll explain all three highlights in one go: For all three highlight types, the first setting is the color. Usually, you can leave the primary at white/gray, set the secondary to the basic color of your hair, and the translucent to a more saturated color than the secondary. The second option is the shininess. The highlight is a simple gaussian bell curve, centered on the optimal reflection direction. It is a bit different from other shading models in that the peak intensity does not increase when you raise the shininess, only the extent of the highlight gets smaller. This makes it easier to tune the highlight size while keeping the brightness constant. The final option is the deviation. This specifies a shifting of the highlight from the specular reflection position towards the root or the tip of the hair. Negative values shift towards the root, while positive values shift the highlight towards the tip of the hair. The primary highlight is caused by light directly reflected from the hair, before it can scatter into the fibre itself. It therefore is normally pure white (the hair pigments in the medulla didn't have any chance to affect the color yet). It is also very narrow, and shifted slightly towards the tip of the hair. The translucent highlight is caused by light that entered hair on one side, and exits it at the backside. It is tinted towards the hair color, and shifted a bit towards the root of the hair. The secondary highlight is light that enters the hair, gets reflected at the backside and exits again at the front. It is very broad, and shifted towards the root of the hair. |
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| Enable? | Simple on/off switch to quickly make hair opaque if needed. |
| Transparency | A transparency color input, so you can mix your own opacity maps with the transparency options that come with the shader. The color values here are composited through a minimum operation, meaning that whatever is more transparent (either the input, or the computed transparency) will be used. If you set this to white, your hair will disappear, and become completely transparent. |
| Root | This sets the amount of transparency at the root. |
| Tip | This sets the amount of transparency at the tip. |
| Crossover | This specifies the center of a smoothstep function between the root and the tip transparency. Values close to 0 move the crossover towards the root, while values close to 1 move it towards the tip. At the crossover point, exactly the average of the root and tip transparency values is reached. |
| Range | This specifies the size of the smooth transition between the root and tip transparencies. A value of 0 will cause an abrupt transition. The default settings of 0.8 for crossover and 0.2 for range mean: The root transparency will be used for the first 60% (0.8 - 0.2 = 0.6) of the hair from the root towards the tip, and then a smooth transition will take place until the tip transparency is reached at 100% (0.8 + 0.2 = 1.0) of the hair length, right at the tip. |
| Border Transparency | This allows you to specify a simple border fading effect. A value of 0 means that the hair will be solid across it's entire width, while 1 means that the hair will have 100% transparency right at the border edges. The falloff isn't linear from the center, but cosine, more like a cylinder. |
| Multiplicity | This simply multiplies the apparent number of hair strands by tiling the border transparency (and the Fresnel effect) several times across the hair width. A very cheap trick, but can be effective with lots of dense hairs. |
| Color | A simple shadow color, with black being opaque and white being completely translucent. |
| Scale | This allows you to scale the shadow color down so you can get more light seeping into the hair. Lower values mean that less and less light is filtered by each hair, with 0 meaning that hairs don't cast any shadows at all. |
| Transparency effect | This sets how much the shadow color is affected by the transparency color and options. The default of 1 means full, effect, while a setting of 0 means that the shadow has no relation to the transparency settings at all. |
| Radiance | Standard radiance setting. This controls how much the hair shading is affected by global illumination. |
| Irradiance | This sets how much hair actually contributes to final gathering. You can tune this down if the hair emits too much illumination during final gathering, and causes surrounding objects to light up. |
| Incandescence | Standard incandescence (self illumination) color. |
| Intensity | The accompanying intensity setting for the incandescence color. |
| Index of Refraction | Sets the hair IOR. Normal hair has an IOR of about 1.55 - 1.6. The IOR is only used for computing a Fresnel falloff for the primary and secondary specular components. |
| Fresnel strength | Primary and secondary highlights are affected by a Fresnel falloff. The primary specular will have maximum intensity at the borders, and fade towards the center, while the secondary specular will have maximum intensity at the center and fade towards the borders. The strength sets how much the highlights are actually affected by this falloff, with 0 meaning they aren't affected at all, and are constant across the hair width, while 1 means full Fresnel falloff. If you set this too high, you will likely get bad aliasing effects, so try to keep it low. |
| Direction Attenuation | This sets a direction attenuation for the specular highlights. The primary and secondary
highlights will have maximum strength when you look at the hair from the same direction the light
is coming from, and minimum intensity when you look at the light through the hair. For the translucent
highlight, it's just the other way round: Maximum intensity when the light is shining through
the hair towards the camera, and minimum intensity when you are lookin at the hair from the same
direction the light is coming from. The direction attenuation setting allows you to set the amount of fading that occurs. 1 means full attenuation, and 0 means no attenuation. |
| Normal blend | This has a similar effect for diffuse and specular components. The normal blend allows you to smoothly interpolate between a long and short hair shading model. A value of 0 means that a long hair model will be used, and maximum highlight/diffuse shading is achieved at glancing angles. A value of 1 means that a short hair model will be used, and diffuse/specular shading will approximately follow that of the underlying surface. It's difficult to explain, try it out on a simple sphere covered in hair :-) |
| Optimization Limit | To reduce the number of transparency/shadow rays and therefore the rendering time, the shader
keeps track of the transparency values that already have applied to a ray passing through the hair.
If the amount of contribution a certain shading element has to the final result drops below the limit
value, it is omitted. This means:
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| Optimization Depth | This is a hard cutoff for transparency/shadow tracing. If more than [depth] hairs already have been intersected, stop. This will normally cause heavy artifacts unless set to very high numbers. Try to leave it at 0, which disables it. |
| Detail shadow hack / bias | There seems to be a little bug in XSI/mental ray which causes detail shadows not to render on surfaces which use
a certain optimization to get rid of another certain optimization =D Long story short, if you want detail shadows to affect the hair, check the checkbox. If you don't need detail shadows, leave it unchecked, since it can cause self intersections and therefore artifacts. The bias allows you to push shaded points more towards the light source so you can get rid of those self intersections. |