Learn how to groom hair and fur in Houdini 16 with Joshua Matthews.
1) Resample the guides
For curly hair, add a Guide Process frizz to your guide groom, then add a Resample node. Be sure to turn on Maximum Segments and turn off Maximum Segment Length. Change Treat Polygons As to Subdivision Curves. In your hairgen, add a Hair Clump node and turn on Curling. Increasing Amplitude gives your hair smooth, tight curls.
2) Use Noise Masks
For hair variation, place a Guide Process with the Set Length operation, then add a Noise Mask. It works as a multiplier to your original value. If length is 3, it randomises between 0 and 3. You can further tweak by changing Frequency, Amplitude, Bend, Frizz, Direction, Wave, Smooth and Straighten.
3) Speed up cooking
Creating VDB for preventing hair intersecting geometry can be slow. For quicker cooking, place a Guide Collide with VDB at the end of the Node Tree. Merge in skin geometry and use a VDB from polygons to make a manual collision skin. Guidecollidevdb lets you control how much the guides avoid the skin.
4) Control hair length
Use VEX for a procedurally faded haircut. Connect a point Wrangle with the input and output of the skin. Add f@hairLength = fit(chramp(‘Ramp’,relbbox(0,@P.y)),0,1,0,1);. This sets the hair length of each point equal to the relative y position of the first input. Chramp creates a ramp parameter for more control and ‘fit’ keeps values between zero and one. In the Set Length node, set Length to be controlled by the skin attribute. It must match the name of the attribute created in the Wrangle.
5) More accurate simulations
To have guides collide with each other accurately, select your Guide Simulate node and click Create External DOP Network to place a dopnet that’s linked to all guide sim parameters for greater control. Select Wire Object>Guides and turn on Width Visualization to see each guide’s collision radius. In your guide sim, turn on self collide.
6) Place guide processes
Think about cooking time when choosing where to put processes. It’s preferable to wait for hairgen to cook than guides, as usually the guides need to be simulated. If you build your fur system mostly in the hairgen, you will
find that your simulation times will be much more manageable.
7) Transmission vs Opacity
Transmission controls how much light passes through a surface, making beautiful effects on hair when backlit. Opacity doesn’t affect light passing through. Opacity with falloff can help emulate softness in fur, specifically on the ends. Here I chose transmission, as with opacity falloff you may lose integrity of the strands at the ends.
8) Caching out hairgen
The act of caching out curves bypasses vm procedural rendering, so you no longer need to visualise your guides and hairgen at the same time. This also means you don’t need to recook the guides every time you change something in the Shader settings. Caching out the hairgen is great for still renders of fur, especially turntables.
9) Use good UVs
UVs aren’t necessary for hair and fur, but can make a huge difference in the flow and direction of your curves. Inside the Guide Initialize node, there are options called UV Blend and UV Rotation. This will group the guides into sections by UV patch, and change the direction and rotation of each patch individually.
10) Use Point Colour
To create different coloured hair patches, use a simple paint SOP on the skin and an Attribute Transfer in the hairgen to overwrite the Cd attribute. Be sure to transfer from points to primitives. In the Shader turn on Tint with Point Color so the Shader is affected by the Cd attribute. Remember, Cd value is multiplied with the base colour rather than replacing.