Make hair collide

You can make hairs in a hair system collide with themselves and with other objects. As an alternative or in addition to setting objects to collide with hair, you can use hair constraints that act as implicit collision objects. These can be helpful if hairs are getting stuck in surfaces, since these implicit collision objects are volumes, not shells. Also, you can set the collision object constraints to affect specific hair curves, whereas Make Collide and the other collision options affect the entire hair system. The combination of collision constraints and geometry collisions can provide robust collision, while at the same time preserving fidelity of the collide object surface. For more information about the Collide Sphere and Collide Cube implicit collision objects, see Set up hair constraints.

Note

Iterations (Dynamics) increases the number of collision tests. A setting of 8 does twice as many as 4. This can improve the results of collisions in some cases, but will also slow down the simulation calculation. If you increase Iterations you need to lower Stiffness to compensate.

Fine tune collisions

If hairs are getting stuck in colliding surfaces, consider the following:

Avoid sharp or open edges. Rounded, closed objects collide better with hair. The collisions are with discreet points (like particles), not lines, so thin edges can slice between vertices. The more CVs there are on the hair the more likely it is to collide with a small edge. The number of hair CVs is determined by the number of CVs on the Start curve times the Sample Density on the follicle node. Collide Oversample values of 1 and 2 will insert extra midpoint CVs at collide time.
If the effective collision width is zero then hairs are less likely to fall through surfaces. The effective collision width is:
Clump Width * Clump Width Scale + Hair Width + Collide Width Offset

By making the Collide Width Offset a large negative number, such as -1000, you can force the collision width to always be zero. This will cause hair clumps to penetrate half way into surfaces. However they will be less likely to fall through a surface.
Increasing the Collide Oversample value can help. Collide Oversample values greater than 2 behave in a different manner. Values of one or two insert new test points between the simulation CVs. This is good when there are not enough CVs on the curve to catch collisions with thin edges. For values greater than this the extra sample points are in a ring about the CV center. This is better for handling wide hair clumps and avoiding sticking. So to avoid zero width hair clumps being sliced by thin edges, use values of 1 or 2; making the Collide Width Offset negative in this case further helps. To better handle wide clumps try Collide Oversamples of 3 or greater.

To make hair collide with the ground

In the hair system’s Attribute Editor, turn on Collide Ground in the Collisions section. The “Ground” is the ground plane (or the grid).
Optionally, adjust the Ground Height value, which is relative to the ground plane.

To make hair collide with objects

Select the hair system. (Window > Outliner)
Shift-click to select the object or surface you want the hair to collide with.
Select Hair > Make Collide. A geoConnector node is added to your collision object.
If your collisions are not working, make sure Collide is turned on in the hair system Attribute Editor.

If your hair is penetrating the collision object, then you can try the following:
- Increase the Tessellation value in the collision object’s geoConnector node.
- Increase the Collide Over Sample value in the Collisions section of the hairSystemShape.

To make hairs collide with themselves

In the hair system’s Attribute Editor, turn on Self Collide in the Collisions section.
Adjust collision settings, including the Collide Width Offset.

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