Density is calculated as the number of hairs per-unit-area of the mesh surface the description is bound to. This-unit-per-area calculation does not change based on the set the Working Units of your scene. For example, the same number of hair per-face are generated in a scene that is modeled in meters as one modeled in centimeters.
After you set the base Density value when initially create the hairs in the Create Interactive Groom Splines Options, there are a number of different ways to control the groom's density value.
The description_base node stores the global hair density value of the description. You can use the Density Multiplier attribute on the description_base node to scale the initial density value of the description. For example, if the initial density is100, setting the Density Multiplier to 0.5 reduces hair density by 50 percent.
You can add another multiplier for density by adjusting the Density Mask attribute. Use the control slider or assign a texture map, either by painting one on the surface of the mesh, or by using an existing texture file, to set this attribute. see Work with masks.
The Density brush acts as a local multiplier for hair density, which updates as you drag the brush across the surface of the mesh. You can use it to add or remove hairs directly by adjusting its brush settings.
In the Density brush Tool Settings window, you can set the following:
The hairs modified by the Density brush respect areas masked out by a Density Mask. For example, you cannot add hairs where the map value is 0.
You can use the Place Brush, to place individual hairs exactly where they are needed. Placed hairs ignore any Density Mask that has been added to the description.
For more information about local and global density values, see Generator and Density Grid in the XGen base_description node attributes topic.