Hair from Guides Operator
The Hair from Guides operator generates dense hair from a set of guides. It takes the sparse guides you have shaped and interpolates between them to produce the full-density hair that is rendered. This is the operator that turns a handful of control strands into the hundreds of thousands of hairs in a finished groom.
Distribution and count
Like Guides from Mesh Operator, this operator distributes hair roots across the surface, with the same family of methods:
- Uniform
- Random (area-based)
- Random (face-based)
- Random (UV-based)
- Vertex
- Even
- Blue Noise - roots spaced as evenly as with Even, without its regular pattern. Changing the count or density keeps the existing roots in place. New Hair from Guides operators use Blue Noise; grooms and presets keep the method they were saved with.
- Guides as hair - convert the incoming guides directly into hair without generating new strands.
The count parameter sets how many hairs are produced. Because the hair roots are placed independently of the guide roots, you can keep guides sparse for easy editing while still generating very dense hair.
Placement masks
Three masks under Root Generator thin out the roots by where they would land on the surface. They work with every distribution method, combine with distribution maps, and are off by default. A mask is measured when the roots are planted, so a mesh that only deforms afterwards keeps its roots.
- Slope Mask - plants roots only where the angle between the surface and Up Direction lies between Minimum Angle and Maximum Angle. 0 degrees is ground facing up, 90 is a wall. Falloff fades the density out beyond both angles; 0 gives a hard edge. Up Direction is in world space. The angle follows the mesh's smoothed vertex normals, so give hard-edged parts of a mesh their own vertices.
- Occlusion Mask - plants fewer roots where the distribution mesh blocks the sky around the surface, such as in crevices and under overhangs. Each root casts Ray Count rays; once more than Threshold of them hit the mesh, the density fades out over Falloff. Distance ignores anything farther away than that, so distant hills do not darken a valley; 0 counts any distance.
- Avoidance Mask - keeps roots at least Footprint away from the roots of the strand sets connected to Strands, for example to keep grass from growing under tree trunks and rocks. Connect as many strand sets as needed. Falloff fades the density back in beyond the footprint. Moving the connected strands plants the roots again.
The masks thin out the requested count rather than redistributing it, so raise the count when a mask removes a large part of the surface.
Guide proximity
With Use Distance on, only hairs within Distance of a guide root are generated, so hair grows around the guides and nowhere else. Falloff adds a soft edge beyond that distance: between Distance and Distance + Falloff the hair thins out gradually until none remains, and zero keeps the hard edge. Falloff is a surface-varying parameter, so a map, a channel or an expression can scale its width over the surface, and its curve sets the hair density across the edge, from the distance on the left to the end of the falloff on the right. Which hairs survive inside the edge is decided per hair, so the result stays the same between evaluations and preview fractions.
Interpolation
The shape of each generated hair is blended from the nearby guides. Even, well-spread guides give the smoothest interpolation; clustered or uneven guides can produce artifacts. If the interpolated hair does not follow the guides as expected, revisit the guide distribution rather than adding more hairs.
Guide Area > Method chooses which guides each hair blends between:
- Barycentric (default) - the three guides of the guide triangle around the hair.
- N Closest Guides - the nearest guides.
- Circle - all guides within a radius.
- Biharmonic - smooth weights solved over the distribution mesh, so the blend follows the surface and stays the same when the mesh deforms. Weight Count sets how many guides each hair blends between.
- Natural Neighbour - the guides around the hair, each weighted by the area it would give up to the hair. The blend changes smoothly across the groom, without the creases barycentric triangles can show.
Placement in the stack
Hair from Guides usually comes after guide creation and guide styling. Styling operators placed above it act on dense hair; operators that require guides should sit below it. See Hair and Guides for why this ordering matters, and The Operator Stack for managing the stack.
Performance
Hair count drives viewport and render cost. You can work with a lighter preview count while grooming and raise it for final output, keeping interaction fast. See Hair and Guide Viewport Display.