Rotating Strands
The Rotate Strands operator sets the twist of individual strands - the rotation of each strand around its own stem. For ordinary cylindrical hair this rotation does not matter, but it becomes important whenever a strand has a defined facing direction or when other operators act around the strand's stem.
When twist matters
Two common cases need a defined twist:
- Skinning geometry along strands: when you use Rendering Hair as Mesh Geometry to attach a flat billboard or proxy mesh along each strand, the twist decides which way that geometry faces. This is essential for cards, feathers, and leaves.
- Operations around the stem: effects such as curling are computed relative to the strand's starting rotation, so setting the twist controls how they sit.
Controlling rotation
You can orient strands in a particular direction - for example facing toward the distribution surface - or give them random orientations for a more natural, scattered look. As with other operators, a strand group limits the effect, and the rotation can be varied with maps and strand channels.
Twisting along the strand
Twist rotates each strand progressively along its length, from no twist at the root to the full amount at the tip. It is set in full turns: 0.5 is a half turn, 1 a full turn, and negative values twist the other way.
Twist Bias changes where along the strand the twist happens. At 0 the twist is spread evenly. Positive values keep the strand flat for longer and gather the twist towards the tip, negative values twist it near the root and leave the rest flat. The total twist at the tip stays the same. This works like the Bias of the 3ds Max Twist modifier and is mostly useful for twisting thin flyaway hair cards.
Related
- Rendering Hair as Mesh Geometry - where strand facing is most visible.
- Curl Operator - depends on strand starting rotation.