Motion & simulation

Motion without a cacheCopy Anchor Link

Two nodes (Add Node > Motion) move the groom with no simulation and nothing cached. Each frame is worked out from the time and the scalp's last few frames, so shots scrub in any order and render on the farm frame by frame, the same every time.

• Play the timeline to see them.
• Both keep roots and segment lengths, and work on strands or guides.
• Bypass (B) or delete either and the groom is exactly as it was.
• Key their parameters like any other (right-click > Set Key, or S over a field): a wind that picks up, or turns (direction keys as X Y Z). See Animating parameters.

groomlab.add_wind(groom, direction=(1, 0, 0), strength=0.3)
groomlab.add_jiggle(groom, amount=1.0)

Simulate the guidesCopy Anchor Link

The Simulate node (Add Node > Motion) is GroomLab's own hair simulation: put it between the guides and Interpolate and the guides swing under gravity and the head's motion, drag in the air and the wind, and stay out of the scalp and a collider; the strands grown from them follow. It runs in Maya, in the standalone app, in groom files and on the farm, with the same results.

• Play to see it. Playing or scrubbing past the frames simulated steps it on from the last one; scrubbing back reads the frames kept. Changing a setting, or grooming the guides, simulates again from the start frame.
• Simulate, bake, render. Simulate Range fills the frames of the playback range; Bake Simulation writes them to an Alembic guide cache (one per Simulate node, beside the scene or the groom file) that renders, exports and the farm read instead of simulating. A bake made with other settings than the node has now is ignored (it says so) and the node simulates.
• Bypass (B) gives the groom exactly as it was; before its start frame the guides are as groomed.

Water and wet hairCopy Anchor Link

With water on, hair below the surface floats (buoyancy), slows and swirls (waterDrag) and drifts with the current; the forces fade in over a thin band at the surface, so nothing jumps as hair crosses it. The simulation writes a wetness channel on the strands, per point, 0 dry to 1 soaked: it rises within a quarter second under water (or in the rain) and dries over dryTime. The look reads it like any channel: map a Clump's strength to {"attribute": "wetness"} for wet hair gathering into locks, and HairColor's mask (with a dark tipColor and a flat tipProfile) to darken it. Bakes carry the wetness, so renders read it too. The underwater_hair preset does all of this.

In Maya: GroomLab > Simulation > Add Simulate Node, Simulate Range, Bake Simulation, Clear Simulation. In the standalone app the timeline grows Simulate, Bake and Clear while the graph has a Simulate node, and a strip under the scrubber shows the frames simulated (emerald) and baked (blue); a saved groom names its bakes.

from groomlab import sim

node = sim.add_simulate(groom, bend=0.6, startFrame=-10)
sim.simulate(groom, end=120)           # Simulate Range
sim.bake_simulation(groom)             # the bake renders and exports read
sim.invalidate_simulation(groom)       # simulate again from the start frame

Follow the scalpCopy Anchor Link

On an animated head the groom normally grows again on every frame, on the scalp as it is. Turn on Follow Scalp (Attribute Editor > Inputs, followScalp) and it grows once, on the rest mesh, and each strand is then carried to the moving scalp by its root: a turn and a move of the skin under the root (its face's edge and normal), no stretch. Every strand keeps its rest shape, and only the carrying runs from frame to frame: playback, exports and every motion blur key are much cheaper (the evaluation at rest is shared by all keys).

• Needs a rest mesh (below). Without one, or on a frame at rest, the groom is exactly what it is with Follow Scalp off.
• What keeps its place: guide curves, simulated guides and guide caches are world-space positions of the current frame: each is taken back to rest through its root, so the strands they shape come out where they are.
• What is read at rest: colliders (Collide), ImportCurves' curves, Wind's direction and its gusts, and Noise and other effects that work in world space are evaluated on the groom at rest and then carried, so they turn with the head instead of staying put in the world. Collide against a moving body, or wind blowing one way in the shot, look different: turn Follow Scalp off for those grooms.
• Jiggle reacts to the head's motion as it does unfollowed (its scalp history is turned into rest space).
• No stretch: a scalp that stretches carries the roots apart but the strands keep their rest size.
• Exports, Arnold (both render modes) and groom files follow too: the flag is stored in the .groom file.

groomlab.set_follow_scalp(groom, True)

Simulate in Maya (nHair)Copy Anchor Link

nHair is still there beside the Simulate node: GroomLab > Simulation > Simulate Guides (nHair) builds nHair curves for the primary guides, with follicles on the scalp. Children follow their simulated parents, so the whole groom moves.

• Presets: fur, short, long, bouncy and wet.
• Bake Simulation to Cache writes the posed guides to Alembic and drops the live sim, so playback is fast and scrubbable.
• Remove Simulation returns to the groom as authored. The authored guides never change.

from groomlab import sim

made = sim.create_sim_curves(groom, "primary", preset="long", collide=True, preroll=20)
sim.bake_sim(groom, "shot010.guides.abc", made)

Round trip to Houdini and othersCopy Anchor Link

1. Export Guides for Simulation... Each curve carries its guide ID as strandId; keep that attribute through the simulation.
2. Simulate in your package of choice.
3. Load Guide Cache... brings the result back. The cache blends between samples, so motion-blur subframes are smooth.

sim.export_guides(groom, "guides.abc", "primary", start=1, end=100)
sim.set_guide_cache(groom, "sim.abc")

Shot fixesCopy Anchor Link

Correctives (Groom Tools > Sculpt) sit on top of the groom like sculpt layers. Fix Frame makes one that blends in and out around the current frame, with keyable weights, and renders include them.