Moving and deforming emitters and colliders
Emitters, colliders and other objects that move, turn and change shape: what the bake follows, Inherit Motion, moving walls, and what marks a bake old.
Any object a simulation reads can move: the emitter, colliders, a force's object, a curve, a goal. Keyframes, parents, constraints, armatures, modifiers and shape keys all count. The bake reads where everything is on each frame and moves it smoothly from one frame to the next, so nothing jumps within a frame.
How the bake follows an object
- Moving, turning, scaling: read on every frame, and blended smoothly over the substeps between.
- Changing shape: a mesh with modifiers, shape keys or animated mesh data is read again on every frame, its vertices moved smoothly between. Curves, text and metaballs are read every frame too.
- Changing its faces: a mesh whose faces change from frame to frame (a Remesh, a Boolean, Geometry Nodes that build it anew) jumps to its new shape at the frame. Particles held to it are held to the nearest of its new faces.
Emitters
Position on a Box, Surface or Volume follows its object as it moves, turns and scales: particles are born wherever it is.
To have them leave going as the object goes, give the Position Inherit Motion (on a surface or a curve): the share of the surface's motion where each one starts. At 1, sparks fly off a spinning wheel along its rim, and drops are flung off a shaken branch. Put any Speed above the Position: Inherit Motion adds to the speed they have, and a Speed below it sets their speed afresh.
A Position on a Surface can also read an Image through a UV Map: with Drives Rate, births are kept where the image is bright and dropped where it's dark (a logo on a plane pours liquid in its shape); Rate and Speed also lets them leave faster where it's brighter; Mask places each only on the bright part, so none are lost.
Colliders
A Collide's objects can move, turn and deform as much as you like. Each step follows a collider from where it was to where it's going, so neither fast particles nor fast colliders pass through.
- A paddle stirs a liquid, a tipping bucket pours it, a hull pushes a wake: moving walls carry what they touch.
- A plane with a Wave modifier makes a rippling floor for sand or balls.
- A character's body, moved by its armature, pushes through falling snow.
- Smoke's Obstacles push the gas as they move: see Smoke.
Spinning things
An object must turn less than half a turn each frame. Faster, and the bake sees it turn the short way round, as a camera sees a wagon wheel turn backwards: a propeller or a fast wheel turns the wrong way, or slower than it does. Keep fast spins under half a turn a frame.
When the bake is old
Moving an object the bake reads, keyframing it, its parent's motion, editing its mesh, its modifiers or shape keys: each marks the bake Changed since the bake. The old bake keeps playing until you bake again, or Auto Bake does.
Constraints and drivers are followed during a bake, but changing one doesn't mark the bake old: VecFlow can't tell. Bake again yourself after changing one. Moving to another frame and playing never mark a bake old. See What marks a bake old.
What goes wrong
- Particles don't fly off a spinning wheel. Inherit Motion is 0, or a Speed below the Position sets their speed afresh.
- A propeller turns the wrong way. More than half a turn a frame: slow it down.
- The bake didn't change after you changed a constraint. It can't tell: bake again.
- Particles stuck to a mesh jump. Its faces changed that frame. Keep the face count the same (shape keys, a Wave or an armature do).
Related
NVIDIA Flow or VecFlow's own solver
Smoke and fire can be worked by VecFlow's own gas solver or by NVIDIA Flow: what each is good at, and what changes when you switch.
Shapes and materials
Particles shown as your own models, picked with Shape, turned and sized by Rotation and Scale, and coloured in a material from the particle's colour and id.