Effectors

Effectors are how you add your own controls for shaping a tree. An effector is a directional influence, such as gravity, a magnet or a noise field: it gives every point a direction, and the node it is added to responds to that direction. Mapping turns attributes into single numbers; effectors do the same for directions, so a new kind of control doesn't need a new parameter on every node.

Shaping shows what each effector does to a tree; this page covers how they work.

Where effectors go

Each of these nodes has an effector stack of up to 16 rows:

• Grow, under Parameters > Shape > Effectors, where effectors steer the growth.
• Assembly Decoration, under Transform, where they bend the placed parts through their rig.
• Surface Decoration, under Surface Mesh > Surface Effectors, where they push the surface.
• Simulate, under Postprocess (Experimental) > Substeps (Experimental) > Substep Effectors, where they refine the grown shape before the decorations run. This stack only runs with Enable Postprocess on (under Globals, on by default).

Add an effector

Click Add Effector on a Grow or Assembly Decoration (Add Surface Effector on a Surface Decoration, Add Substep Effector on a Simulate), or select a Grow or Assembly Decoration and click Effectors on the toolbar. The list lets you:

• create a new effector of any type,
• link an effector that already exists, so one effector can drive several nodes,
• pick one from the network,
• use another attribute from an effector that is already on this node,
• use a vector the tree already carries, with no effector node behind it.

Each row then has a mode, a Scale with a map button, and a menu with Jump to effector, Duplicate effector, Unlink effector and Help. Duplicate effector adds another row that uses the same effector, with default settings, so one effector can drive the node in two modes. If the effector has been deleted, the menu offers Remove redundant row and Replace with another effector… instead.

Response modes

The mode sets how the node responds to the effector's direction:

• Attract and Repel steer the growth by the effector's direction, in opposite senses.
• Flatten grows in the plane at right angles to it.
• Orbit grows around it, which makes spirals.
• Align (up) and Level (up) act on the growth's up vector rather than its direction.
• Disabled switches the row off.

Align (up) and Level (up) change the spiral of the growth, and with it the arrangement of the branches that spawn from it. The Assembly Decoration has the same modes except Orbit. The Surface Decoration has its own, Tangent and Surface +. The Simulate's Substep Effectors have Tangent Push (the default), Tangent Pull, Push and Pull.

Map the strength

An effector at a constant Scale acts the same everywhere. Map the Scale to vary it: mapped from u with Self, it grows from the base of each branch to its tip. On an Assembly Decoration, u with Self is the part's own, so an effector can act on the ends of each part only, and the parts stay instances. The Effectors Cookbook has worked examples.

The effectors

• Gravity Effector: one direction everywhere, Gravity Direction, down by default.
• Direction Effector: one direction everywhere, for any bias other than gravity, such as a prevailing wind.
• Magnet Effector: a direction relative to the geometry wired into its first input, such as a sphere, within its Range. With nothing wired, it does nothing.
• Noise Effector: a direction that changes from place to place, to break up regular growth.
• Ancestor Effector: follows a direction stored on the parent or an earlier ancestor, such as the parent's own noise, so branches echo the shape of the branches they grow from.
• Deflection Effector: a direction to the nearest parts of the same tree, for keeping branches from crowding each other. Set the row to the mode that moves them apart.
• Effector: write your own in VEX (see below).

To respond to light and nearby surfaces, plug the environment into the Simulate's Geometry input. The tree then carries Photo Direction (phototropism_direction), for steering toward or away from light, and Thigmo Direction (thigmotropism_direction), the direction to the nearest surface, and you add them to a Grow's stack from Add Effector, among the vectors the tree carries. See Situation and environment. See Simulate.

Write your own

The Effector node, in the Effectors list, takes a VEX snippet that writes a vector attribute per point. List the attribute under Export, and the effector appears in Add Effector like the shipped ones. The shipped effectors are built the same way, so opening one shows the pattern to copy. Effector Template, hidden from the Tab menu in 0.7.3, has commented example code; see Effector Template.

When an effector does nothing

Check that the row's mode isn't Disabled and its Scale or map isn't 0, that the effector is linked to this node, and that the attribute it writes exists on the points at that stage. A mapped Scale reading the wrong level (Self or Parent) is a common cause.

Status

Most shipped effectors are marked experimental: Gravity, Direction, Magnet, Noise, Ancestor, Deflection and the Effector node. Effectors are being added to more nodes, and more viewport controls, such as curve influences, are planned.