Drivers Panel

Reference

Editor: Graph editor
Mode: Drivers
Panel: Sidebar region ‣ Drivers
Shortcut: N

Reference

Menu: Context menu ‣ Edit Driver
Shortcut: Ctrl-D

This panel is visible in Sidebar of the Drivers Editor or as a popover when adding a driver to a property.

It shows the property that is being driven, followed by a series of settings that determine how the driver works.

Driver Settings

Type

There are two categories of drivers:

Built-in functions (Average, Sum, Min and Max)

The driven property will have the value of the average, sum, lowest or highest (respectively) of the values of the referenced Driver Variables. If there is only one driver variable, these functions will yield the same result.
Custom (Scripted Expression).

An arbitrary Python expression that can refer to the Driver Variables by name. See Expressions.

Driver Value

The current result of the driver setup. Useful for debug purposes.

Variables

See Driver Variables.

Update Dependencies

Forces an update for the Driver Value dependencies.

Show in Drivers Editor

Opens the fully featured Drivers Editor. This button only appears in the popover version of the Drivers panel.

Driver Variables

Variables are references to properties, transformation channels, or the result of a comparison between transformations of two objects.

Drivers should access object data via Driver Variables, rather than direct references in the Python expression, in order for dependencies to be correctly tracked.

../../_images/animation_drivers_drivers-panel_add-variable.png — Add, Copy, Paste buttons.

Add Input Variable: Adds a new Driver Variable.
Copy/Paste Variables: Copies the current variable list so it can be pasted into another driver's variable list.

../../_images/animation_drivers_drivers-panel_single-property.png — Single property.

../../_images/animation_drivers_drivers-panel_transform-channel2.png — Transform channel.

../../_images/animation_drivers_drivers-panel_distance.png — Distance.

Name

Name for use in scripted expressions. The name must start with a letter, and only contain letters, digits, or underscores.

Variable Type

The type of variable to use.

Single Property

Retrieves the value of an RNA property, specified by a data-block reference and a path string.

In case of transform properties, this will return the exact value of the UI property, while Transform Channel will take parenting and/or constraints into account as needed.

Rotation Channel Modes

Rotation Transform Channels support a number of operation modes, including:

Auto Euler

Uses the Euler order of the target to decompose rotation into channels.

XYZ Euler, ...

Explicitly specifies the Euler rotation order to use.

Quaternion

Provides the Quaternion representation of the rotation.

Swing and X/Y/Z Twist

Decomposes the rotation into two parts: a Swing rotation that aims the specified axis in its final direction, followed by a Twist rotation around that axis. This is often necessary for driving corrective Shape Keys and bones for organic joint rotation.

This decomposition is often produced in rigs by using a helper bone with a Damped Track Constraint to extract the swing part, and its child with Copy Transforms to extract the twist component.

The channels values for Swing and Y Twist are:

../../_images/animation_drivers_drivers-panel_angle-curve.png — Falloff curves for weighted angles.

Y Rotation

True angle of the twist rotation.

W Rotation

True angle of the swing rotation, independent of its direction.

X Rotation, Z Rotation

Weighted angles that represent the amount of swing around the X/Z axis.

The magnitude of the angle equals W Rotation when the rotation is purely around that axis, and fades out to zero as the direction changes toward the other axis, following the falloff curves from the graph on the right.

Mathematically, the swing angles are computed from quaternion components, using \(2 \arccos(w)\) for W and \(2 \arcsin(x)\) etc. for the others. The component of the swing rotation that corresponds to the twist axis is always 0, and is replaced by the twist angle.

Expressions

Expression

A text field where you can enter an arbitrary Python expression that refers to Driver Variables by their names.

The expression has access to a set of standard constants and math functions from math, bl_math and other modules, provided in the Driver Namespace. For an example of adding a custom function to the namespace, see the driver namespace example.

For performance reasons it is best to use the Simple Expressions subset as much as possible.

Use Self

If this option is enabled, the variable self can be used for drivers to reference their own data. Useful for objects and bones to avoid having creating a Driver Variable pointing to itself.

Example: self.location.x applied to the Y rotation property of the same object will make the object tumble when moving.

Note that dependencies for properties accessed via self may not be fully tracked.

Simple Expressions

Blender can evaluate a useful subset of Python driver expressions directly, which significantly improves performance, especially on multi-core systems. To take advantage of this, the driver expression must only use the following features:

Variable Names: Use only ASCII characters.
Literals: Floating-point and decimal integer.
Globals: frame
Constants: pi, True, False
Operators: +, -, *, /, ==, !=, <, <=, >, >=, and, or, not, conditional operator/ ternary if
Standard Functions: min, max, radians, degrees, abs, fabs, floor, ceil, trunc, round, int, sin, cos, tan, asin, acos, atan, atan2, exp, log, sqrt, pow, fmod
Blender Provided Functions: lerp, clamp, smoothstep

Simple expressions are evaluated even when Python script execution is disabled.

When an expression outside of this subset is used, Blender displays a "Slow Python expression" warning. However, as long as the majority of drivers use simple expressions, using a complex expression in select few is OK.

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