The Reaction controller
is a procedural controller that lets a parameter react to changes
in any other parameter in 3ds Max. Typically, most of the setup
involving Reaction controllers is done with the
Manager dialog. You use the dialog to
define a master, which is an object that
controls other objects, and, for each master, any number of slaves,
which are objects the master controls. Alternatively, you can assign
a Reaction controller directly to a slave object using Track View
or the Motion panel (as you would any other controller), and then
use Reaction Manager to specify its master and other parameters.
come in five different types: Position Reaction, Rotation Reaction,
Point3 Reaction, Scale Reaction, and Float Reaction. You can assign
a Reaction controller to any animatable track in the scene. Reaction
is not based on time, but rather on other variables in your scene
such as position or rotation.
You can use a Reaction
controller to turn on a light as an object nears a given point.
Muscles can bulge as an arm bone rotates. A ball can squash automatically
as the ball's Z position nears the ground plane. Feet can rotate
as their heels are lifted from the floor.
A particle system can
be triggered by any given event. Morph target percentages can be
controlled by events.
NoteThe Reaction controllers
replace the Reactor controllers found in previous versions of 3ds Max.
If you load a file containing a Reactor controller into this version
of 3ds Max, the Reactor controller is converted to a Reaction
controller, with all settings and reactions fully intact.
Example: To make the position of a sphere
react to the position of a box:
This procedure shows
an alternate way to use a Reaction controller. The recommended workflow
is to do most of the setup in the Reaction Manager dialog: See
Example: To use the Reaction Manager dialog:.
NoteObjects don't need
to be animated to use Reaction controllers. This procedure starts
with animation to make the reaction easier to see.
- On the left side of the Top viewport,
create a box about 30 units on a side.
- Animate the box from position (-100,0,0) at
frame 0 to position (100,0,0) frame 100.
- In the Front viewport, create a sphere
slightly above the box. Position the sphere at (–100,0,50).
- Go to the Motion
panel, and on the Assign Controller rollout, click the Position
track to highlight it.
- On the Assign Controller rollout, click (Assign Controller).
- On the Assign Position Controller dialog,
click Position Reaction to highlight it, and then click OK.
This opens the
Manager dialog. The Reactions list shows
that the sphere is assigned as a slave, but no master is assigned.
A Reaction controller uses a master's motion to control any number
- Right-click the “Unassigned” line at
the top of the Reactions list. From the menu, choose Replace Master.
This places you in a
Pick mode where you can choose a motion track from any object in
the scene to act as master.
- In any viewport, click the box (Box01).
A pop-up menu appears.
- From the pop-up menu, choose Transform Position X Position.
The “Unassigned” text
is replaced by a master track labeled “Box01 / X Position”.
Also, a new state, State01,
appears in the States list.
This is an example of
the basis of the control mechanism used by the Reaction controller:
For each master/slave combination, you can specify any number of
states defined by values for the master and slave tracks. The first
state, created automatically when you added the master, specifies that
when the box is at –100 on the X axis, the sphere should be at (–100,0,50).
In this case, you're controlling three parameters (the sphere position
on all three axes) with one (the box position on the X axis). More typically
you'd use one-to-one master/slave-parameter ratios.
also defines a state automatically when you assign a new slave to
Next you'll create a
second state that tells the sphere how to move on multiple axes
as the box moves on one.
- Drag the time slider to frame 50.
The box moves to the
midpoint of its animated trajectory. The sphere remains where it
- On the Reaction Manager dialog, click (Create State).
This creates a new state
(State02) using the current positions of the box and sphere.
- Try moving the sphere in the Front viewport.
You can't move the sphere
because it's under the full control of its master, the box. Similarly,
you can't change its position using the Coordinate Display fields
on the status bar. However, you can change the sphere's position
using the Reaction Manager's editing tools.
- On the Reaction Manager dialog, click
the Sphere01 / Position entry to highlight it, if necessary, and
then click the Edit Mode button.
- Position the sphere at (0,0,100).
The position updates
in the Sphere01 / Position line under State02. Also, the Reaction
Manager graph shows the change in values of the slave's (sphere)
X and Z position values as the master box's X position changes.
On the graph, as in Track View, red=X, green=Y, and blue=Z.
- Click the Edit Mode button again to turn
it off, and then scrub the time slider.
As the box moves between
frames 0 and 50, the sphere rises to the position specified in the
For the third and final
state, you'll have the slave move in a different direction over
the second half of the master's motion.
- Go to frame 100, and then create a new
- In the States list, click the new state
or its slave track to highlight it.
- Turn on Edit Mode. Position the sphere
at (0,–100,100), and then turn off
- Scrub the time slider between frames
0 and 100.
Over the first 50 frames,
the sphere moves between the first two states, and over the second
half of the animation, the sphere moves between the second and third
To conclude the procedure,
you'll demonstrate that the sphere is responding only to the box's
X position, regardless of animation.
- Go to frame 0. Select the box, and then
press and hold the Alt key
and right-click the box.
This opens the Animation
- From the Set (lower-right) quadrant,
choose Delete Selected Animation.
This command deletes
all animation keys for the current selection.
- Now move the box in all three
Any change in the Y or
Z position has no effect on the sphere. However, moving the box
between –100 and 100 on the X axis, no matter what the Y or Z position,
results in changing the sphere's position as specified in the Reaction
This procedure gives
you a hint of the Reaction controller's power. You can find another
procedure showing additional aspects of the Reaction Manager in:
Example: To use the Reaction Manager dialog:.
After assigning a Reaction
controller, right-click the track and then choose Properties, or
choose Animation Reaction Manager,
to open the