Handles

Houdini displays controls on objects and components in the viewer called handles, which you can manipulate to set parameters.

Handles are equivalent to manipulators in other packages.

Show handles for an object in the viewer

Select an object or components and then click the Move, Scale, or Rotate tools.
or
Select the object and then click the Handles tool to get the combined or operation-specific handles.

See how to use the transform handles, camera and light handles, and compositing handles.

Move, rotate, and scale handles

Most operators and tools that have handles use the transform handles: move (A.K.A. translate), rotate, and scale.

To switch between these handles press RMB on the handle and choose the handle type from the pop-up menu, or press T for move, R for rotate, or E for scale.

Move handle (T)

Rotate handle (R)

Scale handle (E)

General how to and tips

To...	Do this
Align the handles to world, object, camera, or parent space	on the handle and choose an option from the Align submenu.
Turn off axis coloring on handles	This makes Houdini draw handles all red (as was the default in versions of Houdini prior to 9.0), instead of using red, green, and blue to color the different axes. Choose Edit > Preferences > Handles. Turn off Color transform handles.
Show the plane movement handle	Ctrl-click an axis of the transform handle to show or hide a box handle at the center of the handle. Dragging this box moves the handle across the plane perpendicular to the axis you clicked.
Show the handle for the currently selected node	Press Enter.
Show a context menu of handle commands	Press on the handle.
Adjust handle values in small or large increments	Press on a handle part to adjust its value with a value ladder.
Key the handle parameters	Press K.
Move an object/selection’s pivot point	Press Ins to switch to the pivot handle. Move the pivot point. Press Ins again to return to the regular handle.
Detach and move a handle from its default position	Press ' (apostrophe) to detach the handle. Use the transform handles to move the handle. For example, you can move the handle off to the side if it’s obscuring the geometry you're working on. Press ' again to finish. This does not move the object’s pivot point permanently – it changes the position of the handle, which scales and rotates are relative to. So it has the same effect as moving the pivot point, but is more transitory, for when you want to scale or rotate relative to a certain point, but don’t want to mess with the actual pivot.

Using gimbal handles and fixing gimbal lock

What is gimbal lock?

Gimbal lock is when two rotation axes overlap, making it impossible to rotate them independently. This can cause problems in animation, where it can seem like rotations no longer have the proper effect.

This happens because Houdini can’t rotate all three axes at once, but must apply rotations to one axis at a time (this is a fundamental property of Euler angles). So, if the order is “X rotation, then Y rotation, then Z rotation” (the default), Y and Z necessarily rotate independently of X (that is, rotating in Y does not change the X rotation), and so you can make the axes overlap:

A camera begins with rotations set to 0,0,0.

Camera rotated -45 degrees around Y (the green ring). Because Y rotation is applied after X, rotating around Y does not rotate X (the red ring).

Camera rotated -90 degrees around Y (the green ring). The X axis (red ring) and Z axis (blue ring) now overlap.

Since the axes overlap, changes to X and Y rotation values (in this example) do not rotate the camera around two axes… they rotate along the same axis (and may in fact cancel each other out).

Gimbal mode handles

Houdini’s transform handle has a “Gimbal mode” option to control how the rotation handles work.

When Gimbal mode is off, Houdini does background work so you always see three rotation ring handles, and when you drag a ring, the other rings rotate with it, despite the order of rotations.
The benefit is that it’s impossible to gimbal lock the object by dragging the rotation handles. The downside is that in this mode Houdini will change all three rotation components, not just the one you're dragging. And doing rotations this way can lead to very bad animation curves.
When Gimbal mode is on, Houdini displays the true position of the rotation handles, and dragging the rotation handles changes the rotation components directly, without Houdini trying to fix gimbal lock.
The benefit is that you have direct, independent control over the three rotation components, and so you can use the handles to set up animation without worrying about Houdini changing the other components. The downside is the possibility of gimbal lock.

To turn Gimbal mode on or off, press RMB on a rotation handle and choose Gimbal mode.

How to avoid or get out of gimbal lock

Use the following strategies to avoid gimbal lock:

Change the rotation order	Changing the rotation order changes which axes affect the others. By selecting a different order of rotations, you may be able to do the same rotations without gimbal locking. Set the order of rotations for an object in the object’s parameter editor (the right pop-up menu beside Transform order).
Use a parented null to separate rotations	Lock the object’s Rotate X and Rotate Y parameters. Create a Null object, parent it to the object, and lock its Rotate Z parameter. Rotate the Null to control the object’s X and Y rotation, and rotate the object to control its Z rotation. Because the rotations are on different objects, there is no chance for the axes to overlap, and so the object cannot gimbal lock. (If it is more convenient, you can also stack up X and Y nulls and control Z on the object, or stack up X, Y, and Z nulls and lock all rotation on the object.)
Turn off gimbal mode on the handle	(See above.) When Gimbal mode is off, Houdini will do extra math in the background to let you rotate the handles without worrying about gimbal lock, by changing all three rotation components.
Control rotation with look-at and up vector	Instead of controlling an object’s rotation with the rotation handles, you can use a look-at object. Create a Null object and set it in your object’s Look at parameter. Transform the Null to control your object’s rotation. Use your object’s Rotate Z or Up vector parameters to roll the object. This lets you rotate the object without directly setting rotations, avoiding gimbal lock, and it doesn’t require parenting. The downside is if your look-at Null passes through the exact position of your object, the object will flip across the axis.
Use `qlinear` expression function on the channel	The qlinear expression function to control the channels does a quaternion, gimbal-free equivalent of the linear function. `qlinear` must be on all rotation channels with names ending with `rx`, `ry`, and `rz`, and the keys must exist at the same frame.

Persistent handles

You can make an operator’s handle persistent, so they stay constantly visible in the viewer pane even when the operator is not active. This gives you convenient access to visually edit parameters controlled by the handle at any time.

You can also create a persistent handle from scratch that is not tied to any particular node, and edit which parameter a particular handle of a persistent handle.

To...	Do this
To make a handle persistent	Press on the handle and turn on Persistent.
Create or edit a persistent handle	Use the persistent handle editor.
Manage persistent handles	Use the handle list pane .

How to manage groups of handles

If you do not have a handle list pane visible, create a new pane and make it a handle list.

To...	Do this
Group handles together	Select the handles you want to group together in the lower list. Click Add Group.
Show/hide a handle group	To hide a handle group, click the “H” box next to the group’s name in the upper part of the handle list. To show a handle group, click the “E” box next to the group’s name in the upper part of the handle list.

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Compositing operation handles

Use the camera and light handles

Use the transform handles