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
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 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)
You can also press hotkey Y to cycle through these handles as well as any custom handles of the tool.
General how to and tips
|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.
|Show the plane movement handle|
|Show the handle for the currently selected node|
|Show a context menu of handle commands|
|Adjust handle values in small or large increments|
|Key the handle parameters|
|Move an object/selection’s pivot point|
|Detach and move a handle from its default position|
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 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.
|Use a parented null to separate rotations|
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.
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.
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 make a handle persistent|
|Create or edit a persistent handle|
|Manage persistent handles|
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.
|Group handles together|
|Show/hide a handle group|