Hierarchy is defined as a series of ordered groupings. In 3D animation these hierarchies are also known as 'Parenting' or 'Parent/Child' relationships, modo uses these grouping hierarchies to define how multi tiered objects relate to each other. Without them, it would be far too cumbersome to animate anything beyond simple single item transforms.
Hierarchies are essential to animation as they define the orders of inheritance for transformations in a chain of items. Consider an arm, if you move your shoulder (or torso), the arm follows along, as does the wrist and hand. So the shoulder bending is passing its transforms on to the elbow, which can also bend, both of those transforms are passed on to the hand so on. modo visualizes these hierarchy chains as cascading items within the item list.
Note the indentations of each subsequent item in the list. The top locator item is the 'parent' item, the next its 'child', followed by siblings in lower orders. Proper ordering of items in a hierarchy is imperative as it defines how the transforms relate to items further down the chain. Creating hierarchies in the item list is a simple process covered in the 'Item List' section of the documentation. The animation tab setup provides some additional tools for creating and modifying hierarchies.
Compensation: When parenting items together that have prior transforms applied, you might find your items popping out of position, what is happening is the child is inheriting the transforms of the parent, instantly sending it off to a position you likely don't want. Enabling the 'Compensate' button which is positioned in the top area of the animation layout, next to the mode buttons, changes the Parent/Unparent function to Parent in Place/Unparent in Place. What this does is ignores prior parental transforms, so items retain their initial positions in relation to their parent.
Parent/Unparent: When in item mode, the 'Parent' and 'Unparent' commands produce the same results as the item list, but work with selections in the 3D viewport. Selection order is important when using this function as it sets the relationship properly. The last item selected becomes the parent while everything else becomes the child of the parent. Multiple items selections won't create an entire chain, to do that, you would need to apply the parenting command multiple times. To use the parent command first make sure you are in 'Item' mode. Select an item in the 3D viewport by clicking on it, this will be the child item, hold down the shift key and select the second item, with both items selected, click the parent button. And that's it. You can see that the first item selected, has now become the child of the second item. Unparent works the same way, select any item or items and click the unparent button to remove them from the hierarchy. Keep in mind when setting up hierarchies that any item can be parented to any other item, cameras, lights, and locators.
Insert:The 'Insert' command is a useful function for adding items into an existing hierarchy without breaking or upsetting the existing chain. 'Insert' will add the first selection as a child of the second selection, inheriting the latter selections transforms and siblings.
Insert at Parent: The 'Insert at Parent' command reverses the insert command, inserting an item as a parent instead of a child, while still inheriting transforms and siblings. Useful for inserting an item into the top of a hierarchy.
Extract: The Extract command will remove an item from within a hierarchy, without disrupting the rest of the hierarchy chain, by automatically parenting children of the extracted item, to its parent, once the item is removed.
3D Viewport Parenting Actions
While in 'Items' selection mode users can use keyboard shortcuts to quickly set up basic hierarchies simply using selection. The last item selected always represents the parent item. While in any 3D viewport, users can select two or more items and use the 'P' keyboard shortcut to parent in place, 'Ctrl+P' performs normal parenting, 'Shift+P' unparents in place, and 'Ctrl+Shift+P' unparents the selected items. For more complex actions users can utilize the 'Drop Action' function.
Users can also utilize drag and drop actions directly in the 3D viewport to define more complex hierarchies, the 'Drop Action' menu in the 'Animate' and 'Layout' interface tabs define what action happens when an item is dropped onto another. Users must be in 'Items' mode with no active tools for the actions to work. Simply LMB+click on an element and drag it toward the target item. The actual element in the interface won't move until the target item is reached and the mouse button is released, producing the desired action.
None- No action is performed on the elements.
Parent- Source item is made a child of the Destination item.
Parent in Place- Source item is parented as above, but with 'Compensation' applied.
Match- Source item is modified to match position scale and rotation of destination.
Match Position- Source item is modified to match the position only of the destination item.
Match Rotation- Dropped item is modified to match the rotation only of the destination item.
Match Scale- Dropped item is modified to match the scale only of the destination item.
Insert- Inserts the source item into the hierarchy of the destination item. The item will be positioned and orientated to match the destination item and will become the parent of the destination item.
Insert at Parent- Similar to 'Insert' but the source item will match it's position and orientation to the destination item's parent (if a parent item is present).
Insert in Place- The source item will be inserted into the destination item's hierarchy as it's parent but will retain it's current position and orientation.
Place- Positions the selected item at the intersection of the mouse pointer and surface.
Place and Align- Positions the selected item at the intersection of the mouse pointer and surface rotating the item to match the surfaces normal direction.
