Q100389: Importing an Alembic file with multiple geometry objects as seperate ReadGeo nodes
Nuke has the ability to read and write Alembic files (.abc), either created in Nuke or imported from an external 3D program. Reading and writing Alembic files within Nuke is a pretty straight forward process, the ReadGeo node is used to import Alembic data and the WriteGeo node is used to export Alembic data.
One advantage of the Alembic format is the ability to store multiple geometry objects into a single Alembic file, and share this data between scripts or other programs.
This article shows the correct way to import an Alembic file that contains multiple geometry objects, using the ReadGeo node in Nuke.
The ReadGeo node's main purpose is to bring in a variety of created data from other 3D programs into Nuke. One format often used is Alembic (.abc). This is quite a popular format as it provides a universally accepted open computer graphics interchange framework.
Within Nuke, you can read and write a variety of data from or into an Alembic file, including geometry, cameras, point clouds and axis's information.
When using the WriteGeo node to write 3D data to an Alembic file, you have the following export options:
When using the ReadGeo node to import Alembic data into Nuke, you can import files containing a single geometry or multiple geometry objects.
Importing Alembic files with multiple geometry objects
If you have multiple geometry objects in the same Alembic file, then you will get the following dialog box when trying to import the file using a ReadGeo node:
In this dialog box, you have two options:
- 'Create all-in-one node'
- 'Create parents as separate nodes'
Using 'Create all-in-one' creates one ReadGeo node for everything that’s selected in the dialog box. Using 'Create parents as separate nodes' allows creating one ReadGeo node for each parent item in the selection tree.
If you proceed with the current Alembic object selection as above, clicking either of these options will have the same result: A single ReadGeo node will be used with all the geometry objects combined into one object, with no ability to split it within Nuke.
To have multiple ReadGeo nodes used when dealing with multiple geometry objects in the same Alembic file, you need to first define a parent and child setup in the dialog box, before selecting the 'Create parents as separate nodes' option.
To have each geometry object represented correctly, each independent piece of geometry will need to be given a parent value. By default, when importing independent geometry, the 'root' directory is the parent (indicated by the yellow circle on the far right) and the geometry and/or other sub folders are set as children (indicated by the yellow line on the far right).
To explain the usage of the parent and child assignment, below are a series of steps that show how the import process should be done:
1) Using a ReadGeo node, import an Alembic file that contains multiple geometry objects
2) In the ReadGeo dialog box displayed, select all the geometry objects you would like as independent ReadGeo nodes, then right-click and select 'select as parent'
3) This sets these geometry objects to parents, and will be indicated with an yellow circle
4) The last step to complete the setup is to click 'Create parents as separate nodes'.
The result in the Viewer is that each geometry will be imported into Nuke using an independent ReadGeo node. For the above example, 3 such nodes will be created like:
If you want to work with these geometry objects as one group, there are two methods that can be used:
1) Combine using a MergeGeo node
2) Combine using a Scene node
The advantage of having the geometry objects independent as opposed to combined into one group or piece of geometry, is the ability to alter the geometry at an individual level. This allows the transforming, reshaping or removal of geometry easier within Nuke.
More information about importing Alembic geometry in Nuke and the basics of the parent/child relationship, can be found in the Nuke Online Help > Importing Geometry and Point Clouds from Alembic Files section.
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