User guide

be used. Under each of these locations you have separate branch of the hierarchy that declares the actual geometry

used for that LOD representation of the asset.

The most common metadata used to determine which level of detail to use are tags or weights. Tags allow each level

of detail to be given a 'tag' name with a string. Selection of which level of detail to use can be done using this tag

name, such as select the level of detail called 'high' or 'boundingVolume'.

Weights allow a floating point value to be assigned to each level of detail. Selection can then be done by choosing the

closest level of detail to a given weight value. This allows sparse population of levels of detail, for example not every

asset might have a 'medium' level of detail, but if you select them by weight then the most appropriate LOD from

whatever representations exist can be selected.

LodSelect can be used to selection of which LOD to use using either tags or weight values. This uses a CEL expression

to specify the LOD Groups you want to do the selection on.

Some renderers, such a Pixar's RenderMan, have features to handle multiple LODs themselves. Selection of which

LOD to use, and potential blending between the LODs as they transition, is done at render-time. This is specified by

having range data associated with each LOD that describes the range of distances from camera to use that LOD for,

and the transition range for any blending. LOD range data can be set up using LodGroupCreate or LodValuesAssign

nodes.

Alembic and Other Input Data Formats

It is potentially possible to bring in 3D scene data from any source. However, due to the way that filters can get

called recursively on-demand it is best to work with formats that can be efficiently accessed in this manner. This is

one of the reasons that we recommend Alembic as being ideally suited for delivering assets to Katana.

If you want to write a custom plug-in to read in data from a new source, such as using an in-house geometry caching

format, you can write a Scene Graph Generator plug-in. This is a C++ API that allows you to create new locations in

the scene graph hierarchy and set attribute value. For more details about the Scene Graph Generator API, and the

associated Attribute Modifier API (for C++ plug-ins to modify attributes on existing scene graph locations) please

look at sections 12.1 and 12.2.

10 STRUCTURED SCENE GRAPH DATA | ALEMBIC AND OTHER INPUT DATA FORMATS