In SMTK, operations performed on geometric or conceptual models are represented by instances of the Operator class. Each operation, such as creating an edge, is a subclass that contains the code to perform the operation by overriding the operate() and ableToOperate() methods. These subclasses register themselves (via SMTK’s auto-init macros) in a way that allows a list of applicable subclasses to be enumerated for any given type of model entity. Operators perform work on a model in situ: they modify the model in place. If you are familiar with VTK, this can be confusing, since VTK’s pipeline filters leave their input datasets untouched while SMTK’s operators do not. However, this in-place operation is a standard practice among solid modeling software.


Describe SMTK’s auto-init macros.

The next sections describe in detail: first, how operators specify the inputs they require and outputs they produce; and second, how operators register themselves for intropspection.

Inputs and Outputs

The inputs that an operator requires in order to run and the outputs that an operator may produce are each modeled as attribute definitions. These definitions reside in an attribute resource owned by an instance of a modeling session.

Each operator’s inputs are specified by a Definition that inherits a base attribute-definition named “operator” while the its outputs are specified by a Definition that inherits a base attribute-definition named “result”. Furthermore, if an operator’s input specification is named “foo”, its output specification must be named “result(foo)”.

SMTK uses this naming convention to construct results automatically. Given an instance of an Operator subclass, you can call its specification() method to obtain the Attribute which models its inputs. When the operator is executed, it returns an Attribute instance that models its result.

Recall that the attribute.Resource holds Definitions which contain ItemDefinitions. When a Definition is instantiated as an Attribute, all its active ItemDefinitions are instantiated as Items. The Definitions themselves specify what types of model-entities may be associated with their Attributes using a membership mask. Associations between an operator’s Attribute and model entities (i.e., subclasses of EntityRef) denote the entities that the operator will act upon. The operator-attribute’s Items specify input parameters such as point locations, geometric distances, etc.

The fact that inputs and outputs are specified using SMTK’s own attribute resource means that one need not construct an instance of the operator’s C++ class in order to obtain information about it; instead, simply call operatorSystem() on the session and ask for all the definitions which inherit “operator”.

The next sections go over conventions that SMTK uses for its inputs and outputs.


There are no naming conventions for input parameters, however:

  • operators should use model-entity associations as the primary means for selecting geometry that an operator will act upon;

  • also, by using the attribute resource to hold operator specifications, simply checking whether an attribute is in a valid state is usually enough to guarantee that the operator can run successfully. This is what the default implementation of ableToOperate() does. Subclasses may override the method to provide additional checks (i.e., whether a file is writable);

  • when an operator needs only an associated set of model inputs, either because it has no parameters or they all take on valid default values, it can be run without further user input and applications are encouraged to provide a way to execute them immediately. However, when ableToOperate() returns false or applications wish to provide users a chance to override default parameters, some interface must be provided. The qtOperatorView class is provided for this purpose.


Output attributes have several items with standard names used to tell applications what changes an operator has made to the model:

  • created (ModelEntityItem) An array of model entities that were created during the operation. These items may have tessellations (meaning there are new renderable entities) or relationships to existing entities (meaning there are new items to insert in the tree view).

  • modified (ModelEntityItem) An array of model entities that were modified during the operation. This does not imply that an entity’s tessellation has changed; the tess_changed entry below is used for that.

  • expunged (ModelEntityItem) An array of model entities that were removed entirely from the model manager during the operation.

  • tess_changed (ModelEntityItem) An array of model entities whose geometric tessellations changed during the operation. This is signaled separately from modified above to minimize the overhead in rendering when only topological changes have occurred.

  • cleanse entities (VoidItem) When present and enabled, this operator marks the modified and created entities as “clean” (meaning that they do not need to be saved; they are at exactly the state present in their owning-model’s URL).

  • allow camera reset (VoidItem) When present and enabled, this operator will allow (but not force) the camera of the active render view to be reset. A reset will actually occur when no renderable entities existed prior to the operation but at least one renderable entity exists afterward. Operators which load data from files are encouraged to include this item in their result attribute while operators which let users create or modify entities interactively — especially through interaction in render-views — are discouraged from allowing camera resets.


  • How to enumerate operators: ask the session.

  • Operators are registered with a particular session via the operator’s use of the smtkDeclareModelOperator and smtkImplementsModelOperator macros.

  • Operators registered with the base session are inherited unless the session’s constructor prevents it explicitly.