The attribute resource is composed of C++ classes, also accessible in Python, whose instances perform the following functions:
instances represent a dictionary of named values. The values are all subclasses of the Item class. The entries that may appear in an attribute’s dictionary are constrained by the attribute’s Definition. In addition to holding a set of Items, an attribute may optionally be attached to (or associated with in SMTK’s parlance) a set of geometric model entities from SMTK’s geometric modeling system.
instances hold the set of possible key-value pairs that must be present in Attribute instances that reference them. A definition may inherit another definition as a base type. For instance, deflection, temperature, and voltage boundary condition definitions might all inherit a Dirichlet boundary condition definition. Even when the base class provides no requirements, this is useful for fetching attributes that meet a specific condition.
instances hold values in an attribute key-value pair. The particular subclass of Item determines the type of storage used to hold the value (e.g. Int, Double, String, RefItem, ModelEntityItem). Each item references an ItemDefinition that constrains the values that may be held in storage, in much the same way that an Attribute has a Definition. Some items (those derived from ValueItem) can have other items as children; this is used to implement conditional items, where the presence of children is predicated on the value taken on by their parent item.
instances constrain the number of values that an Item instance may contain as well as the particular values that are considered valid. For example, an ItemDefinition for temperature could specify that temperature is a scalar (i.e., only a single value may be held in the Item), that it is a floating point value, and that it must be positive.
instances hold resources of attributes associated with a particular purpose such as
defining a simulation’s input deck (see the simulation workflows repository for examples);
specifying locations where input and output files are located during the export process (SMTK’s simulation subsystem creates an attribute resource for this purpose); and
defining operations that can be performed on a geometric model (SMTK’s geometric modeling system uses an attribute resource to hold definitions for each modeling operation that can be performed by each of its modeling kernels).
Because it can be tedious to programmatically create a bunch of instances of the classes above to represent a particular simulation’s input deck, SMTK provides an XML file format for serializing and deserializing all of the attributes, definitions, items, and item-definitions stored in an attribute resource.
Interfaces to the attribute resource¶
The attribute resource has three interfaces:
1. An XML file syntax for specifying the kinds data to be modeled for individual simulation codes and problem domains. This file syntax is used to specify the unique inputs for a simulation code, including, for example, data types, valid ranges, and default values. Specific data can be associated with geometric model entities, and structural features can be used to group and classify simulation data.
2. A set of user-interface panels for end users to create and edit simulation data. Using standard windows components (buttons, text boxes, selection lists, etc.), users can enter all of the detailed data needed to specify a desired simulation. These user-interface panels are automatically produced by SMTK at runtime, based on the XML file(s) provided to the system.
3. An API for accessing the simulation data produced by end users. Once an end-user has completed specifying the simulation, application software can be used to traverse that data and generate the simulation input files. The native SMTK API is C++, and python bindings are also included. In practice, python scripts are typically used to access the simulation data and generate the simulation input files.
SMTK can be used in a broad range of scientific and engineering simulation applications. In physics-based applications, such as structural mechanics, computational fluid dynamics (CFD), and electromagnetic (EM) modeling, simulations are often performed relative to a geometric model. This model may be created using computer-aided design (CAD), or computationally constructed/reconstructed from empirical data. In either case, a discretization process is typically performed with the model to generate geometric input suitable for the simulation code, often in the form of a finite element or finite volume mesh. To complete the simulation inputs, the non-geometric inputs are generated to enumerate the specific boundary conditions, material properties, solver parameters, etc.
With SMTK, the process of generating simulation input data can be automated, providing the dual benefits of work-product reuse and error reduction. Once the simulation-specific data are defined in an XML template, domain experts or other end users can create the simulation data, or attribute data, for specific problems using the SMTK user interface panels. Then simulation-specific python scripts can be used to traverse the attribute data and write the simulation input files.
<Definitions> <AttDef Type="Example1" Label="Example 1" BaseType="" Version="0" Unique="true" Associations=""> <ItemDefinitions> <String Name="ExampleString" Label="String item:" Version="0" NumberOfRequiredValues="1"> <BriefDescription>Enter some string of import</BriefDescription> <DefaultValue>Yellow denotes default value</DefaultValue> </String> <Int Name="ExampleInteger" Label="Integer item:" Version="0" NumberOfRequiredValues="1"> <BriefDescription>For some integer value</BriefDescription> <DefaultValue>42</DefaultValue> </Int> <Double Name="ExampleDouble" Label="Double item:" Version="0" NumberOfRequiredValues="1"> <BriefDescription>For floating-point precision values</BriefDescription> <DefaultValue>3.14159</DefaultValue> </Double> <Double Name="ExampleVector" Label="Double item w/3 values:" Version="0" NumberOfRequiredValues="3"> <BriefDescription>Number of components is set to 3</BriefDescription> <ComponentLabels> <Label>x</Label> <Label>y</Label> <Label>z</Label> </ComponentLabels> <DefaultValue>0</DefaultValue> </Double> <String Name="SecondString" Label="Another string item:" Version="0" NumberOfRequiredValues="1"> <BriefDescription>Enter some string of import</BriefDescription> <DefaultValue>whatever</DefaultValue> </String> </ItemDefinitions> </AttDef> <!-- Remaining content not shown -->
One particular workflow SMTK supports is attributes that can be defined by multiple, distinct sets of values. For example, there are many ways to define a circle, four of which are: using 3 points on the circle, using a center and radius, using 2 points (a center plus a point on the circle), and by inscribing a circle inside a triangle. There are times when you will want to represent an attribute that can accept any of these definitions instead of constraining the user to work with a single construction technique.
SMTK accommodates this by having you create an auxiliary item whose value enumerates the different possible definitions. This auxiliary item owns all of the items that are active depending on the auxiliary item’s value. For our example of a circle, the attribute definition would be
<Definitions> <AttDef Type="circle" Label="Circle" BaseType="" Version="0"> <ItemDefinitions> <!-- Here is our auxiliary item --> <Int Name="construction method"> <!-- First we list *all* of the child items --> <ChildrenDefinitions> <Double Name="radius" NumberOfRequiredValues="1"> <Min Inclusive="false">0</Min> <DefaultValue>0.5</DefaultValue> </Double> <Double Name="center" NumberOfRequiredValues="3"> <DefaultValue>0.0</DefaultValue> </Double> <Double Name="point 1" NumberOfRequiredValues="3"/> <Double Name="point 2" NumberOfRequiredValues="3"/> <Double Name="point 3" NumberOfRequiredValues="3"/> </ChildrenDefinitions> <!-- Now we enumerate the acceptable auxiliary values along with the children used in each case. --> <DiscreteInfo DefaultIndex="1"> <Structure> <Value Enum="3 points">0</Value> <Items> <Item>point 1</Item> <Item>point 2</Item> <Item>point 3</Item> </Items> </Structure> <Structure> <Value Enum="2 points">1</Value> <Items> <Item>center</Item> <Item>point 1</Item> </Items> </Structure> <Structure> <Value Enum="center and radius">2</Value> <Items> <Item>center</Item> <Item>radius</Item> </Items> </Structure> <Structure> <Value Enum="inscribe in triangle">3</Value> <Items> <Item>point 1</Item> <Item>point 2</Item> <Item>point 3</Item> </Items> </Structure> </DiscreteInfo> </Int> </ItemDefinitions> </AttDef> </Definitions>
You can see that each “Structure” section describes a particular way to define the circle. Different subsets of the items are active depending on whether the auxiliary “construction method” value is 0, 1, 2, or 3.
When SMTK generates a user interface for the attribute above, the “construction method” value is represented as a tabbed widget with 1 tab for each of the “Structure” sections above. The default tab will be “2 points”.