# Data model

OVITO organizes the information it processes into data objects, each representing a specific fragment of a dataset. For example, a dataset may be composed of a `SimulationCell` object holding the box dimensions and boundary conditions, a `Particles` object storing information associated with the particles, and a `Bonds` sub-object storing the list of bonds between particles. For each type of data object you will find a corresponding Python class in the `ovito.data` module. All of them derive from one common base class: `DataObject`.

Data objects can contain other data objects, forming a nested structure with parent-child relationships. For example, the `Particles` object is a container, which manages a number of `Property` objects, each being an array of property values associated with the particles. Furthermore, the `Particles` object can also contain a `Bonds` object, which in turn is a container for the `Property` objects storing the per-bond property values:

At the topmost level of this hierarchy of nested objects is always the `DataCollection` class. It is the fundamental unit representing a complete dataset that was loaded from one or more input simulation files, and which then gets processed by the modifier steps of a data pipeline. Modifiers may alter individual data objects within the `DataCollection`, add new data objects to the top-level collection, or create additional sub-objects in nested container objects.

When you call the `Pipeline.compute()` method, you receive back a `DataCollection` holding the computation results of the pipeline. The `DataCollection` class provides various property fields for accessing the different kinds of sub-objects it contains.

It is important to note that a `DataCollection` object represents just a single animation frame and not an entire simulation trajectory. Thus, in OVITO’s data model, a simulation trajectory is rather represented as a series of `DataCollection` instances. A data pipeline operates on and produces only a single `DataCollection` at a time, i.e., it works on a frame-by-frame basis.

## Particles

The `Particles` data object, which is accessible via the `DataCollection.particles` field, holds all particle or molecule-related data. OVITO uses a property-centered representation of particles, where information is stored as a set of uniform memory arrays, all being of the same length. Each array represents one particle property such as position, type, mass, color, etc., and holds the values for all N particles in the system. A property data array is an instance of the `Property` data object class, which is not only used by OVITO for storing particle properties but also bond properties, voxel grid properties, and more.

Thus, a system of particles is nothing else than a loose collection of `Property` objects, which are held together by a container, the `Particles` object, which is a specialization of the generic `PropertyContainer` base class. Each particle property has a unique name that identifies the meaning of the property. OVITO defines a set of standard property names, which have a specific meaning to the program and a prescribed data format. The `Position` standard property, for example, holds the XYZ coordinates of all particles and is mandatory. Other standard properties, such as `Color` or `Mass`, are optional and may or may not be present in a `Particles` container. Furthermore, `Property` objects with non-standard names are supported, representing user-defined particle properties.

The `Particles` container object mimics the programming interface of a Python dictionary, which lets you look up properties by name. To find out which properties are present, you can query the dictionary for its keys:

    >>> data = pipeline.compute()
    >>> list(data.particles.keys())
    ['Particle Identifier', 'Particle Type', 'Position', 'Color']

Individual particle properties can be looked up by their name:

    >>> color_property = data.particles['Color']

Some standard properties can also be accessed through convenient getter fields, for example the `Particles.colors` field:

    >>> color_property = data.particles.colors

The `Particles` class is a sub-class of the generic `PropertyContainer` base class. OVITO defines several property container types, such as the `Bonds`, `DataTable` and `VoxelGrid` types, which all work like the `Particles` type. They all have in common that they represent an array of uniform data elements, which may be associated with an arbitrary set of properties.

## Property objects

A `PropertyContainer` manages a variable set of `Property` objects, each `Property` storing the values for one particular property of all data elements in an array. A `Property` object behaves pretty much like a standard NumPy array:

    >>> coordinates = data.particles.positions

    >>> print(coordinates[...])
    [[ 73.24230194  -5.77583981  -0.87618297]
     [-49.00170135 -35.47610092 -27.92519951]
     [-50.36349869 -39.02569962 -25.61310005]
     ...,
     [ 42.71210098  59.44919968  38.6432991 ]
     [ 42.9917984   63.53770065  36.33330154]
     [ 44.17670059  61.49860001  37.5401001 ]]

Property arrays can be one-dimensional (in case of scalar properties) or two-dimensional (in case of vector properties). The size of the first array dimension is always equal to the number of data elements (e.g. particles) stored in the parent `PropertyContainer`. The container reports the current number of elements via its `count` attribute:

    >>> data.particles.count  # Number of particles
    28655
    >>> data.particles['Mass'].shape   # 1-dim. array
    (28655,)
    >>> data.particles['Color'].shape  # 2-dim. array
    (28655, 3)
    >>> data.particles['Color'].dtype  # Data type of property array
    float64

OVITO currently supports three different numeric data types for property arrays: `float64`, `int32` and `int64`. For built-in standard properties the data type and the dimensionality are prescribed by OVITO. For user-defined properties they can be chosen by the user when creating a new property.

## Global attributes

Global attributes are simple tokens of information associated with a `DataCollection` as a whole, organized as key-value pairs in the Python dictionary `DataCollection.attributes`. File readers automatically generate certain global attributes at the source of a data pipeline to associate the imported dataset with relevant information such as the current simulation timestep number or the name of the input file. In the graphical user interface of OVITO you can inspect the current set of global attributes by opening the Data Inspector panel.

Modifiers in a data pipeline may associate a `DataCollection` with additional attributes to report their computation results. For example, the `ClusterAnalysisModifier` outputs the attribute named `ClusterAnalysis.cluster_count`, which reflects the total number of particles clusters that have been found by the clustering algorithm at the current timestep. OVITO provides functions to export such attributes to an output text file, or to embed them in rendered images and animations as a dynamic `TextLabelOverlay`.

Please refer to the `DataCollection.attributes` documentation for a more extensive overview of how to work with global attributes.

## Data tables

Tabulated data is represented in OVITO by `DataTable` objects, which are a specialized type of `PropertyContainer`. A `DataTable` consists of a variable number of rows and columns. Each column is an instance of the `Property` class. Data tables are typically generated dynamically by modifiers performing computations, for example the `HistogramModifier` or the `CommonNeighborAnalysisModifier`, to store their results. In the graphical user interface of OVITO, data tables are rendered as graphs or charts (line, scatter, histogram plots), found in the data inspector panel.

In Python, all data tables generated by the modifiers in the current pipeline can be accessed from the `DataCollection.tables` dictionary returned by the pipeline. Each table has a unique `identifier` string, which serves as lookup key in that dictionary. Furthermore, OVITO provides export functions for writing data tables to an output text file. Please see the `DataTable` class for further details.

## Surface meshes

OVITO can import or generate surface mesh data structures for visualization purposes and other applications. For instance, the `ConstructSurfaceModifier` can be inserted into a data pipeline to construct a triangulated surface mesh to represent the spatial region filled with particles. The output of this modifier is a `SurfaceMesh` object, which holds the vertices and faces of the mesh. See also corresponding section in the user manual.

In Python, surface meshes generated by the modifiers in the current pipeline can be accessed from the `DataCollection.surfaces` dictionary returned by the pipeline. Each surface mesh has a unique `identifier` string, which serves as lookup key in that dictionary. Please see the `SurfaceMesh` class for further details.

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> - Manipulating data