MD Workspace¶

Description of MDWorkspace¶

• Dimensions: A MDWorkspace can have between 1 and 9 dimensions.
  • Each dimension is defined with a name, units, and minimum/maximum extents.
• MDEvent: A MDEvent is simply a point in space defined by its coordinates, plus a signal (weight) and error.
  • The MDLeanEvent type contains only coordinates, signal and error.
  • The MDEvent type also contains a run index (for multiple runs summed into one workspace) and a detector ID, allowing for more information to be extracted.
• The class is named MDEventWorkspace.

Structure¶

The MDWorkspace is a container that can hold a large number of MDEvents. The events are organized into “boxes”: types are MDBox and MDGridBox. At the simplest level, an MDWorkspace will be a single MDBox with an unsorted bunch of events.

In order to allow for efficient searching and binning of these events, the boxes are organized into a recursive boxing structure (adaptive mesh refinement). During MDWorkspace construction, if a MDBox is found to contain too many events, it will be split into smaller boxes.

MDWorkspace_structure.png

The threshold for splitting is defined in CreateMDWorkspace as the SplitThreshold parameter. Each parent box will get split into N sub-boxes in each dimension. For example, in a 2D workspace, you might split a parent box into 4x4 sub-boxes, creating 16 MDBoxes under the parent box (which becomes a MDGridBox). The level of splitting is defined in the SplitInto parameter.

Creating a MDWorkspace¶

There are several algorithms that will create a MDWorkspace:

• CreateMDWorkspace creates a blank MDWorkspace with any arbitrary set of dimensions.
• ConvertToDiffractionMDWorkspace converts an EventWorkspace or Workspace2D from detector space to reciprocal space, for elastic single-crystal or powder diffraction experiments.
• ConvertToMD converts workspaces for inelastic experiments.
• SliceMD takes a slice out of a MDWorkspace to create a new one.
• LoadSQW converts from the SQW format.

File-Backed MDWorkspaces¶

For workspaces with a large number of events that would not fit in memory, it is possible to use a NXS file back-end as a data store. The box structure will always remain in memory, but the underlying events will be stored in a file and retrieved only when required. This can be set at creation (CreateMDWorkspace) or when loading from a file, or an in-memory MDWorkspace can be converted to file-backed with the SaveMD algorithm.

Because of disk IO, file-backed MDWorkspaces are slower to process for some operations (e.g. binning or slicing). Some types of visualization and analysis, however, are just as fast with file-backed MDWorkspaces as their in-memory equivalent.

Viewing MDWorkspaces¶

• Right-click on a MDWorkspace and select:
  • Show Vates Simple Interface: to open a simplified 3D view based on Paraview.
  • Show Slice Viewer: to open the Slice Viewer, which shows 2D slices of the multiple-dimensional workspace.
• You can also use Python script to open the SliceViewer.

Or, you can load a MDWorkspace .nxs file in Paraview if the proper plugin is installed.

Category: Concepts