3.3.1.49. NXtransformations

Status:

contributed definition, extends NXobject, version 1.1

Description:

Collection of axis-based translations and rotations to describe a geometry. May also contain general axes that do not move.

A nested sequence of transformations lists the offset and rotation steps needed to describe the position and orientation of any movable or fixed device.

There will be one or more transformations (axes) defined by one or more fields for each transformation. The all-caps name AXISNAME designates the particular axis generating a transformation (e.g. a rotation axis or a translation axis or a general axis). The all-caps name AXISUNITS designates the units appropriate to the transformation_type value, i.e. NX_LENGTH for translation, NX_ANGLE for rotation, and NX_UNITLESS for general.

This class will usually contain all axes of a sample stage or goniometer or a detector. The NeXus default McSTAS coordinate frame is assumed, but additional useful coordinate axes may be defined as general axes.

The entry point (depends_on) will be outside of this class and point to a field in here. Following the chain may also require following depends_on links to transformations outside, for example to a common base table. If a relative path is given, it is relative to the group enclosing the depends_on specification.

TODO: this needs an equation (issue #484)

Symbols:

No symbol table

Groups cited:

none

Structure:

AXISNAME: NX_NUMBER {units=AXISUNITS}

Units need to be appropriate for translation or rotation

The name of this field is not forced. The user is free to use any name that does not cause confusion. When using more than one AXISNAME field, make sure that each field name is unique in the same group, as required by HDF5.

@transformation_type: NX_CHAR

The transformation_type may be translation, in which case the values are linear displacements along the axis, rotation, in which case the values are angular rotations around the axis, or general for an axis that simply helps to define a coordinate frame and does not have any displacement values.

Any of these values: translation | rotation | general

@vector: NX_NUMBER {units=NX_DIMENSIONLESS}

Three values that define the axis for this transformation. The axis should be normalized to unit length, making it dimensionless. For rotation axes, the direction should be chosen for a right-handed rotation with increasing angle. For translation axes the direction should be chosen for increasing displacement.

@offset: NX_NUMBER

A fixed offset applied before the transformation (three vector components).

@offset_units: NX_CHAR

Units of the offset. Values should be consistent with NX_LENGTH.

@depends_on: NX_CHAR

Points to the path to a field defining the axis on which this depends or the string ”.”.

AXISNAME_end: (optional) NX_NUMBER {units=AXISUNITS}

AXISNAME_end is a placeholder for a name constructed from the actual name of an axis to which _end has been appended.

The values in this field are the end points of the motions that start at the corresponding positions given in the AXISNAME field.

AXISNAME_range: (optional) NX_NUMBER {units=AXISUNITS}

AXISNAME_range is a placeholder for a name constructed from the actual name of an axis to which _range has been appended.

The values in this optional field are differences between the values in the field AXISNAME_end and the field AXISNAME.

AXISNAME_average_range: (optional) NX_NUMBER {units=AXISUNITS}

AXISNAME_average_range is a placeholder for a name constructed from the actual name of an axis to which _average_range has been appended.

The value of this optional field is the average of the differences between the values in the field AXISNAME_end and the field AXISNAME.

NXDL Source:

https://github.com/nexusformat/definitions/blob/master/contributed_definitions/NXtransformations.nxdl.xml