RFC-5: Coordinate Systems and Transformations#

Add named coordinate systems and expand and clarify coordinate transformations. This document represents the updated proposal following the original RFC5 proposal and incorporates feedback from reviewers and implementers.

Status#

This RFC is currently in RFC state R4 (authors prepare responses).

Role	Name	GitHub Handle	Institution	Date	Status
Author	John Bogovic	@bogovicj	HHMI Janelia	2024-07-30	(Corresponding Author) Implemented
Author	Davis Bennett	@d-v-b		2024-07-30	Implemented validation
Author	Luca Marconato	@LucaMarconato	EMBL	2024-07-30	Implemented
Author	Matt McCormick	@thewtex	ITK	2024-07-30	Implemented
Author	Stephan Saalfeld	@axtimwalde	HHMI Janelia	2024-07-30	Implemented (with JB)
Author	Johannes Soltwedel	@jo-mueller	German Bioimaging e.V.	2025-10-07	(Corresponding Author) Implemented
Endorser	Will Moore	@will-moore	University of Dundee	2025-10-23	Implemented
Endorser	David Stansby	@dstansby	University College London	2025-10-23	Implemented
Endorser	Norman Rzepka	@normanrz	Scalable Minds	2024-08-22
Reviewer	Dan Toloudis, David Feng, Forrest Collman, Nathalie GAudreault, Gideon Dunster	toloudis, dyf, fcollman	Allen Institutes	2024-11-28	Review
Reviewer	Will Moore, Jean-Marie Burel, Jason Swedlow	will-moore, jburel, jrswedlow	University of Dundee	2025-01-22	Review

Overview#

This RFC provides first-class support for spatial and coordinate transformations in OME-Zarr.

Working version title: 0.6dev2

Background#

Coordinate and spatial transformation are vitally important for neuro and bio-imaging and broader scientific imaging practices to enable:

Reproducibility and Consistency: Supporting spatial transformations explicitly in a file format ensures that transformations are applied consistently across different platforms and applications. This FAIR capability is a cornerstone of scientific research, and having standardized formats and tools facilitates verification of results by independent researchers.
Integration with Analysis Workflows: Having spatial transformations as a first-class citizen within file formats allows for seamless integration with various image analysis workflows. Registration transformations can be used in subsequent image analysis steps without requiring additional conversion.
Efficiency and Accuracy: Storing transformations within the file format avoids the need for re-sampling each time the data is processed. This reduces sampling errors and preserves the accuracy of subsequent analyses. Standardization enables on-demand transformation, critical for the massive volumes collected by modern microscopy techniques.
Flexibility in Analysis: A file format that natively supports spatial transformations allows researchers to apply, modify, or reverse transformations as needed for different analysis purposes. This flexibility is critical for tasks such as longitudinal studies, multi-modal imaging, and comparative analysis across different subjects or experimental conditions.

Toward these goals, this RFC expands the set of transformations in the OME-Zarr spec covering many of the use cases requested in this github issue. It also adds “coordinate systems” - named sets of “axes.” Related the relationship of discrete arrays to physical coordinates and the interpretation and motivation for axis types.

Proposal#

Below is a complete copy of the proposed changes including suggestions from reviewers and contributors of the previously associated github pull request. The changes, if approved, shall be translated into bikeshed syntax and added to the ngff repository in a separate PR. This PR will then comprise complete json schemas when the RFC enters the SPEC phase (see RFC1).

“coordinateSystems” metadata#

A coordinateSystem is a JSON object with a “name” field and a “axes” field. Every coordinate system:

MUST contain the field “name”. The value MUST be a non-empty string that is unique among all entries under coordinateSystems.
MUST contain the field “axes”, whose value is an array of valid “axes” (see below).

The elements of "axes" correspond to the index of each array dimension and coordinates for points in that coordinate system. For the below example, the "x" dimension is the last dimension. The “dimensionality” of a coordinate system is indicated by the length of its “axes” array. The “volume_micrometers” example coordinate system below is three dimensional (3D).

Example

Coordinate Systems metadata example

{
    "name" : "volume_micrometers",
    "axes" : [
        {"name": "z", "type": "space", "unit": "micrometer"},
        {"name": "y", "type": "space", "unit": "micrometer"},
        {"name": "x", "type": "space", "unit": "micrometer"}
    ]
}

The axes of a coordinate system (see below) give information about the types, units, and other properties of the coordinate system’s dimensions. Axis names may contain semantically meaningful information, but can be arbitrary. As a result, two coordinate systems that have identical axes in the same order may not be “the same” in the sense that measurements at the same point refer to different physical entities and therefore should not be analyzed jointly. Tasks that require images, annotations, regions of interest, etc., SHOULD ensure that they are in the same coordinate system (same name and location within the Zarr hierarchy, with identical axes) or can be transformed to the same coordinate system before doing analysis. See the example below.

“axes” metadata#

“axes” describes the dimensions of a coordinate systems and adds an interpretation to the samples along that dimension.

It is a list of dictionaries, where each dictionary describes a dimension (axis) and:

MUST contain the field “name” that gives the name for this dimension. The values MUST be unique across all “name” fields in the same coordinate system.
SHOULD contain the field “type”. It SHOULD be one of the strings “array”, “space”, “time”, “channel”, “coordinate”, or “displacement” but MAY take other string values for custom axis types that are not part of this specification yet.
MAY contain the field “discrete”. The value MUST be a boolean, and is true if the axis represents a discrete dimension (see below for details).
SHOULD contain the field “unit” to specify the physical unit of this dimension. The value SHOULD be one of the following strings, which are valid units according to UDUNITS-2.
- Units for “space” axes: ‘angstrom’, ‘attometer’, ‘centimeter’, ‘decimeter’, ‘exameter’, ‘femtometer’, ‘foot’, ‘gigameter’, ‘hectometer’, ‘inch’, ‘kilometer’, ‘megameter’, ‘meter’, ‘micrometer’, ‘mile’, ‘millimeter’, ‘nanometer’, ‘parsec’, ‘petameter’, ‘picometer’, ‘terameter’, ‘yard’, ‘yoctometer’, ‘yottameter’, ‘zeptometer’, ‘zettameter’
- Units for “time” axes: ‘attosecond’, ‘centisecond’, ‘day’, ‘decisecond’, ‘exasecond’, ‘femtosecond’, ‘gigasecond’, ‘hectosecond’, ‘hour’, ‘kilosecond’, ‘megasecond’, ‘microsecond’, ‘millisecond’, ‘minute’, ‘nanosecond’, ‘petasecond’, ‘picosecond’, ‘second’, ‘terasecond’, ‘yoctosecond’, ‘yottasecond’, ‘zeptosecond’, ‘zettasecond’
MAY contain the field “longName”. The value MUST be a string, and can provide a longer name or description of an axis and its properties.

The length of “axes” MUST be equal to the number of dimensions of the arrays that contain the image data.

Arrays are inherently discrete (see Array coordinate systems, below) but are often used to store discrete samples of a continuous variable. The continuous values “in between” discrete samples can be retrieved using an interpolation method. If an axis is continuous ("discrete" : false), it indicates that interpolation is well-defined. Axes representing space and time are usually continuous. Similarly, joint interpolation across axes is well-defined only for axes of the same type. In contrast, discrete axes ("discrete" : true) may be indexed only by integers. Axes representing a channel, coordinate, or displacement are usually discrete.

Note

The most common methods for interpolation are “nearest neighbor”, “linear”, “cubic”, and “windowed sinc”. Here, we refer to any method that obtains values at real-valued coordinates using discrete samples as an “interpolator”. As such, label images may be interpolated using “nearest neighbor” to obtain labels at points along the continuum.

Array coordinate systems#

The dimensions of an array do not have an interpretation until they are associated with a coordinate system via a coordinate transformation. Nevertheless, it can be useful to refer to the “raw” coordinates of the array. Some applications might prefer to define points or regions-of-interest in “pixel coordinates” rather than “physical coordinates,” for example. Indicating that choice explicitly will be important for interoperability. This is possible by using array coordinate systems.

Every array has a default coordinate system whose parameters need not be explicitly defined. The dimensionality of each array coordinate system equals the dimensionality of its corresponding Zarr array. Its name is the path to the array in the container, its axes have "type": "array", are unitless, and have default names. The i-th axis has "name": "dim_i" (these are the same default names used by xarray). As with all coordinate systems, the dimension names must be unique and non-null.

Example

{
  "arrayCoordinateSystem" : {
    "name" : "myDataArray",
    "axes" : [
      {"name": "dim_0", "type": "array"},
      {"name": "dim_1", "type": "array"},
      {"name": "dim_2", "type": "array"}
    ]
  }
}

For example, if 0/zarr.json contains:

{
    "zarr_format": 3,
    "node_type": "array",
    "shape": [4, 3, 5],
    //...
}

Then dim_0 has length 4, dim_1 has length 3, and dim_2 has length 5.

The axes and their order align with the shape of the corresponding zarr array, and whose data depends on the byte order used to store chunks. As described in the Zarr array metadata, the last dimension of an array in “C” order are stored contiguously on disk or in-memory when directly loaded.

The name and axes names MAY be customized by including a arrayCoordinateSystem field in the user-defined attributes of the array whose value is a coordinate system object. The length of axes MUST be equal to the dimensionality. The value of "type" for each object in the axes array MUST equal "array".

Coordinate convention#

The pixel/voxel center is the origin of the continuous coordinate system.

It is vital to consistently define relationship between the discrete/array and continuous/interpolated coordinate systems. A pixel/voxel is the continuous region (rectangle) that corresponds to a single sample in the discrete array, i.e., the area corresponding to nearest-neighbor (NN) interpolation of that sample. The center of a 2d pixel corresponding to the origin (0,0) in the discrete array is the origin of the continuous coordinate system (0.0, 0.0) (when the transformation is the identity). The continuous rectangle of the pixel is given by the half-open interval [-0.5, 0.5) x [-0.5, 0.5) (i.e., -0.5 is included, +0.5 is excluded). See chapter 4 and figure 4.1 of the ITK Software Guide.

“coordinateTransformations” metadata#

“coordinateTransformations” describe the mapping between two coordinate systems (defined by “coordinateSystems”). For example, to map an array’s discrete coordinate system to its corresponding physical coordinates. Coordinate transforms are in the “forward” direction. This means they represent functions from points in the input space to points in the output space (see example below).

They:

MUST contain the field “type” (string).
MUST contain any other fields required by the given “type” (see table below).
MUST contain the field “output” (string), unless part of a sequence or inverseOf (see details).
MUST contain the field “input” (string), unless part of a sequence or inverseOf (see details).
MAY contain the field “name” (string). Its value MUST be unique across all “name” fields for coordinate transformations.
Parameter values MUST be compatible with input and output space dimensionality (see details).

The following transformations are supported:

Type	Fields	Description
`identity`		The identity transformation is the do-nothing transformation and is typically not explicitly defined.
`mapAxis`	`"mapAxis":List[number]`	an axis permutation as a transpose array of integer indices that refer to the ordering of the axes in the respective coordinate system.
`translation`	one of: `"translation":List[number]`, `"path":str`	Translation vector, stored either as a list of numbers (`"translation"`) or as a zarr array at a location in this container (`path`).
`scale`	one of: `"scale":List[number]`, `"path":str`	Scale vector, stored either as a list of numbers (`scale`) or as a zarr array at a location in this container (`path`).
`affine`	one of: `"affine":List[List[number]]`, `"path":str`	2D affine transformation matrix stored either with JSON (`affine`) or as a zarr array at a location in this container (`path`).
`rotation`	one of: `"rotation":List[List[number]]`, `"path":str`	2D rotation transformation matrix stored as an array stored either with json (`rotation`) or as a zarr array at a location in this container (`path`).
`sequence`	`"transformations":List[Transformation]`	sequence of transformations. Applying the sequence applies the composition of all transforms in the list, in order.
`displacements`	`"path":str` `"interpolation":str`	Displacement field transformation located at `path`.
`coordinates`	`"path":str` `"interpolation":str`	Coordinate field transformation located at `path`.
`inverseOf`	`"transformation":Transformation`	The inverse of a transformation. Useful if a transform is not closed-form invertible. See forward and inverse of bijections for details and examples.
`bijection`	`"forward":Transformation` `"inverse":Transformation`	An invertible transformation providing an explicit forward transformation and its inverse.
`byDimension`	`"transformations":List[Transformation]`, `"input_axes": List[str]`, `"output_axes": List[str]`	A high dimensional transformation using lower dimensional transformations on subsets of dimensions.

Implementations SHOULD prefer to store transformations as a sequence of less expressive transformations where possible (e.g., sequence[translation, rotation], instead of affine transformation with translation/rotation).

Example

{
  "coordinateSystems": [
    { "name": "in", "axes": [{"name": "j"}, {"name": "i"}] },
    { "name": "out", "axes": [{"name": "y"}, {"name": "x"}] }
  ],
  "coordinateTransformations": [ 
    {
      "type": "scale",
      "scale": [2, 3.12],
      "input": "in",
      "output": "out"
    }
  ]
}

For example, the scale transformation above defines the function:

x = 3.12 * i
y = 2 * j

i.e., the mapping from the first input axis to the first output axis is determined by the first scale parameter.

Conforming readers:

MUST parse identity, scale, translation transformations;
SHOULD parse mapAxis, affine, rotation transformations;
SHOULD display an informative warning if encountering transformations that cannot be parsed or displayed by a viewer;
SHOULD be able to apply transformations to points;
SHOULD be able to apply transformations to images;

Coordinate transformations can be stored in multiple places to reflect different usecases.

Transformations in individual multiscale datasets represent a special case of transformations and are explained below.
Additional transformations for single multiscale images MUST be stored under a field coordinateTransformations in the multiscales dictionaries. This coordinateTransformations field MUST contain a list of valid transformations.
Transformations between two or more images MUST be stored in the attributes of a parent zarr group. For transformations that store data or parameters in a zarr array, those zarr arrays SHOULD be stored in a zarr group called “coordinateTransformations”.

store.zarr                      # Root folder of the zarr store
│
├── zarr.json                   # coordinate transformations describing the relationship between two image coordinate systems
│                               # are stored in the attributes of their parent group.
│                               # transformations between coordinate systems in the 'volume' and 'crop' multiscale images are stored here.
│
├── coordinateTransformations   # transformations that use array storage for their parameters should go in a zarr group named "coordinateTransformations".
│   └── displacements           # for example, a zarr array containing a displacement field
│       └── zarr.json
│
├── volume
│   ├── zarr.json              # group level attributes (multiscales)
│   └── 0                      # a group containing the 0th scale
│       └── image              # a zarr array
│           └── zarr.json      # physical coordinate system and transformations here
└── crop
    ├── zarr.json              # group level attributes (multiscales)
    └── 0                      # a group containing the 0th scale
        └── image              # a zarr array
            └── zarr.json      # physical coordinate system and transformations here

Example

Two instruments simultaneously image the same sample from two different angles, and the 3D data from both instruments are calibrated to “micrometer” units. An analysis of sample A requires measurements from images taken from both instruments at certain points in space. Suppose a region of interest (ROI) is determined from the image obtained from instrument 2, but quantification from that region is needed for instrument 1. Since measurements were collected at different angles, a measurement by instrument 1 at the point with image array coordinates (x,y,z) may not correspond to the measurement at the same array coordinates in instrument 2 (i.e., it may not be the same physical location in the sample). To analyze both images together, they must be transformed to a common coordinate system.

The set of coordinate transformations encodes relationships between coordinate systems, specifically, how to convert points from one coordinate system to another. Implementations can apply the coordinate transform to images or points in coordinate system “sampleA_instrument2” to bring them into the “sampleA_instrument1” coordinate system. In this case, image data within the ROI defined in image2 should be transformed to the “sampleA_image1” coordinate system, then used for quantification with the instrument 1 image.

The coordinateTransformations in the parent-level metadata would contain the following data. The transformation parameters are stored in a separate zarr-group under coordinateTransformations/sampleA_instrument2-to-instrument1 as shown above.

"coordinateTransformations": [
    {
        "type": "affine",
        "path": "coordinateTransformations/sampleA_instrument2-to-instrument1",
        "input": "sampleA_instrument2",
        "output": "sampleA_instrument1"
    }
]

And the image at the path sampleA_instrument1 would have the following as the first coordinate system:

"coordinateSystems": [
    {
        "name": "sampleA-instrument1",
        "axes": [
            {"name": "z", "type": "space", "unit": "micrometer"},
            {"name": "y", "type": "space", "unit": "micrometer"},
            {"name": "x", "type": "space", "unit": "micrometer"}
        ]
    },
]

The image at path sampleA_instrument2 would have this as the first listed coordinate system:

[
    {
        "name": "sampleA-instrument2",
        "axes": [
            {"name": "z", "type": "space", "unit": "micrometer"},
            {"name": "y", "type": "space", "unit": "micrometer"},
            {"name": "x", "type": "space", "unit": "micrometer"}
        ]
    }
],

Additional details#

Most coordinate transformations MUST specify their input and output coordinate systems using input and output with a string value that MUST correspond to the name of a coordinate system or the path to a multiscales group. Exceptions are if the coordinate transformation is wrapped in another transformation, e.g. as part of a transformations list of a sequence or as transformation of an inverseOf transformation. In these two cases input and output could, in some cases, be omitted (see below for details). If unused, the input and output fields MAY be null.

If used in a parent-level zarr-group, the input and output fields can be the name of a coordinateSystem in the same parent-level group or the path to a multiscale image group. If either input or output is a path to a multiscale image group, the authoritative coordinate system for the respective image is the first coordinateSystem defined therein. If the names of input or output correspond to both an existing path to a multiscale image group and the name of a coordinateSystem defined in the same metadata document, the coordinateSystem MUST take precedent.

For usage in multiscales, see the multiscales section for details.

Coordinate transformations are functions of points in the input space to points in the output space. We call this the “forward” direction. Points are ordered lists of coordinates, where a coordinate is the location/value of that point along its corresponding axis. The indexes of axis dimensions correspond to indexes into transformation parameter arrays.

When rendering transformed images and interpolating, implementations may need the “inverse” transformation - from the output to the input coordinate system. Inverse transformations will not be explicitly specified when they can be computed in closed form from the forward transformation. Inverse transformations used for image rendering may be specified using the inverseOf transformation type, for example:

{
    "type": "inverseOf",
    "transformation" : {
        "type": "displacements",
        "path": "/path/to/displacements",
    },
    "input": "input_image",
    "output": "output_image",
}

Implementations SHOULD be able to compute and apply the inverse of some coordinate transformations when they are computable in closed-form (as the Transformation types section below indicates). If an operation is requested that requires the inverse of a transformation that can not be inverted in closed-form, implementations MAY estimate an inverse, or MAY output a warning that the requested operation is unsupported.

Matrix transformations#

Two transformation types (affine and rotation) are parametrized by matrices. Matrices are applied to column vectors that represent points in the input coordinate system. The first and last axes in a coordinate system correspond to the top and bottom entries in the column vector, respectively. Matrices are stored as two-dimensional arrays, either as json or in a zarr array. When stored as a 2D zarr array, the first dimension indexes rows and the second dimension indexes columns (e.g., an array of "shape":[3,4] has 3 rows and 4 columns). When stored as a 2D json array, the inner array contains rows (e.g. [[1,2,3], [4,5,6]] has 2 rows and 3 columns).

Transformation types#

Input and output dimensionality may be determined by the coordinate system referred to by the input and output fields, respectively. If the value of input is a path to an array, its shape gives the input dimension, otherwise it is given by the length of axes for the coordinate system with the name of the input. If the value of output is an array, its shape gives the output dimension, otherwise it is given by the length of axes for the coordinate system with the name of the output.

identity #

identity transformations map input coordinates to output coordinates without modification. The position of the i-th axis of the output coordinate system is set to the position of the ith axis of the input coordinate system. identity transformations are invertible.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

mapAxis #

mapAxis transformations describe axis permutations as a transpose vector of integers. Transformations MUST include a mapAxis field whose value is an array of integers that specifies the new ordering in terms of indices of the old order. The length of the array MUST equal the number of dimensions in both the input and output coordinate systems. Each integer in the array MUST be a valid zero-based index into the input coordinate system’s axes (i.e., between 0 and N-1 for an N-dimensional input). Each index MUST appear exactly once in the array. The value at position i in the array indicates which input axis becomes the i-th output axis. mapAxis transforms are invertible.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

translation #

translation transformations are special cases of affine transformations. When possible, a translation transformation should be preferred to its equivalent affine. Input and output dimensionality MUST be identical and MUST equal the the length of the “translation” array (N). translation transformations are invertible.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

path: The path to a zarr-array containing the translation parameters. The array at this path MUST be 1D, and its length MUST be N.
translation: The translation parameters stored as a JSON list of numbers. The list MUST have length N.

scale #

scale transformations are special cases of affine transformations. When possible, a scale transformation SHOULD be preferred to its equivalent affine. Input and output dimensionality MUST be identical and MUST equal the the length of the “scale” array (N). Values in the scale array SHOULD be non-zero; in that case, scale transformations are invertible.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

path: The path to a zarr-array containing the scale parameters. The array at this path MUST be 1D, and its length MUST be N.
scale: The scale parameters are stored as a JSON list of numbers. The list MUST have length N.

affine #

affines are matrix transformations from N-dimensional inputs to M-dimensional outputs. They are represented as the upper (M)x(N+1) sub-matrix of a (M+1)x(N+1) matrix in homogeneous coordinates (see examples). This transformation type may be (but is not necessarily) invertible when N equals M. The matrix MUST be stored as a 2D array either as json or as a zarr array.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

path: The path to a zarr-array containing the affine parameters. The array at this path MUST be 2D whose shape MUST be (M)x(N+1).
affine: The affine parameters stored in JSON. The matrix MUST be stored as 2D nested array (an array of arrays of numbers) where the outer array MUST be length M and the inner arrays MUST be length N+1.

rotation #

rotations are matrix transformations that are special cases of affine transformations. When possible, a rotation transformation SHOULD be used instead of an equivalent affine. Input and output dimensionality (N) MUST be identical. Rotations are stored as NxN matrices, see below, and MUST have determinant equal to one, with orthonormal rows and columns. The matrix MUST be stored as a 2D array either as json or in a zarr array. rotation transformations are invertible.

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

path: The path to an array containing the affine parameters. The array at this path MUST be 2D whose shape MUST be N x N.
rotation: The parameters stored in JSON. The matrix MUST be stored as a 2D nested array (an array of arrays of numbers) where the outer array MUST be length N and the inner arrays MUST be length N.

inverseOf #

An inverseOf transformation contains another transformation (often non-linear), and indicates that transforming points from output to input coordinate systems is possible using the contained transformation. Transforming points from the input to the output coordinate systems requires the inverse of the contained transformation (if it exists).

The input and output fields MAY be omitted for inverseOf transformations if those fields may be omitted for the transformation it wraps.

Note

Software libraries that perform image registration often return the transformation from fixed image coordinates to moving image coordinates, because this “inverse” transformation is most often required when rendering the transformed moving image. Results such as this may be enclosed in an inverseOf transformation. This enables the “outer” coordinate transformation to specify the moving image coordinates as input and fixed image coordinates as output, a choice that many users and developers find intuitive.

sequence #

A sequence transformation consists of an ordered array of coordinate transformations, and is invertible if every coordinate transform in the array is invertible (though could be invertible in other cases as well). To apply a sequence transformation to a point in the input coordinate system, apply the first transformation in the list of transformations. Next, apply the second transformation to the result. Repeat until every transformation has been applied. The output of the last transformation is the result of the sequence.

A sequence transformation MUST NOT be part of another sequence transformation. The input and output fields MUST be included for sequence transformations.

transformations: A non-empty array of transformations.

coordinates and displacements #

coordinates and displacements transformations store coordinates or displacements in an array and interpret them as a vector field that defines a transformation. The arrays must have a dimension corresponding to every axis of the input coordinate system and one additional dimension to hold components of the vector. Applying the transformation amounts to looking up the appropriate vector in the array, interpolating if necessary, and treating it either as a position directly (coordinates) or a displacement of the input point (displacements).

These transformation types refer to an array at location specified by the "path" parameter. The input and output coordinate systems for these transformations (“input / output coordinate systems”) constrain the array size and the coordinate system metadata for the array (“field coordinate system”).

The input and output fields MAY be omitted if wrapped in another transformation that provides input/output (e.g., sequence, inverseOf, ‘byDimension or bijection).

If the input coordinate system has N axes, the array at location path MUST have N+1 dimensions
The field coordinate system MUST contain an axis identical to every axis of its input coordinate system in the same order.
The field coordinate system MUST contain an axis with type coordinate or displacement, respectively, for transformations of type coordinates or displacements.
- This SHOULD be the last axis (contiguous on disk when c-order).
If the output coordinate system has M axes, the length of the array along the coordinate/displacement dimension MUST be of length M.

The ith value of the array along the coordinate or displacement axis refers to the coordinate or displacement of the ith output axis. See the example below.

coordinates and displacements transformations are not invertible in general, but implementations MAY approximate their inverses. Metadata for these coordinate transforms have the following fields:

path

The location of the coordinate array in this (or another) container.

interpolation

The interpolation attributes MAY be provided. Its value indicates the interpolation to use if transforming points not on the array's discrete grid. Values could be:

linear (default)
nearest
cubic

For both coordinates and displacements, the array data at referred to by path MUST define coordinate system and coordinate transform metadata:

Every axis name in the coordinateTransform’s input MUST appear in the coordinate system.
The array dimension corresponding to the coordinate or displacement axis MUST have length equal to the number of dimensions of the coordinateTransform output
If the input coordinate system N axes, then the array data at path MUST have (N + 1) dimensions.
SHOULD have a name identical to the name of the corresponding coordinateTransform.

For coordinates:

coordinateSystem metadata MUST have exactly one axis with "type" : "coordinate"
the shape of the array along the “coordinate” axis must be exactly N

For displacements:

coordinateSystem metadata MUST have exactly one axis with "type" : "displacement"
the shape of the array along the “displacement” axis must be exactly N
input and output MUST have an equal number of dimensions.

byDimension #

byDimension transformations build a high dimensional transformation using lower dimensional transformations on subsets of dimensions. The input and output fields MUST always be included for this transformations type.

transformations: Each child transformation MUST contain input_axes and output_axes fields whose values are arrays of strings. Every axis name in a child transformation's input_axes MUST correspond to a name of some axis in this parent object's input coordinate system. Every axis name in the parent byDimension's output coordinate system MUST appear in exactly one child transformation's output_axes array. Each child transformation's input_axes and output_axes arrays MUST have the same length as that transformation's parameter arrays.

bijection #

A bijection transformation is an invertible transformation in which both the forward and inverse transformations are explicitly defined. Each direction SHOULD be a transformation type that is not closed-form invertible. Its input and output spaces MUST have equal dimension. The input and output dimensions for the both the forward and inverse transformations MUST match bijection’s input and output space dimensions.

input and output fields MAY be omitted for the forward and inverse transformations, in which case the forward transformation’s input and output are understood to match the bijection’s, and the inverse transformation’s input (output) matches the bijection’s output (input), see the example below.

The input and output fields MAY be omitted for bijection transformations if the fields may be omitted for both its forward and inverse transformations

Practically, non-invertible transformations have finite extents, so bijection transforms should only be expected to be correct / consistent for points that fall within those extents. It may not be correct for any point of appropriate dimensionality.

“multiscales” metadata#

Metadata about an image can be found under the multiscales key in the group-level OME-Zarr Metadata. Here, “image” refers to 2 to 5 dimensional data representing image or volumetric data with optional time or channel axes. It is stored in a multiple resolution representation.

multiscales contains a list of dictionaries where each entry describes a multiscale image.

Each multiscales dictionary MUST contain the field “coordinateSystems”, whose value is an array containing coordinate system metadata (see coordinate systems). The last entry of this array is the “intrinsic” coordinate system and MUST contain axis information pertaining to physical coordinates. It should be used for viewing and processing unless a use case dictates otherwise. It will generally be a representation of the image in its native physical coordinate system.

The following MUST hold for all coordinate systems inside multiscales metadata. The length of “axes” must be between 2 and 5 and MUST be equal to the dimensionality of the Zarr arrays storing the image data (see “datasets:path”). The “axes” MUST contain 2 or 3 entries of “type:space” and MAY contain one additional entry of “type:time” and MAY contain one additional entry of “type:channel” or a null / custom type. In addition, the entries MUST be ordered by “type” where the “time” axis must come first (if present), followed by the “channel” or custom axis (if present) and the axes of type “space”. If there are three spatial axes where two correspond to the image plane (“yx”) and images are stacked along the other (anisotropic) axis (“z”), the spatial axes SHOULD be ordered as “zyx”.

Each multiscales dictionary MUST contain the field datasets, which is a list of dictionaries describing the arrays storing the individual resolution levels. Each dictionary in datasets MUST contain the field path, whose value is a string containing the path to the Zarr array for this resolution relative to the current Zarr group. The paths MUST be ordered from largest (i.e. highest resolution) to smallest. Every Zarr array referred to by a path MUST have the same number of dimensions and MUST NOT have more than 5 dimensions. The number of dimensions and order MUST correspond to number and order of axes.

Each dictionary in datasets MUST contain the field coordinateTransformations, whose value is a list of dictionaries that define a transformation that maps Zarr array coordinates for this resolution level to the “intrinsic” coordinate system (the last entry of the coordinateSystems array). The transformation is defined according to transformations metadata. The transformation MUST take as input points in the array coordinate system corresponding to the Zarr array at location path. The value of “input” MUST equal the value of path, implementations should always treat the value of input as if it were equal to the value of path. The value of the transformation’s output MUST be the name of the “intrinsic” coordinate system.

This transformation MUST be one of the following:

A single scale or identity transformation
A sequence transformation containing one scale and one translation transformation.

In these cases, the scale transformation specifies the pixel size in physical units or time duration. If scaling information is not available or applicable for one of the axes, the value MUST express the scaling factor between the current resolution and the first resolution for the given axis, defaulting to 1.0 if there is no downsampling along the axis. This is strongly recommended so that the the “intrinsic” coordinate system of the image avoids more complex transformations.

If applications require additional transformations, each multiscales dictionary MAY contain the field coordinateTransformations, describing transformations that are applied to all resolution levels in the same manner. The value of input MUST equal the name of the “intrinsic” coordinate system. The value of output MUST be the name of the output coordinate System which is different from the “intrinsic” coordinate system.

Each multiscales dictionary SHOULD contain the field name.

Each multiscales dictionary SHOULD contain the field type, which gives the type of downscaling method used to generate the multiscale image pyramid. It SHOULD contain the field “metadata”, which contains a dictionary with additional information about the downscaling method.

Example

A complete example of json-file for a 5D (TCZYX) multiscales with 3 resolution levels could look like this:

{
  "zarr_format": 3,
  "node_type": "group",
  "attributes": {
    "ome": {
      "version": "0.5",
      "multiscales": [
        {
          "name": "example",
          "coordinateSystems": [
              {
                "name": "intrinsic",
                "axes": [
                  { "name": "t", "type": "time", "unit": "millisecond" },
                  { "name": "c", "type": "channel" },
                  { "name": "z", "type": "space", "unit": "micrometer" },
                  { "name": "y", "type": "space", "unit": "micrometer" },
                  { "name": "x", "type": "space", "unit": "micrometer" }
                ]
              }
          ],
          "datasets": [
            {
              "path": "0",
              "coordinateTransformations": [
                {
                  // the voxel size for the first scale level (0.5 micrometer)
                  // and the time unit (0.1 milliseconds), which is the same for each scale level
                  "type": "scale",
                  "scale": [0.1, 1.0, 0.5, 0.5, 0.5],
                  "input": "0",
                  "output": "intrinsic"
                }
              ]
            },
            {
              "path": "1",
              "coordinateTransformations": [
                {
                  // the voxel size for the second scale level (downscaled by a factor of 2 -> 1 micrometer)
                  // and the time unit (0.1 milliseconds), which is the same for each scale level
                  "type": "scale",
                  "scale": [0.1, 1.0, 1.0, 1.0, 1.0],
                  "input": "1",
                  "output": "intrinsic"
                }
              ]
            },
            {
              "path": "2",
              "coordinateTransformations": [
                {
                  // the voxel size for the third scale level (downscaled by a factor of 4 -> 2 micrometer)
                  // and the time unit (0.1 milliseconds), which is the same for each scale level
                  "type": "scale",
                  "scale": [0.1, 1.0, 2.0, 2.0, 2.0],
                  "input": "2",
                  "output": "intrinsic"
                }
              ]
            }
          ],
          "type": "gaussian",
          "metadata": {
            "description": "the fields in metadata depend on the downscaling implementation. Here, the parameters passed to the skimage function are given",
            "method": "skimage.transform.pyramid_gaussian",
            "version": "0.16.1",
            "args": "[true]",
            "kwargs": { "multichannel": true }
          }
        }
      ]
    }
  }
}

If only one multiscale is provided, use it. Otherwise, the user can choose by name, using the first multiscale as a fallback:

datasets = []
for named in multiscales:
    if named["name"] == "3D":
        datasets = [x["path"] for x in named["datasets"]]
        break
if not datasets:
    # Use the first by default. Or perhaps choose based on chunk size.
    datasets = [x["path"] for x in multiscales[0]["datasets"]]

Requirements#

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [IETF RFC 2119][IETF RFC 2119]

Stakeholders#

People who need to represent the result of image registration algorithms, or any imaging scientist in need of affine or non-linear transformations.

This RFC has been discussed in:

PR 138
Issues 84, 94, 101, and 146
Several OME-Zarr community calls (one example)

Implementation#

Many RFCs have an “implementation” section which details how the implementation will work. This section should explain the rough specification changes. The goal is to give an idea to reviewers about the subsystems that require change and the surface area of those changes.

This knowledge can result in recommendations for alternate approaches that perhaps are idiomatic to the project or result in less packages touched. Or, it may result in the realization that the proposed solution in this RFC is too complex given the problem.

For the RFC author, typing out the implementation in a high-level often serves as “[rubber duck debugging][rubber duck debugging]” and you can catch a lot of issues or unknown unknowns prior to writing any real code.

Drawbacks, risks, alternatives, and unknowns#

Adopting this proposal will add an implementation burden because it adds more transformation types. Though this drawback is softened by the fact that implementations will be able to choose which transformations to support (e.g., implementations may choose not to support non-linear transformations).

An alternative to this proposal would be not to add support transformations directly and instead recommend software use an existing format (e.g., ITK’s). The downside of that is that alternative formats will not integrate well with OME-NGFF as they do not use JSON or Zarr.

In all, we believe the benefits of this proposal (outlined in the Background section) far outweigh these drawbacks, and will better promote software interoperability than alternatives.

Prior art and references#

ITK represents many types of transformations, and can serialize them to either plain text or to an HDF5 file. This is a practical approach that works well for software that depend on ITK, the proposed solution encoding transformations will be more interoperable.

Displacement fields are typically stored in formats designed for medical imaging (e.g. Nifti). While effective, they can only describe one type of non-linear transformation.

The Saalfeld lab at Janelia developed a custom format for storing affine and displacement field transformations in HDF5 which is similarly less interoperable than would be ideal.

Abandoned Ideas#

One consideration was to change (reverse) the order of parameters for transformations to match the convention used by many libraries. We opted not to make this change for two reasons. First, to maintain backward-compatibility. Second, the convention used by the libraries generally applies for 2D and 3D spatial transformations, but the specification should be applicable to transformations of arbitrary dimension and axis type, where there is not a strong convention we are aware of.

An early consideration was to use axis names to indicate correspondence across different coordinate systems (i.e. if two coordinate systems both have the “x” axis, then it is “the same” axis. We abandoned this for several reasons. It was restrictive - it is useful to have many coordinate systems with an “x” axis without requiring that they be “identical.” Under our early idea, every set of spatial axes would need unique names (“x1”, “x2”, …), and this seemed burdensome. As well, this approach would have also made transformations less explicit and likely would have required more complicated implementations. For example, points in two coordinate systems with re-ordered axis names ["x","y"] vs ["y","x"] would need to be axis-permuted, even if such a permutation was not explicitly specified.

Future possibilities#

Additional transformation types should be added in the future. Top candidates include:

thin-plate spline
b-spline
velocity fields
by-coordinate

Performance#

This proposal adds new features, and has no effect on performance for existing functionality.

Backwards Compatibility#

Adds new transformations, but existing transformations (scale, translation) are backward-compatible.

Adds coordinate systems, these contain axes which are backward-compatible with the axis specification for version 0.4. This proposal adds new fields to the axis metadata.

Testing#

Public examples of transformations with expected input/output pairs are provided here

UI/UX#

Implementations SHOULD communicate if it encounters an unsupported transformation (e.g. some software may opt not to support non-linear transformations), and inform users what action will be taken. The details of this choice should be software / application dependent, but ignoring the unsupported transformation or falling back to a simpler transformation are likely to be common choices.

Implementations SHOULD communicate if and when an image can be displayed in multiple coordinate systems. Users might choose between different options, or software could choose a default (e.g. the first or last listed coordinate system). The multiscales in version 0.4 has a similar consideration.

Changelog#

Date	Description	Link
2024-10-08	RFC assigned and published	ome/ngff#255

RFC-5: Coordinate Systems and Transformations

Contents

RFC-5: Coordinate Systems and Transformations#

Status#

Overview#

Background#

Proposal#

“coordinateSystems” metadata#

“axes” metadata#

Array coordinate systems#

Coordinate convention#

“coordinateTransformations” metadata#

Additional details#

Matrix transformations#

Transformation types#

“multiscales” metadata#

Requirements#

Stakeholders#

Implementation#

Drawbacks, risks, alternatives, and unknowns#

Prior art and references#

Abandoned Ideas#

Future possibilities#

Performance#

Backwards Compatibility#

Testing#

UI/UX#

Changelog#