import os
import abc
import numpy as np
__all__ = ['BaseLowLevelWCS', 'validate_physical_types']
[docs]class BaseLowLevelWCS(metaclass=abc.ABCMeta):
"""
Abstract base class for the low-level WCS interface.
This is described in `APE 14: A shared Python interface for World Coordinate
Systems <https://doi.org/10.5281/zenodo.1188875>`_.
"""
@property
@abc.abstractmethod
def pixel_n_dim(self):
"""
The number of axes in the pixel coordinate system.
"""
@property
@abc.abstractmethod
def world_n_dim(self):
"""
The number of axes in the world coordinate system.
"""
@property
@abc.abstractmethod
def world_axis_physical_types(self):
"""
An iterable of strings describing the physical type for each world axis.
These should be names from the VO UCD1+ controlled Vocabulary
(http://www.ivoa.net/documents/latest/UCDlist.html). If no matching UCD
type exists, this can instead be ``"custom:xxx"``, where ``xxx`` is an
arbitrary string. Alternatively, if the physical type is
unknown/undefined, an element can be `None`.
"""
@property
@abc.abstractmethod
def world_axis_units(self):
"""
An iterable of strings given the units of the world coordinates for each
axis.
The strings should follow the `IVOA VOUnit standard
<http://ivoa.net/documents/VOUnits/>`_ (though as noted in the VOUnit
specification document, units that do not follow this standard are still
allowed, but just not recommended).
"""
[docs] @abc.abstractmethod
def pixel_to_world_values(self, *pixel_arrays):
"""
Convert pixel coordinates to world coordinates.
This method takes `~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim` scalars or arrays as
input, and pixel coordinates should be zero-based. Returns
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_n_dim` scalars or arrays in units given by
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_units`. Note that pixel coordinates are
assumed to be 0 at the center of the first pixel in each dimension. If a
pixel is in a region where the WCS is not defined, NaN can be returned.
The coordinates should be specified in the ``(x, y)`` order, where for
an image, ``x`` is the horizontal coordinate and ``y`` is the vertical
coordinate.
"""
[docs] @abc.abstractmethod
def array_index_to_world_values(self, *index_arrays):
"""
Convert array indices to world coordinates.
This is the same as `~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_to_world_values` except that
the indices should be given in ``(i, j)`` order, where for an image
``i`` is the row and ``j`` is the column (i.e. the opposite order to
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_to_world_values`).
"""
[docs] @abc.abstractmethod
def world_to_pixel_values(self, *world_arrays):
"""
Convert world coordinates to pixel coordinates.
This method takes `~astropy.wcs.wcsapi.BaseLowLevelWCS.world_n_dim` scalars or arrays as
input in units given by `~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_units`. Returns
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim` scalars or arrays. Note that pixel
coordinates are assumed to be 0 at the center of the first pixel in each
dimension. If a world coordinate does not have a matching pixel
coordinate, NaN can be returned. The coordinates should be returned in
the ``(x, y)`` order, where for an image, ``x`` is the horizontal
coordinate and ``y`` is the vertical coordinate.
"""
[docs] @abc.abstractmethod
def world_to_array_index_values(self, *world_arrays):
"""
Convert world coordinates to array indices.
This is the same as `~astropy.wcs.wcsapi.BaseLowLevelWCS.world_to_pixel_values` except that
the indices should be returned in ``(i, j)`` order, where for an image
``i`` is the row and ``j`` is the column (i.e. the opposite order to
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_to_world_values`). The indices should be
returned as rounded integers.
"""
@property
@abc.abstractmethod
def world_axis_object_components(self):
"""
A list with `~astropy.wcs.wcsapi.BaseLowLevelWCS.world_n_dim` elements giving information
on constructing high-level objects for the world coordinates.
Each element of the list is a tuple with three items:
* The first is a name for the world object this world array
corresponds to, which *must* match the string names used in
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_object_classes`. Note that names might
appear twice because two world arrays might correspond to a single
world object (e.g. a celestial coordinate might have both “ra” and
“dec” arrays, which correspond to a single sky coordinate object).
* The second element is either a string keyword argument name or a
positional index for the corresponding class from
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_object_classes`.
* The third argument is a string giving the name of the property
to access on the corresponding class from
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_object_classes` in order to get numerical
values.
See the document
`APE 14: A shared Python interface for World Coordinate Systems
<https://doi.org/10.5281/zenodo.1188875>`_ for examples.
"""
@property
@abc.abstractmethod
def world_axis_object_classes(self):
"""
A dictionary giving information on constructing high-level objects for
the world coordinates.
Each key of the dictionary is a string key from
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_object_components`, and each value is a
tuple with three elements:
* The first element of the tuple must be a class or a string specifying
the fully-qualified name of a class, which will specify the actual
Python object to be created.
* The second element, should be a tuple specifying the positional
arguments required to initialize the class. If
`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_axis_object_components` specifies that the
world coordinates should be passed as a positional argument, this this
tuple should include `None` placeholders for the world coordinates.
* The last tuple element must be a dictionary with the keyword
arguments required to initialize the class.
Note that we don't require the classes to be Astropy classes since there
is no guarantee that Astropy will have all the classes to represent all
kinds of world coordinates. Furthermore, we recommend that the output be
kept as human-readable as possible.
The classes used here should have the ability to do conversions by
passing an instance as the first argument to the same class with
different arguments (e.g. ``Time(Time(...), scale='tai')``). This is
a requirement for the implementation of the high-level interface.
The second and third tuple elements for each value of this dictionary
can in turn contain either instances of classes, or if necessary can
contain serialized versions that should take the same form as the main
classes described above (a tuple with three elements with the fully
qualified name of the class, then the positional arguments and the
keyword arguments). For low-level API objects implemented in Python, we
recommend simply returning the actual objects (not the serialized form)
for optimal performance. Implementations should either always or never
use serialized classes to represent Python objects, and should indicate
which of these they follow using the
`~astropy.wcs.wcsapi.BaseLowLevelWCS.serialized_classes` attribute.
See the document
`APE 14: A shared Python interface for World Coordinate Systems
<https://doi.org/10.5281/zenodo.1188875>`_ for examples .
"""
# The following three properties have default fallback implementations, so
# they are not abstract.
@property
def array_shape(self):
"""
The shape of the data that the WCS applies to as a tuple of length
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim` in ``(row, column)``
order (the convention for arrays in Python).
If the WCS is valid in the context of a dataset with a particular
shape, then this property can be used to store the shape of the
data. This can be used for example if implementing slicing of WCS
objects. This is an optional property, and it should return `None`
if a shape is not known or relevant.
"""
return None
@property
def pixel_shape(self):
"""
The shape of the data that the WCS applies to as a tuple of length
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim` in ``(x, y)``
order (where for an image, ``x`` is the horizontal coordinate and ``y``
is the vertical coordinate).
If the WCS is valid in the context of a dataset with a particular
shape, then this property can be used to store the shape of the
data. This can be used for example if implementing slicing of WCS
objects. This is an optional property, and it should return `None`
if a shape is not known or relevant.
If you are interested in getting a shape that is comparable to that of
a Numpy array, you should use
`~astropy.wcs.wcsapi.BaseLowLevelWCS.array_shape` instead.
"""
return None
@property
def pixel_bounds(self):
"""
The bounds (in pixel coordinates) inside which the WCS is defined,
as a list with `~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim` ``(min, max)`` tuples.
The bounds should be given in ``[(xmin, xmax), (ymin, ymax)]``
order. WCS solutions are sometimes only guaranteed to be accurate
within a certain range of pixel values, for example when defining a
WCS that includes fitted distortions. This is an optional property,
and it should return `None` if a shape is not known or relevant.
"""
return None
@property
def axis_correlation_matrix(self):
"""
Returns an (`~astropy.wcs.wcsapi.BaseLowLevelWCS.world_n_dim`,
`~astropy.wcs.wcsapi.BaseLowLevelWCS.pixel_n_dim`) matrix that indicates using booleans
whether a given world coordinate depends on a given pixel coordinate.
This defaults to a matrix where all elements are `True` in the absence of
any further information. For completely independent axes, the diagonal
would be `True` and all other entries `False`.
"""
return np.ones((self.world_n_dim, self.pixel_n_dim), dtype=bool)
@property
def serialized_classes(self):
"""
Indicates whether Python objects are given in serialized form or as
actual Python objects.
"""
return False
UCDS_FILE = os.path.join(os.path.dirname(__file__), 'ucds.txt')
with open(UCDS_FILE) as f:
VALID_UCDS = set([x.strip() for x in f.read().splitlines()[1:]])
[docs]def validate_physical_types(physical_types):
"""
Validate a list of physical types against the UCD1+ standard
"""
for physical_type in physical_types:
if (physical_type is not None and
physical_type not in VALID_UCDS and
not physical_type.startswith('custom:')):
raise ValueError("Invalid physical type: {0}".format(physical_type))