.. _astropy_nddata:
*****************************************
N-Dimensional Datasets (`astropy.nddata`)
*****************************************
Introduction
============
The `~astropy.nddata` package provides classes to represent images and other
gridded data, some essential functions for manipulating images, and the
infrastructure for package developers who wish to include support for the
image classes. This subpackage was developed based on `APE 7`_.
.. _astropy_nddata_getting_started:
Getting Started
===============
NDData
------
The primary purpose of `~astropy.nddata.NDData` is to act as a *container* for
data, metadata, and other related information like a mask.
An `~astropy.nddata.NDData` object can be instantiated by passing it an
n-dimensional `numpy` array::
>>> import numpy as np
>>> from astropy.nddata import NDData
>>> array = np.zeros((12, 12, 12)) # a 3-dimensional array with all zeros
>>> ndd1 = NDData(array)
Or something that can be converted to a `numpy.ndarray`::
>>> ndd2 = NDData([1, 2, 3, 4])
>>> ndd2
NDData([1, 2, 3, 4])
And can be accessed again via the ``data`` attribute::
>>> ndd2.data
array([1, 2, 3, 4])
It also supports additional properties like a ``unit`` or ``mask`` for the
data, a ``wcs`` (World Coordinate System) and ``uncertainty`` of the data and
additional ``meta`` attributes:
>>> data = np.array([1,2,3,4])
>>> mask = data > 2
>>> unit = 'erg / s'
>>> from astropy.nddata import StdDevUncertainty
>>> uncertainty = StdDevUncertainty(np.sqrt(data)) # representing standard deviation
>>> meta = {'object': 'fictional data.'}
>>> ndd = NDData(data, mask=mask, unit=unit, uncertainty=uncertainty,
... meta=meta)
>>> ndd
NDData([1, 2, 3, 4], unit='erg / s')
The representation only displays the ``data``; the other attributes need to be
accessed directly, for example, ``ndd.mask`` to access the mask.
NDDataRef
---------
Building upon this pure container, `~astropy.nddata.NDDataRef` implements:
+ A ``read`` and ``write`` method to access ``astropy``'s unified file I/O
interface.
+ Simple arithmetics like addition, subtraction, division, and multiplication.
+ Slicing.
Instances are created in the same way::
>>> from astropy.nddata import NDDataRef
>>> ndd = NDDataRef(ndd)
>>> ndd
NDDataRef([1, 2, 3, 4], unit='erg / s')
But also support arithmetic (:ref:`nddata_arithmetic`) like addition::
>>> import astropy.units as u
>>> ndd2 = ndd.add([4, -3.5, 3, 2.5] * u.erg / u.s)
>>> ndd2
NDDataRef([ 5. , -1.5, 6. , 6.5], unit='erg / s')
Because these operations have a wide range of options, these are not available
using arithmetic operators like ``+``.
Slicing or indexing (:ref:`nddata_slicing`) is possible (with warnings issued if
some attribute cannot be sliced)::
>>> ndd2[2:] # discard the first two elements # doctest: +FLOAT_CMP
NDDataRef([6. , 6.5], unit='erg / s')
>>> ndd2[1] # get the second element # doctest: +FLOAT_CMP
NDDataRef(-1.5, unit='erg / s')
Working with Two-Dimensional Data Like Images
---------------------------------------------
Though the `~astropy.nddata` package supports any kind of gridded data, this
introduction will focus on the use of `~astropy.nddata` for two-dimensional
images. To get started, we will construct a two-dimensional image with a few
sources, some Gaussian noise, and a "cosmic ray" which we will later mask out.
Examples
^^^^^^^^
..
EXAMPLE START
Working with Two-Dimensional Data Using NDData
First, construct a two-dimensional image with a few sources, some Gaussian
noise, and a "cosmic ray"::
>>> import numpy as np
>>> from astropy.modeling.models import Gaussian2D
>>> rng = np.random.default_rng()
>>> y, x = np.mgrid[0:500, 0:600]
>>> data = (Gaussian2D(1, 150, 100, 20, 10, theta=0.5)(x, y) +
... Gaussian2D(0.5, 400, 300, 8, 12, theta=1.2)(x,y) +
... Gaussian2D(0.75, 250, 400, 5, 7, theta=0.23)(x,y) +
... Gaussian2D(0.9, 525, 150, 3, 3)(x,y) +
... Gaussian2D(0.6, 200, 225, 3, 3)(x,y))
>>> data += 0.01 * rng.standard_normal((500, 600))
>>> cosmic_ray_value = 0.997
>>> data[100, 300:310] = cosmic_ray_value
This image has a large "galaxy" in the lower left and the "cosmic ray" is the
horizontal line in the lower middle of the image:
.. doctest-skip::
>>> import matplotlib.pyplot as plt
>>> plt.imshow(data, origin='lower')
.. plot::
import numpy as np
import matplotlib.pyplot as plt
from astropy.modeling.models import Gaussian2D
y, x = np.mgrid[0:500, 0:600]
data = (Gaussian2D(1, 150, 100, 20, 10, theta=0.5)(x, y) +
Gaussian2D(0.5, 400, 300, 8, 12, theta=1.2)(x,y) +
Gaussian2D(0.75, 250, 400, 5, 7, theta=0.23)(x,y) +
Gaussian2D(0.9, 525, 150, 3, 3)(x,y) +
Gaussian2D(0.6, 200, 225, 3, 3)(x,y))
rng = np.random.default_rng(123456)
data += 0.01 * rng.standard_normal((500, 600))
cosmic_ray_value = 0.997
data[100, 300:310] = cosmic_ray_value
plt.imshow(data, origin='lower')
The "cosmic ray" can be masked out in this test image, like this::
>>> mask = (data == cosmic_ray_value)
..
EXAMPLE END
`~astropy.nddata.CCDData` Class for Images
------------------------------------------
The `~astropy.nddata.CCDData` object, like the other objects in this package,
can store the data, a mask, and metadata. The `~astropy.nddata.CCDData` object
requires that a unit be specified::
>>> from astropy.nddata import CCDData
>>> ccd = CCDData(data, mask=mask,
... meta={'object': 'fake galaxy', 'filter': 'R'},
... unit='adu')
Slicing
-------
Slicing works the way you would expect with the mask and, if present,
WCS, sliced appropriately::
>>> ccd2 = ccd[:200, :]
>>> ccd2.data.shape
(200, 600)
>>> ccd2.mask.shape
(200, 600)
>>> # Show the mask in a region around the cosmic ray:
>>> ccd2.mask[99:102, 299:311]
array([[False, False, False, False, False, False, False, False, False,
False, False, False],
[False, True, True, True, True, True, True, True, True,
True, True, False],
[False, False, False, False, False, False, False, False, False,
False, False, False]]...)
For many applications it may be more convenient to use
`~astropy.nddata.Cutout2D`, described in `image_utilities`_.
Image Arithmetic, Including Uncertainty
---------------------------------------
Methods are provided for basic arithmetic operations between images, including
propagation of uncertainties. Three uncertainty types are supported: variance
(`~astropy.nddata.VarianceUncertainty`), standard deviation
(`~astropy.nddata.StdDevUncertainty`), and inverse variance
(`~astropy.nddata.InverseVariance`).
Examples
^^^^^^^^
..
EXAMPLE START
Image Arithmetic Including Uncertainty in NDData
This example creates an uncertainty that is Poisson error, stored as a
variance::
>>> from astropy.nddata import VarianceUncertainty
>>> poisson_noise = np.ma.sqrt(np.ma.abs(ccd.data))
>>> ccd.uncertainty = VarianceUncertainty(poisson_noise ** 2)
As a convenience, the uncertainty can also be set with a ``numpy`` array. In
that case, the uncertainty is assumed to be the standard deviation::
>>> ccd.uncertainty = poisson_noise
INFO: array provided for uncertainty; assuming it is a StdDevUncertainty. [astropy.nddata.ccddata]
If we make a copy of the image and add that to the original, the uncertainty
changes as expected::
>>> ccd2 = ccd.copy()
>>> added_ccds = ccd.add(ccd2, handle_meta='first_found')
>>> added_ccds.uncertainty.array[0, 0] / ccd.uncertainty.array[0, 0] / np.sqrt(2) # doctest: +FLOAT_CMP
0.99999999999999989
..
EXAMPLE END
Reading and Writing
-------------------
A `~astropy.nddata.CCDData` can be saved to a FITS file::
>>> ccd.write('test_file.fits')
And can also be read in from a FITS file::
>>> ccd2 = CCDData.read('test_file.fits')
Note the unit is stored in the ``BUNIT`` keyword in the header on saving, and is
read from the header if it is present.
Detailed help on the available keyword arguments for reading and writing
can be obtained via the ``help()`` method as follows:
.. doctest-skip::
>>> CCDData.read.help('fits') # Get help on the CCDData FITS reader
>>> CCDData.writer.help('fits') # Get help on the CCDData FITS writer
.. _image_utilities:
Image Utilities
---------------
Cutouts
^^^^^^^
Though slicing directly is one way to extract a subframe,
`~astropy.nddata.Cutout2D` provides more convenient access to cutouts from the
data.
Examples
~~~~~~~~
..
EXAMPLE START
Accessing Cutouts in NDData
This example pulls out the large "galaxy" in the lower left of the image, with
the center of the cutout at ``position``::
>>> from astropy.nddata import Cutout2D
>>> position = (149.7, 100.1)
>>> size = (81, 101) # pixels
>>> cutout = Cutout2D(ccd, position, size)
>>> plt.imshow(cutout.data, origin='lower') # doctest: +SKIP
.. plot::
import numpy as np
import matplotlib.pyplot as plt
from astropy.modeling.models import Gaussian2D
from astropy.nddata import CCDData
from astropy.nddata import Cutout2D
y, x = np.mgrid[0:500, 0:600]
data = (Gaussian2D(1, 150, 100, 20, 10, theta=0.5)(x, y) +
Gaussian2D(0.5, 400, 300, 8, 12, theta=1.2)(x,y) +
Gaussian2D(0.75, 250, 400, 5, 7, theta=0.23)(x,y) +
Gaussian2D(0.9, 525, 150, 3, 3)(x,y) +
Gaussian2D(0.6, 200, 225, 3, 3)(x,y))
rng = np.random.default_rng(123456)
data += 0.01 * rng.standard_normal((500, 600))
cosmic_ray_value = 0.997
data[100, 300:310] = cosmic_ray_value
mask = (data == cosmic_ray_value)
ccd = CCDData(data, mask=mask,
meta={'object': 'fake galaxy', 'filter': 'R'},
unit='adu')
position = (149.7, 100.1)
size = (81, 101) # pixels
cutout = Cutout2D(ccd, position, size)
plt.imshow(cutout.data, origin='lower')
This cutout can also plot itself on the original image::
>>> plt.imshow(ccd, origin='lower') # doctest: +SKIP
>>> cutout.plot_on_original(color='white') # doctest: +SKIP
.. plot::
import numpy as np
import matplotlib.pyplot as plt
from astropy.modeling.models import Gaussian2D
from astropy.nddata import CCDData, Cutout2D
y, x = np.mgrid[0:500, 0:600]
data = (Gaussian2D(1, 150, 100, 20, 10, theta=0.5)(x, y) +
Gaussian2D(0.5, 400, 300, 8, 12, theta=1.2)(x,y) +
Gaussian2D(0.75, 250, 400, 5, 7, theta=0.23)(x,y) +
Gaussian2D(0.9, 525, 150, 3, 3)(x,y) +
Gaussian2D(0.6, 200, 225, 3, 3)(x,y))
rng = np.random.default_rng(123456)
data += 0.01 * rng.standard_normal((500, 600))
cosmic_ray_value = 0.997
data[100, 300:310] = cosmic_ray_value
mask = (data == cosmic_ray_value)
ccd = CCDData(data, mask=mask,
meta={'object': 'fake galaxy', 'filter': 'R'},
unit='adu')
position = (149.7, 100.1)
size = (81, 101) # pixels
cutout = Cutout2D(ccd, position, size)
plt.imshow(ccd, origin='lower')
cutout.plot_on_original(color='white')
The cutout also provides methods for finding pixel coordinates in the original
or in the cutout; recall that ``position`` is the center of the cutout in the
original image::
>>> position
(149.7, 100.1)
>>> cutout.to_cutout_position(position) # doctest: +FLOAT_CMP
(49.7, 40.099999999999994)
>>> cutout.to_original_position((49.7, 40.099999999999994)) # doctest: +FLOAT_CMP
(149.7, 100.1)
For more details, including constructing a cutout from World Coordinates and
the options for handling cutouts that go beyond the bounds of the original
image, see :ref:`cutout_images`.
..
EXAMPLE END
Image Resizing
^^^^^^^^^^^^^^
The functions `~astropy.nddata.block_reduce` and
`~astropy.nddata.block_replicate` resize images.
Example
~~~~~~~
..
EXAMPLE START
Image Resizing in NDData
This example reduces the size of the image by a factor of 4. Note that the
result is a `numpy.ndarray`; the mask, metadata, etc. are discarded:
.. doctest-requires:: skimage
>>> from astropy.nddata import block_reduce, block_replicate
>>> smaller = block_reduce(ccd, 4) # doctest: +IGNORE_WARNINGS
>>> smaller
array(...)
>>> plt.imshow(smaller, origin='lower') # doctest: +SKIP
.. plot::
import numpy as np
import matplotlib.pyplot as plt
from astropy.modeling.models import Gaussian2D
from astropy.nddata import block_reduce, block_replicate
from astropy.nddata import CCDData, Cutout2D
y, x = np.mgrid[0:500, 0:600]
data = (Gaussian2D(1, 150, 100, 20, 10, theta=0.5)(x, y) +
Gaussian2D(0.5, 400, 300, 8, 12, theta=1.2)(x,y) +
Gaussian2D(0.75, 250, 400, 5, 7, theta=0.23)(x,y) +
Gaussian2D(0.9, 525, 150, 3, 3)(x,y) +
Gaussian2D(0.6, 200, 225, 3, 3)(x,y))
rng = np.random.default_rng(123456)
data += 0.01 * rng.standard_normal((500, 600))
cosmic_ray_value = 0.997
data[100, 300:310] = cosmic_ray_value
mask = (data == cosmic_ray_value)
ccd = CCDData(data, mask=mask,
meta={'object': 'fake galaxy', 'filter': 'R'},
unit='adu')
smaller = block_reduce(ccd.data, 4)
plt.imshow(smaller, origin='lower')
By default, both `~astropy.nddata.block_reduce` and
`~astropy.nddata.block_replicate` conserve flux.
..
EXAMPLE END
Other Image Classes
-------------------
There are two less restrictive classes, `~astropy.nddata.NDDataArray` and
`~astropy.nddata.NDDataRef`, that can be used to hold image data. They are
primarily of interest to those who may want to create their own image class by
subclassing from one of the classes in the `~astropy.nddata` package. The main
differences between them are:
+ `~astropy.nddata.NDDataRef` can be sliced and has methods for basic
arithmetic operations, but the user needs to use one of the uncertainty
classes to define an uncertainty. See :ref:`NDDataRef` for more detail.
Most of its properties must be set when the object is created because they
are not mutable.
+ `~astropy.nddata.NDDataArray` extends `~astropy.nddata.NDDataRef` by adding
the methods necessary for it to behave like a ``numpy`` array in expressions
and adds setters for several properties. It lacks the ability to
automatically recognize and read data from FITS files and does not attempt
to automatically set the WCS property.
+ `~astropy.nddata.CCDData` extends `~astropy.nddata.NDDataArray` by setting
up a default uncertainty class, setting up straightforward read/write to FITS
files, and automatically setting up a WCS property.
More General Gridded Data Classes
---------------------------------
There are two additional classes in the ``nddata`` package that are of
interest primarily to users who either need a custom image class that goes
beyond the classes discussed so far, or who are working with gridded data that
is not an image.
+ `~astropy.nddata.NDData` is a container class for holding general gridded
data. It includes a handful of basic attributes, but no slicing or arithmetic.
More information about this class is in :ref:`nddata_details`.
+ `~astropy.nddata.NDDataBase` is an abstract base class that developers of new
gridded data classes can subclass to declare that the new class follows the
`~astropy.nddata.NDData` interface. More details are in
:ref:`nddata_subclassing`.
Additional Examples
===================
The list of packages below that use the ``nddata`` framework is intended to be
useful to either users writing their own image classes or those looking
for an image class that goes beyond what `~astropy.nddata.CCDData` does.
+ The `SunPy project `_ uses `~astropy.nddata.NDData` as the
foundation for its
`Map classes `_.
+ The class `~astropy.nddata.NDDataRef` is used in
`specutils `_ as the basis for
`Spectrum1D `_, which adds several methods useful for
spectra.
+ The package `ndmapper `_, which
makes it easy to build reduction pipelines for optical data, uses
`~astropy.nddata.NDDataArray` as its image object.
+ The package `ccdproc `_ uses the
`~astropy.nddata.CCDData` class throughout for implementing optical/IR image
reduction.
Using ``nddata``
================
.. toctree::
:maxdepth: 2
ccddata.rst
utils.rst
bitmask.rst
decorator.rst
nddata.rst
mixins/index.rst
subclassing.rst
.. note that if this section gets too long, it should be moved to a separate
doc page - see the top of performance.inc.rst for the instructions on how to do
that
.. include:: performance.inc.rst
Reference/API
=============
.. automodapi:: astropy.nddata
:no-inheritance-diagram:
.. automodapi:: astropy.nddata.bitmask
:no-inheritance-diagram:
.. automodapi:: astropy.nddata.utils
:no-inheritance-diagram:
.. _APE 7: https://github.com/astropy/astropy-APEs/blob/main/APE7.rst