skyimage¶
The skyimage
module contains algorithms that are used by
skymatch
to manage all of the information for footprints (image outlines)
on the sky as well as perform useful operations on these outlines such as
computing intersections and statistics in the overlap regions.
 Authors:
Mihai Cara (contact: help@stsci.edu)
 class jwst.skymatch.skyimage.DataAccessor[source]¶
Base class for all data accessors. Provides a common interface to access data.
 abstract set_data(data)[source]¶
Sets data.
 Parameters:
data (numpy.ndarray) – Data array to be set.
 class jwst.skymatch.skyimage.NDArrayInMemoryAccessor(data)[source]¶
Acessor for inmemory
numpy.ndarray
data. set_data(data)[source]¶
Sets data.
 Parameters:
data (numpy.ndarray) – Data array to be set.
 class jwst.skymatch.skyimage.NDArrayMappedAccessor(data, tmpfile=None, prefix='tmp_skymatch_', suffix='.npy', tmpdir='')[source]¶
Data accessor for arrays stored in temporary files.
 set_data(data)[source]¶
Sets data.
 Parameters:
data (numpy.ndarray) – Data array to be set.
 class jwst.skymatch.skyimage.SkyGroup(images, id=None, sky=0.0)[source]¶
Holds multiple
SkyImage
objects whose sky background values must be adjusted together.SkyGroup
provides methods for obtaining bounding polygon of the group ofSkyImage
objects and to compute sky value of the group. calc_sky(overlap=None, delta=True)[source]¶
Compute sky background value.
 Parameters:
overlap (SkyImage, SkyGroup, SphericalPolygon, list of tuples, None, optional) – Another
SkyImage
,SkyGroup
,spherical_geometry.polygons.SphericalPolygon
, or a list of tuples of (RA, DEC) of vertices of a spherical polygon. This parameter is used to indicate that sky statistics should computed only in the region of intersection of this image with the polygon indicated byoverlap
. Whenoverlap
isNone
, sky statistics will be computed over the entire image.delta (bool, optional) – Should this function return absolute sky value or the difference between the computed value and the value of the sky stored in the
sky
property.
 Returns:
skyval (float, None) – Computed sky value (absolute or relative to the
sky
attribute). If there are no valid data to perform this computations (e.g., because this image does not overlap with the image indicated byoverlap
),skyval
will be set toNone
.npix (int) – Number of pixels used to compute sky statistics.
polyarea (float) – Area (in srad) of the polygon that bounds data used to compute sky statistics.
 property id¶

While
id
can be of any type, it is preferable thatid
be of a type with nice string representation.
 intersection(skyimage)[source]¶
Compute intersection of this
SkyImage
object and anotherSkyImage
,SkyGroup
, orSphericalPolygon
object.
 property polygon¶
Get image’s bounding polygon.
 class jwst.skymatch.skyimage.SkyImage(image, wcs_fwd, wcs_inv, pix_area=1.0, convf=1.0, mask=None, id=None, skystat=None, stepsize=None, meta=None, reduce_memory_usage=True)[source]¶
Container that holds information about properties of a single image such as:
image data;
WCS of the chip image;
bounding spherical polygon;
id;
pixel area;
sky background value;
sky statistics parameters;
mask associated image data indicating “good” (1) data.
Initializes the SkyImage object.
 Parameters:
image (numpy.ndarray, NDArrayDataAccessor) – A 2D array of image data or a
NDArrayDataAccessor
.wcs_fwd (function) – “forward” pixeltoworld transformation function.
wcs_inv (function) – “inverse” worldtopixel transformation function.
pix_area (float, optional) – Average pixel’s sky area.
convf (float, optional) –
Conversion factor that when multiplied to
image
data converts the data to “uniform” (across multiple images) surface brightness units.Note
The functionality to support this conversion is not yet implemented and at this moment
convf
is ignored.mask (numpy.ndarray, NDArrayDataAccessor) – A 2D array or
NDArrayDataAccessor
of a 2D array that indicates which pixels in the inputimage
should be used for sky computations (1
) and which pixels should not be used for sky computations (0
).id (anything) – The value of this parameter is simple stored within the
SkyImage
object. While it can be of any type, it is preferable thatid
be of a type with nice string representation.skystat (callable, None, optional) –
A callable object that takes a either a 2D image (2D
numpy.ndarray
) or a list of pixel values (a Nx1 array) and returns a tuple of two values: some statistics (e.g., mean, median, etc.) and number of pixels/values from the input image used in computing that statistics.When
skystat
is not set,SkyImage
will useSkyStats
object to perform sky statistics on image data.stepsize (int, None, optional) – Spacing between vertices of the image’s bounding polygon. Default value of
None
creates bounding polygons with four vertices corresponding to the corners of the image.meta (dict, None, optional) – A dictionary of various items to be stored within the
SkyImage
object.reduce_memory_usage (bool, optional) – Indicates whether to attempt to minimize memory usage by attaching input
image
and/ormask
numpy.ndarray
arrays to filemapped accessor. This has no effect when input parametersimage
and/ormask
are already ofNDArrayDataAccessor
objects.
 calc_sky(overlap=None, delta=True)[source]¶
Compute sky background value.
 Parameters:
overlap (SkyImage, SkyGroup, SphericalPolygon, list of tuples, None, optional) – Another
SkyImage
,SkyGroup
,spherical_geometry.polygons.SphericalPolygon
, or a list of tuples of (RA, DEC) of vertices of a spherical polygon. This parameter is used to indicate that sky statistics should computed only in the region of intersection of this image with the polygon indicated byoverlap
. Whenoverlap
isNone
, sky statistics will be computed over the entire image.delta (bool, optional) – Should this function return absolute sky value or the difference between the computed value and the value of the sky stored in the
sky
property.
 Returns:
skyval (float, None) – Computed sky value (absolute or relative to the
sky
attribute). If there are no valid data to perform this computations (e.g., because this image does not overlap with the image indicated byoverlap
),skyval
will be set toNone
.npix (int) – Number of pixels used to compute sky statistics.
polyarea (float) – Area (in srad) of the polygon that bounds data used to compute sky statistics.
 property id¶

While
id
can be of any type, it is preferable thatid
be of a type with nice string representation.
 intersection(skyimage)[source]¶
Compute intersection of this
SkyImage
object and anotherSkyImage
,SkyGroup
, orSphericalPolygon
object.
 property is_sky_valid¶
Indicates whether sky value was successfully computed. Must be set externally.
 property pix_area¶
Set or get mean pixel area.
 property poly_area¶
Get bounding polygon area in srad units.
 property polygon¶
Get image’s bounding polygon.
 property radec¶
Get RA and DEC of the vertices of the bounding polygon as a
ndarray
of shape (N, 2) where N is the number of vertices + 1.
 set_builtin_skystat(skystat='median', lower=None, upper=None, nclip=5, lsigma=4.0, usigma=4.0, binwidth=0.1)[source]¶
Replace already set
skystat
with a “builtin” version of a statistics callable object used to measure sky background.See
SkyStats
for the parameter description.
 property skystat¶
Stores/retrieves a callable object that takes a either a 2D image (2D
numpy.ndarray
) or a list of pixel values (a Nx1 array) and returns a tuple of two values: some statistics (e.g., mean, median, etc.) and number of pixels/values from the input image used in computing that statistics.When
skystat
is not set,SkyImage
will useSkyStats
object to perform sky statistics on image data.