# HpxGeom¶

class gammapy.maps.HpxGeom(nside, nest=True, frame='icrs', region=None, axes=None, sparse=False)[source]

Geometry class for HEALPIX maps.

This class performs mapping between partial-sky indices (pixel number within a HEALPIX region) and all-sky indices (pixel number within an all-sky HEALPIX map). Multi-band HEALPIX geometries use a global indexing scheme that assigns a unique pixel number based on the all-sky index and band index. In the single-band case the global index is the same as the HEALPIX index.

By default the constructor will return an all-sky map. Partial-sky maps can be defined with the region argument.

Parameters
nsidendarray

HEALPIX nside parameter, the total number of pixels is 12*nside*nside. For multi-dimensional maps one can pass either a single nside value or a vector of nside values defining the pixel size for each image plane. If nside is not a scalar then its dimensionality should match that of the non-spatial axes.

nestbool

True -> ‘NESTED’, False -> ‘RING’ indexing scheme

framestr

Coordinate system, “icrs” | “galactic”

regionstr or tuple

Spatial geometry for partial-sky maps. If none the map will encompass the whole sky. String input will be parsed according to HPX_REG header keyword conventions. Tuple input can be used to define an explicit list of pixels encompassed by the geometry.

axeslist

Axes for non-spatial dimensions.

sparsebool

If True defer allocation of partial- to all-sky index mapping arrays. This option is only compatible with partial-sky maps with an analytic geometry (e.g. DISK).

Attributes Summary

 as_energy_true If the geom contains an energy axis rename it to energy true axes List of non-spatial axes. center_coord Map coordinates of the center of the geometry (tuple). center_pix Pixel coordinates of the center of the geometry (tuple). center_skydir Sky coordinate of the center of the geometry. data_shape Shape of the Numpy data array matching this geometry. frame ipix HEALPIX pixel and band indices for every pixel in the map. is_allsky Flag for all-sky maps. is_flat Whether the geom non spatial axes have length 1, i.e. is_hpx is_image Whether the geom is an image without extra dimensions. is_regular Flag identifying whether this geometry is regular in non-spatial dimensions. ndim Number of dimensions (int). nest Is HEALPix order nested? (bool). npix Number of pixels in each band. nside NSIDE in each band. order ORDER in each band (NSIDE = 2 ** ORDER). ordering HEALPix ordering (‘NESTED’ or ‘RING’). projection Map projection. region Region string. shape_axes Shape of non-spatial axes.

Methods Summary

 contains(coords) Check if a given map coordinate is contained in the geometry. Check if a given pixel coordinate is contained in the geometry. coord_to_idx(coords[, clip]) Convert map coordinates to pixel indices. coord_to_pix(coords) Convert map coordinates to pixel coordinates. copy(**kwargs) Copy and overwrite given attributes. create([nside, binsz, nest, frame, region, …]) Create an HpxGeom object. crop(crop_width) Crop the geometry at the edges. downsample(factor) Downsample the spatial dimension of the geometry by a given factor. drop(axis_name) Drop an axis from the geom. energy_mask([energy_min, energy_max, …]) Create a mask for a given energy range. from_hdu(hdu[, hdu_bands]) Create an HPX object from a BinTable HDU. from_hdulist(hdulist[, hdu, hdu_bands]) Load a geometry object from a FITS HDUList. from_header(header[, hdu_bands, format]) Create an HPX object from a FITS header. get_coord([idx, flat]) Get the coordinate array for this geometry. get_idx([idx, local, flat]) Get tuple of pixel indices for this geometry. get_index_list(nside, nest, region) Get list of pixels indices for all the pixels in a region. global_to_local(idx_global[, ravel]) Compute global (all-sky) index from a local (partial-sky) index. local_to_global(idx_local) Compute a local index (partial-sky) from a global (all-sky) index. pad(pad_width) Pad the geometry at the edges. Convert pixel coordinates to map coordinates. pix_to_idx(pix[, clip]) Convert pixel coordinates to pixel indices. resample_axis(axis) Resample geom to a new axis binning. slice_by_idx(slices) Create a new geometry by slicing the non-spatial axes. Solid angle array (Quantity in sr). squash(axis_name) Squash geom axis. to_bands_hdu([hdu, hdu_skymap, format]) to_cube(axes) Append non-spatial axes to create a higher-dimensional geometry. to_header([format]) Build and return FITS header for this HEALPIX map. Create 2D image geometry (drop non-spatial dimensions). to_slice(slices[, drop_axes]) Geometry copy with swapped ORDERING (NEST->RING or vice versa). to_ud_graded(order) Upgrade or downgrade the resolution to the given order. to_wcs_geom([proj, oversample, drop_axes, …]) Make a WCS projection appropriate for this HPX pixelization. upsample(factor) Upsample the spatial dimension of the geometry by a given factor.

Attributes Documentation

as_energy_true

If the geom contains an energy axis rename it to energy true

axes

List of non-spatial axes.

center_coord

Map coordinates of the center of the geometry (tuple).

center_pix

Pixel coordinates of the center of the geometry (tuple).

center_skydir

Sky coordinate of the center of the geometry.

Returns
pixSkyCoord
data_shape

Shape of the Numpy data array matching this geometry.

frame
ipix

HEALPIX pixel and band indices for every pixel in the map.

is_allsky

Flag for all-sky maps.

is_flat

Whether the geom non spatial axes have length 1, i.e. if the geom is equivalent to an image.

is_hpx = True
is_image

Whether the geom is an image without extra dimensions.

is_regular

Flag identifying whether this geometry is regular in non-spatial dimensions.

False for multi-resolution or irregular geometries. If true all image planes have the same pixel geometry.

ndim

Number of dimensions (int).

nest

Is HEALPix order nested? (bool).

npix

Number of pixels in each band.

For partial-sky geometries this can be less than the number of pixels for the band NSIDE.

nside

NSIDE in each band.

order

ORDER in each band (NSIDE = 2 ** ORDER).

Set to -1 for bands with NSIDE that is not a power of 2.

ordering

HEALPix ordering (‘NESTED’ or ‘RING’).

projection

Map projection.

region

Region string.

shape_axes

Shape of non-spatial axes.

Methods Documentation

contains(coords)[source]

Check if a given map coordinate is contained in the geometry.

Parameters
coordstuple or MapCoord

Tuple of map coordinates.

Returns
containmentndarray

Bool array.

contains_pix(pix)

Check if a given pixel coordinate is contained in the geometry.

Parameters
pixtuple

Tuple of pixel coordinates.

Returns
containmentndarray

Bool array.

coord_to_idx(coords, clip=False)

Convert map coordinates to pixel indices.

Parameters
coordstuple or MapCoord

Coordinate values in each dimension of the map. This can either be a tuple of numpy arrays or a MapCoord object. If passed as a tuple then the ordering should be (longitude, latitude, c_0, …, c_N) where c_i is the coordinate vector for axis i.

clipbool

Choose whether to clip indices to the valid range of the geometry. If false then indices for coordinates outside the geometry range will be set -1.

Returns
pixtuple

Tuple of pixel indices in image and band dimensions. Elements set to -1 correspond to coordinates outside the map.

coord_to_pix(coords)[source]

Convert map coordinates to pixel coordinates.

Parameters
coordstuple

Coordinate values in each dimension of the map. This can either be a tuple of numpy arrays or a MapCoord object. If passed as a tuple then the ordering should be (longitude, latitude, c_0, …, c_N) where c_i is the coordinate vector for axis i.

Returns
pixtuple

Tuple of pixel coordinates in image and band dimensions.

copy(**kwargs)

Copy and overwrite given attributes.

Parameters
**kwargsdict

Keyword arguments to overwrite in the map geometry constructor.

Returns
copyGeom

Copied map geometry.

classmethod create(nside=None, binsz=None, nest=True, frame='icrs', region=None, axes=None, skydir=None, width=None)[source]

Create an HpxGeom object.

Parameters
nsideint or ndarray

HEALPix NSIDE parameter. This parameter sets the size of the spatial pixels in the map.

binszfloat or ndarray

Approximate pixel size in degrees. An NSIDE will be chosen that correponds to a pixel size closest to this value. This option is superseded by nside.

nestbool

True for HEALPIX “NESTED” indexing scheme, False for “RING” scheme

frame{“icrs”, “galactic”}, optional

Coordinate system, either Galactic (“galactic”) or Equatorial (“icrs”).

skydirtuple or SkyCoord

Sky position of map center. Can be either a SkyCoord object or a tuple of longitude and latitude in deg in the coordinate system of the map.

regionstr

HPX region string. Allows for partial-sky maps.

widthfloat

Diameter of the map in degrees. If set the map will encompass all pixels within a circular region centered on skydir.

axeslist

List of axes for non-spatial dimensions.

Returns
geomHpxGeom

A HEALPix geometry object.

Examples

>>> from gammapy.maps import HpxGeom, MapAxis
>>> axis = MapAxis.from_bounds(0,1,2)
>>> geom = HpxGeom.create(nside=16)
>>> geom = HpxGeom.create(binsz=0.1, width=10.0)
>>> geom = HpxGeom.create(nside=64, width=10.0, axes=[axis])
>>> geom = HpxGeom.create(nside=[32,64], width=10.0, axes=[axis])

crop(crop_width)[source]

Crop the geometry at the edges.

Parameters
crop_width{sequence, array_like, int}

Number of values cropped from the edges of each axis.

Returns
geomGeom

Cropped geometry.

downsample(factor)[source]

Downsample the spatial dimension of the geometry by a given factor.

Parameters
factorint

Downsampling factor.

axis_namestr

Axis to downsample.

Returns
geomGeom

Downsampled geometry.

drop(axis_name)

Drop an axis from the geom.

Parameters
axis_namestr

Name of the axis to remove.

Returns
geomGeom

New geom with the axis removed.

energy_mask(energy_min=None, energy_max=None, round_to_edge=False)

Create a mask for a given energy range.

The energy bin must be fully contained to be included in the mask.

Parameters
energy_min, energy_maxQuantity

Energy range

Returns
maskndarray

classmethod from_hdu(hdu, hdu_bands=None)[source]

Create an HPX object from a BinTable HDU.

Parameters
hduBinTableHDU

The FITS HDU

hdu_bandsBinTableHDU

The BANDS table HDU

Returns
hpxHpxGeom

HEALPix geometry.

classmethod from_hdulist(hdulist, hdu=None, hdu_bands=None)

Load a geometry object from a FITS HDUList.

Parameters
hdulistHDUList

HDU list containing HDUs for map data and bands.

hdustr

Name or index of the HDU with the map data.

hdu_bandsstr

Name or index of the HDU with the BANDS table. If not defined this will be inferred from the FITS header of the map HDU.

Returns
geomGeom

Geometry object.

classmethod from_header(header, hdu_bands=None, format=None)[source]

Create an HPX object from a FITS header.

Parameters
headerHeader

hdu_bandsBinTableHDU

The BANDS table HDU.

formatstr, optional

FITS convention. If None the format is guessed. The following formats are supported:

• “fgst-ccube”

• “fgst-ltcube”

• “fgst-bexpcube”

• “fgst-srcmap”

• “fgst-template”

• “fgst-srcmap-sparse”

• “galprop”

• “galprop2”

Returns
hpxHpxGeom

HEALPix geometry.

get_coord(idx=None, flat=False)[source]

Get the coordinate array for this geometry.

Returns a coordinate array with the same shape as the data array. Pixels outside the geometry are set to NaN. Coordinates for a single image plane can be accessed by setting idx to the index tuple of a plane.

Parameters
idxtuple, optional

A tuple of indices with one index for each non-spatial dimension. If defined only coordinates for the image plane with this index will be returned. If none then coordinates for all pixels will be returned.

flatbool, optional

Return a flattened array containing only coordinates for pixels contained in the geometry.

Returns
coordstuple

Tuple of coordinate vectors with one vector for each dimension.

get_idx(idx=None, local=False, flat=False)[source]

Get tuple of pixel indices for this geometry.

Returns all pixels in the geometry by default. Pixel indices for a single image plane can be accessed by setting idx to the index tuple of a plane.

Parameters
idxtuple, optional

A tuple of indices with one index for each non-spatial dimension. If defined only pixels for the image plane with this index will be returned. If none then all pixels will be returned.

localbool

Flag to return local or global pixel indices. Local indices run from 0 to the number of pixels in a given image plane.

flatbool, optional

Return a flattened array containing only indices for pixels contained in the geometry.

Returns
idxtuple

Tuple of pixel index vectors with one vector for each dimension.

static get_index_list(nside, nest, region)[source]

Get list of pixels indices for all the pixels in a region.

Parameters
nsideint

HEALPIX nside parameter

nestbool

True for ‘NESTED’, False = ‘RING’

regionstr

HEALPIX region string

Returns
ilistndarray

List of pixel indices.

global_to_local(idx_global, ravel=False)[source]

Compute global (all-sky) index from a local (partial-sky) index.

Parameters
idx_globaltuple

A tuple of pixel indices with global HEALPix pixel indices.

ravelbool

Return a raveled index.

Returns
idx_localtuple

A tuple of pixel indices with local HEALPIX pixel indices.

local_to_global(idx_local)[source]

Compute a local index (partial-sky) from a global (all-sky) index.

Returns
idx_globaltuple

A tuple of pixel index vectors with global HEALPIX pixel indices

pad(pad_width)[source]

Pad the geometry at the edges.

Parameters

Number of values padded to the edges of each axis.

Returns
geomGeom

pix_to_coord(pix)[source]

Convert pixel coordinates to map coordinates.

Parameters
pixtuple

Tuple of pixel coordinates.

Returns
coordstuple

Tuple of map coordinates.

pix_to_idx(pix, clip=False)[source]

Convert pixel coordinates to pixel indices.

Returns -1 for pixel coordinates that lie outside of the map.

Parameters
pixtuple

Tuple of pixel coordinates.

clipbool

Choose whether to clip indices to the valid range of the geometry. If false then indices for coordinates outside the geometry range will be set -1.

Returns
idxtuple

Tuple of pixel indices.

resample_axis(axis)

Resample geom to a new axis binning.

This method groups the existing bins into a new binning.

Parameters
axisMapAxis

New map axis.

Returns
mapGeom

Geom with resampled axis.

slice_by_idx(slices)

Create a new geometry by slicing the non-spatial axes.

Parameters
slicesdict

Dict of axes names and integers or slice object pairs. Contains one element for each non-spatial dimension. For integer indexing the corresponding axes is dropped from the map. Axes not specified in the dict are kept unchanged.

Returns
geomGeom

Sliced geometry.

solid_angle()[source]

Solid angle array (Quantity in sr).

The array has the same dimensionality as map.nside since all pixels have the same solid angle.

squash(axis_name)

Squash geom axis.

Parameters
axis_namestr

Axis to squash.

Returns
geomGeom

Geom with squashed axis.

to_bands_hdu(hdu=None, hdu_skymap=None, format=None)
to_cube(axes)[source]

Append non-spatial axes to create a higher-dimensional geometry.

This will result in a new geometry with N+M dimensions where N is the number of current dimensions and M is the number of axes in the list.

Parameters
axeslist

Axes that will be appended to this geometry.

Returns
geomGeom

Map geometry.

to_header(format='gadf', **kwargs)[source]

Build and return FITS header for this HEALPIX map.

to_image()[source]

Create 2D image geometry (drop non-spatial dimensions).

Returns
geomGeom

Image geometry.

to_slice(slices, drop_axes=True)[source]
to_swapped()[source]

Geometry copy with swapped ORDERING (NEST->RING or vice versa).

Returns
geomHpxGeom

A HEALPix geometry object.

to_ud_graded(order)[source]

This method does not preserve the geometry footprint.

Returns
geomHpxGeom

A HEALPix geometry object.

to_wcs_geom(proj='AIT', oversample=2, drop_axes=True, width_pix=None)[source]

Make a WCS projection appropriate for this HPX pixelization.

Parameters
drop_axesbool

Drop non-spatial axes from the HEALPIX geometry. If False then all dimensions of the HEALPIX geometry will be copied to the WCS geometry.

projstr

Projection type of WCS geometry.

oversamplefloat

Oversampling factor for WCS map. This will be the approximate ratio of the width of a HPX pixel to a WCS pixel. If this parameter is None then the width will be set from width_pix.

width_pixint

Width of the WCS geometry in pixels. The pixel size will be set to the number of pixels satisfying oversample or width_pix whichever is smaller. If this parameter is None then the width will be set from oversample.

Returns
wcsWcsGeom

WCS geometry

upsample(factor)[source]

Upsample the spatial dimension of the geometry by a given factor.

Parameters
factorint

Upsampling factor.

axis_namestr

Axis to upsample.

Returns
geomGeom

Upsampled geometry.