Basic mangle commands
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command
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does
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purpose
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pixelize
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splits polygons in the mask so that each polygon is in only one pixel
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Several mangle commands require comparing each polygon to all of the other
polygons in the mask. If the mask is pixelized first, mangle only needs to
compare a polygon to the other polygons in the same pixel, which speeds up
the processing considerably.
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snap
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identifies almost coincident cap boundaries and snaps them together
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The positions of the intersections of two almost but not exactly coincident
cap boundaries may be subject to significant numerical uncertainty.
To avoid numerical problems, such boundaries must be made exactly coincident.
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balkanize
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subdivides the angular mask into many disjoint polygons
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Polygons must not overlap,
to avoid regions being double-counted.
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weight
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assigns a weight to each polygon
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Define the weight to be applied to each polygon,
for example
in computing the weighted sum of spherical harmonics of polygons.
This command may be avoided in many cases:
see Losing Weight.
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unify
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unifies polygons where possible
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Reduce the number of polygons,
for grace and efficiency.
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Advanced mangle commands
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harmonize
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computes the spherical harmonics of a mask
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Investigations of large scale structure, especially at the largest scales,
often proceed most advantageously through spherical harmonics.
The area of the mask is proportional to the zeroth harmonic.
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map
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maps the mask reconstructed from its spherical harmonics
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So you can see what you got.
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drangle
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computes the angle subtended by that part of a circle
lying within the mask
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This is the basic element needed to compute the cross-correlation
(commonly denoted
<DR>,
for Data-Random, in cosmological parlance),
between a given point and points in the mask
separated from it by a given angle.
If the points are random points generated by
ransack,
then the result will be a Monte Carlo realization of
the auto-correlation
(commonly denoted
<RR>,
for Random-Random, in cosmological parlance),
between pairs of points in the mask
separated by a given angle.
|
|
ransack
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generates random points within the mask
|
Correlation functions are often measured with the help of random backgrounds
of points.
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poly2poly
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converts a polygon file from one format to another
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For example,
you can generate points suitable for plotting the polygons
of your mask by converting to graphics format.
Or,
you can determine the vertices of the polygons of your mask
by converting to vertices format,
or you can find a point inside each polygon of your mask
by converting to midpoint format.
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polyid
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determines which polygon(s) a point lies inside
|
Useful utility.
Is there such a thing as a useless utility?
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rasterize
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pixelizes a mask according to a user-defined pixelization scheme, returns a polygon file describing the area-averaged weight within each pixel
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One direction of the HEALPix ⇔ mangle conversion scheme, although it works with any spherical pixelization scheme. Allows one to take a standard
mangle polygon file and make a discrete approximation to it according to an arbitrary pixelization scheme (e.g., HEALPix); the output can then be converted
to a FITS file (using the weight in each HEALPix pixel) or turned into another polygon file in mangle (using poly2poly).
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pixelmap
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takes a mask pixelized by one of mangle's pixelization schemes, returns a polygon file describing the area-averaged weight within each pixel
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This routine uses the internal mangle pixelization information to produce a discrete approximation to a mask without the additional steps used in rasterize. It can produce a pixelmap using any of the pixelization schemes available in pixelize.
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