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NRAO Home > CASA > CASA Task Reference Manual |
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0.1.38 imdev
Requires:
Synopsis
Create an image that can represent the statistical deviations of the input
image.
Arguments
Inputs |
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imagename |
| Input image name
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| allowed: | string |
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| Default: |
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outfile |
| Output image file name. If left blank (the default), no
image is written but a new image tool referencing the
collapsed image is returned.
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| allowed: | string |
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| Default: |
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region |
| Region selection. Default is to use the full image.
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| allowed: | any |
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| Default: | variant
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box |
| Rectangular region(s) to select in direction plane.
Default is to use the entire direction plane.
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| allowed: | string |
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| Default: |
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chans |
| Channels to use. Default is to use all channels.
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| allowed: | string |
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| Default: |
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stokes |
| Stokes planes to use. Default is to use all Stokes planes.
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| allowed: | string |
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| Default: |
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mask |
| Mask to use. Default setting is none.
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| allowed: | string |
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| Default: |
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overwrite |
| Overwrite (unprompted) pre-existing output file?
Ignored if ”outfile” is left blank.
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| allowed: | bool | |
| Default: | False |
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stretch |
| Stretch the mask if necessary and possible? Default value
is False. | |
| allowed: | bool |
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| Default: | False | |
grid |
| x,y grid spacing. Array of exactly two positive integers.
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| allowed: | intArray |
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| Default: | 11
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anchor |
| x,y anchor pixel location. Either ”ref” to use the image
reference pixel, or an array of exactly two integers.
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| allowed: | variant |
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| Default: | variant ref |
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xlength |
| Either x coordinate length of box, or diameter of circle.
Circle is used if ylength is empty string.
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| allowed: | variant |
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| Default: | variant 1pix |
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ylength |
| y coordinate length of box. Use a circle if ylength is
empty string.
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| allowed: | variant |
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| Default: | variant 1pix |
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interp |
| Interpolation algorithm to use. One of ”nearest”,
”linear”, ”cubic”, or ”lanczos”. Minimum match
supported.
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| allowed: | string |
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| Default: | cubic |
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stattype |
| Statistic to compute. See full description for supported
statistics.
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| allowed: | string |
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| Default: | sigma |
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statalg |
| Statistics computation algorithm to use. Supported
values are ”chauvenet” and ”classic”, Minimum match
is supported.
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| allowed: | string |
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| Default: | classic |
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zscore |
| For chauvenet, this is the target maximum number of
standard deviations data may have to be included. If
negative, use Chauvenet”s criterion. Ignored if algorithm
is not ”chauvenet”.
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| allowed: | double |
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| Default: | -1 |
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maxiter |
| For chauvenet, this is the maximum number of iterations
to attempt. Iterating will stop when either this limit is
reached, or the zscore criterion is met. If negative, iterate
until the zscore criterion is met. Ignored if algortihm is
not ”chauvenet”.
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| allowed: | int |
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| Default: | -1 |
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bool
Example
This application creates an image that reflects the statistics of the input image. The output image has
the same dimensions and coordinate system as the (selected region in) input image. The grid parameter
describes how many pixels apart "grid" pixels are. Statistics are computed around each grid pixel. Grid
pixels are limited to the direction plane only; independent statistics are computed for each direction plane
(ie at each frequency/stokes pixel should the input image happen to have such additional axes). Using the
xlength and ylength parameters, one may specify either a rectangular or circular region around each grid
point that defines which surrounding pixels are used in the statistic computation for individual grid points.
If the ylength parameter is the empty string, then a circle of diameter provided by xlength centered on
the grid point is used. If ylength is not empty, then a rectangular box of dimensions xlength x ylength centered
on the grid pixel is used. These two parameters may be specified in pixels, using either numerical values or
valid quantities with "pix" as the unit (eg "4pix"). Otherwise, they must be specified as valid angular
quantities, with recognized units (eg "4arcsec"). As with other region selections in CASA, full pixels are
included in the computation even if the specified region includes only a fraction of that pixel. BEWARE OF
MACHINE PRECISION ISSUES, because you may get a smaller number of pixels included in a region than you
expect if you specify, eg, an integer number of pixels. In such cases, you probably want to specify that
number plus a small epsilon value (eg "2.0001pix" rather than "2pix") to mitigate machine precision issues
when computing region extents.
The output image is formed by putting the statistics calculated at each grid point at the corresponding
grid point in the output image. Interpolation of these output values is then used to compute values at
non-grid-point pixels. The user may specify which interpolation algorithm to use for this computation
using the interp parameter.
ANCHORING THE GRID
The user may choose at which pixel to "anchor" the grid. For example, if one specifies grid=[4,4] and
anchor=[0,0], grid points will be located at pixels [0,0], [0,4], [0,8] ... [4,0], [4,4], etc. This
is exactly the same grid that would be produced if the user specified anchor=[4,4] or anchor=[20,44].
If the user specifies anchor=[1, 2] and grid=[4,4], then the grid points will be at pixels [1,2], [5,2],
[9,2]... [5,2], [5,6], etc. and the resulting grid is the same as it would be if the user specified eg
anchor=[9,10] or anchor=[21, 18]. The value "ref", which is the default, indicates that the reference
pixel of the input image should be used to anchor the grid. The x and y values of this pixel will be
rounded to the nearest integer if necessary.
SUPPORTED STATISTICS AND STATISTICS ALGORITHMS
One may specify which statistic should be represented using the stattype parameter. The following values
are recognized (minimum match supported):
iqr inner quartile range (q3 - q1)
max maximum
mean mean
medabsdevmed, madm median absolute deviation from the median
median median
min minimum
npts number of points
q1 first quartile
q3 third quartile
rms rms
sigma, std standard deviation
sumsq sum of squares
sum sum
var variance
xmadm median absolute deviation from the median multipied by x, where x is the reciprocal of Phi^-1(3/4),
where Phi^-1 is the reciprocal of the quantile function. Numerically, x = 1.482602218505602. See, eg,
https://en.wikipedia.org/wiki/Median_absolute_deviation#Relation_to_standard_deviation
Using the statalg parameter, one may also select whether to use the Classical or Chauvenet/ZScore statistics algorithm to
compute the desired statistic (see the help for ia.statistics() or imstat for a full description of these algorithms).
# compute standard deviations in circles of diameter 10arcsec around
# grid pixels spaced every 4 x 5 pixels and anchored at pixel [30, 40],
# and use linear interpolation to compute values at non-grid-pixels
imdev("my.im", "sigma.im", grid=[4, 5], anchor=[30, 40], xlength="10arcsec", stattype="sigma", interp="lin", statalg="cl")
# compute median of the absolute deviations from the median values using
# the z-score/Chauvenet algorithm, by fixing the maximum z-score to determine outliers to 5.
# Use cubic interpolation to compute values for non-grid-point pixels. Use a rectangular region
# of dimensions 5arcsec x 20arcsec centered on each grid point as the region in which to include
# pixels for the computation of stats for that grid point.
imdev("my.im", "madm.im", grid=[4, 5], anchor=[30, 40], xlength="5arcsec", ylength="20arcsec, stattype="madm", interp="cub", statalg="ch", zscore=5)
More information about CASA may be found at the
CASA web page
Copyright © 2016 Associated Universities Inc., Washington, D.C.
This code is available under the terms of the GNU General Public Lincense
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