# imdev¶

imdev(imagename, outfile='', region='', box='', chans='', stokes='', mask='', overwrite=False, stretch=False, grid=[1, 1], anchor='ref', xlength='1pix', ylength='1pix', interp='cubic', stattype='sigma', statalg='classic', zscore=- 1, maxiter=- 1)[source]

Create an image that can represent the statistical deviations of the input image.

[Description] [Examples] [Development] [Details]

Parameters
• imagename (string) - Input image name

• outfile (string=’’) - Output image file name. If left blank (the default), no image is written but a new image tool referencing the collapsed image is returned.

• region ({string, record}=’’) - Region selection. Default is to use the full image.

• box (string=’’) - Rectangular region(s) to select in direction plane. Default is to use the entire direction plane.

• chans (string=’’) - Channels to use. Default is to use all channels.

• stokes (string=’’) - Stokes planes to use. Default is to use all Stokes planes.

• stretch (bool=False) - Stretch the mask if necessary and possible? Default value is False.

• overwrite (bool=False) - Overwrite (unprompted) pre-existing output file? Ignored if “outfile” is left blank.

• grid (intArray=[1, 1]) - x,y grid spacing. Array of exactly two positive integers.

• anchor ({string, intArray}=’ref’) - x,y anchor pixel location. Either “ref” to use the image reference pixel, or an array of exactly two integers.

• xlength ({string, int}=’1pix’) - Either x coordinate length of box, or diameter of circle. Circle is used if ylength is empty string.

• ylength ({string, int}=’1pix’) - y coordinate length of box. Use a circle if ylength is empty string.

• interp (string=’cubic’) - Interpolation algorithm to use. One of “nearest”, “linear”, or “cubic”. Minimum match supported.

• stattype (string=’sigma’) - Statistic to compute. See full description for supported statistics.

• statalg (string=’classic’) - Statistics computation algorithm to use. Supported values are “chauvenet” and “classic”, Minimum match is supported.

statalg = chauvenet
• zscore (double=-1) - 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”.

• maxiter (int=-1) - 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”.

Description

Create an image that can represent the statistical deviations of the input image.

This application creates an image that reflects the statistics around specified grid points of the input image. The output image has the same dimensions and coordinate system as the (selected region in the) input image. The grid parameter describes how many pixels apart the grid pixels are from one another and the statistics are computed around each grid pixel. Grid pixels are limited to the direction plane only (typically RA and dec); independent statistics are computed for each direction plane (i.e., 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 (e.g., “4pix”). Otherwise, they must be specified as valid angular quantities, with recognized units (e.g., “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.

Warning

WARNING: Beware of machine precision issues because you may get a smaller number of pixels included in a region than you expect if you specify, e.g., an integer number of pixels. In such cases, you probably want to specify that number plus a small epsilon value (e.g., “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 of the standard interpolation algorithms to use for this computation using the interp parameter. The input image pixel mask is copied to the output image. If interpolation is performed, output pixels are masked where the interpolation fails.

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]. 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’ or ‘madm’ - median absolute deviation from the median

• ‘median’ - median

• ‘min’ - minimum

• ‘npts’ - number of points

• ‘q1’ - first quartile

• ‘q3’ - third quartile

• ‘rms’ - rms

• ‘sigma’ or ‘std’ - standard deviation

• ‘sumsq’ - sum of squares

• ‘sum’ - sum

• ‘var’ - variance

• ‘xmadm’ - median absolute deviation from the median converted to an RMS-equivalent value. Result is MADM multipied by x, where x is the reciprocal of $$Phi^{-1}*(3/4)$$ and $$Phi^{-1}$$ is the reciprocal of the quantile function. Numerically, x = 1.482602218505602. See here for an example.

Using the statalg parameter, one may also select whether to use the default Classical (“classic”, which uses the “framework” statistics method) or Chauvenet/ZScore (“chauvenet”) statistics algorithm to compute the desired statistic (see the help for ia.statistics or imstat for a full description of these algorithms; see this page for further information on Operations and Statistics on CASA images).

Parameter descriptions

imagename

The name of the input image that imdev will use.

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.

region

Region selection. Default is to use the full image.

box

Rectangular region(s) to select in direction plane. Default is to use the entire direction plane.

chans

Channels from the input image to use. Default is to use all channels.

stokes

Stokes planes to use. Default is to use all Stokes planes.

Mask to use. Default setting is none.

stretch

Stretch the mask if necessary and possible. Default value is False.

overwrite

Overwrite (unprompted) pre-existing output file. Ignored if outfile is left blank.

grid

x,y grid spacing. Array of exactly two positive integers.

anchor

x,y anchor pixel location. Either “ref” to use the image reference pixel, or an array of exactly two integers.

xlength

Either x coordinate length of box, or diameter of circle. Circle is used if ylength is empty string.

ylength

y coordinate length of box. Use a circle if ylength is empty string.

interp

Interpolation algorithm to use. One of “nearest”, “linear”, or “cubic”. Minimum match supported.

stattype

Statistic to compute. Accepted values discussed in the section above.

statalg

Statistics computation algorithm to use. Supported values are “chauvenet” and “classic”, Minimum match is supported.

statalg=’chauvenet’ expandable parameters

zscore

This is the target maximum number of standard deviations data may have to be included. If negative, use Chauvenet”s criterion.

maxiter

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.

Examples

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],
interp="cub", statalg="ch", zscore=5)

Development

Parameter Details

Detailed descriptions of each function parameter

imagename (string) - Input image name
outfile (string='') - Output image file name. If left blank (the default), no image is written but a new image tool referencing the collapsed image is returned.
region ({string, record}='') - Region selection. Default is to use the full image.
box (string='') - Rectangular region(s) to select in direction plane. Default is to use the entire direction plane.
chans (string='') - Channels to use. Default is to use all channels.
stokes (string='') - Stokes planes to use. Default is to use all Stokes planes.
mask (string='') - Mask to use. Default setting is none.
overwrite (bool=False) - Overwrite (unprompted) pre-existing output file? Ignored if “outfile” is left blank.
stretch (bool=False) - Stretch the mask if necessary and possible? Default value is False.
grid (intArray=[1, 1]) - x,y grid spacing. Array of exactly two positive integers.
anchor ({string, intArray}='ref') - x,y anchor pixel location. Either “ref” to use the image reference pixel, or an array of exactly two integers.
xlength ({string, int}='1pix') - Either x coordinate length of box, or diameter of circle. Circle is used if ylength is empty string.
ylength ({string, int}='1pix') - y coordinate length of box. Use a circle if ylength is empty string.
interp (string='cubic') - Interpolation algorithm to use. One of “nearest”, “linear”, or “cubic”. Minimum match supported.
stattype (string='sigma') - Statistic to compute. See full description for supported statistics.
statalg (string='classic') - Statistics computation algorithm to use. Supported values are “chauvenet” and “classic”, Minimum match is supported.
zscore (double=-1) - 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”.
maxiter (int=-1) - 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”.