Immath
From CASA Guides
Help on immath task:
Perform math operations on images
This task evaluates mathematical expressions involving existing
image files. The results of the calculations are stored in the
designated output file. Options are available to specify mathematical
expression directly or pre-defined expression for calculation of
spectral index image, and polarization intensity and position angle
images are available. The image file names imbedded in the expression or
specified in the imagename parameter for the pre-defined calculations may
be CASA images or FITS images.
NOTE: Index values start at 0 Use the imhead task to see the range of
index values for each axes.
Keyword arguments:
imagename -- input image name(s)
Default: none;
Examples: mode='evalexpr'; imagename=['image1.im', 'image2.im' ]
The text 'IM0' is replaced by 'image1.im' in the
expression and 'IM1' is repalced with 'image2.im'
mode='spix'; imagename=['image1.im','image2.im'] will calculate
an image of log(S1/S2)/log(f1/f2), where S1 and S2 are fluxes and
f1 and f2 are frequencies
mode='pola'; imagename=['imageQ.im','imageU.im'] will calculate
an image of polarization angle distribution, where imageQ.im and
imageU.im are Stokes Q and U images, respectively. Calculate 0.5*arctan(U/Q).
mode='poli'; imagename=['imageQ.im','imageU.im','imageV.im'] will calculate
total polarization intensity image, where imageQ.im, imageU.im, imageV.im
are Stokes Q, U, and V images, respectively.
mode -- mode for mathematical operation
Default: evalexpr
Options: 'evalexpr' : evalulate a mathematical expression defined in 'expr'
'spix' : spectalindex image
'pola' : polarization position angle image
'poli' : polarization intesity image
>>> mode expandable parameters
sigma - (for mode='poli') standard deviation of noise of Stokes images with unit such as
Jy/beam to correct for bias
Default: '0.0Jy/beam' (= no debiasing)
expr -- (for mode='evalexpr') A mathematical expression with images.
Image file names are specified in the imagenames paramter, and
the variables IM0, IM1, ... are used to represent these files
in the expression. Explicit notations of file names in the
expression are also supported, in which cases the file names must
be enclosed in double quotes (") and imagename is ignored.
Default: none
Examples:
Make an image that is image1.im - image2.im
expr=' (IM0 - IM1 )'
or with an explicit notation,
expr='("image1.im" - "image2.im")'
Clip an image below a value (0.5 in this case)
expr = ' iif( IM0 >=0.5, IM0, 0.0) '
Note: iif (a, b, c) a is the boolian expression
b is the value if true
c is the value if false
Take the rms value of two images
expr = ' sqrt(IM0 * IM0 + IM1 * IM1) '
Note: No exponentiaion available?
Build an image pixel by pixel from the minimum of (image2.im, 2*image1.im)
expr='min(IM1,2*max(IM0))'
outfile -- The file where the results of the image calculations
are stored. Overwriting an existing outfile is not permitted.
Default: immath_results.im; Example: outfile='results.im'
mask -- Name of mask applied to each image in the calculation
Default '' means no mask; Example: mask='orion.mask'.
region -- File path to an ImageRegion file.
An ImageRegion file can be created with the CASA
viewer's region manager. Typically ImageRegion files
will have the suffix '.rgn'. If a region file is given
then the box, chans, and stokes selections whill be
ignored.
Default: none
Example: region='myimage.im.rgn'
box -- A box region on the directional plane
Only pixel values acceptable at this time.
Default: none (whole 2-D plane); Example: box='10,10,50,50'
chans -- channel numbers, velocity, and/or frequency
Only channel numbers acceptable at this time.
Default: none (all); Example: chans='3~20'
stokes -- Stokes parameters to image, may or may not be separated
by commas but best if you use commas.
Default: none (all); Example: stokes='IQUV';
Options: 'I','Q','U','V','RR','RL','LR','LL','XX','YX','XY','YY', ...
Available functions in the expr and mask paramters:
pi(), e(), sin(), sinh(), asinh(), cos(), cosh(), tan(), tanh(),
atan(), exp(), log(), log10(), pow(), sqrt(), complex(), conj()
real(), imag(), abs(), arg(), phase(), aplitude(), min(), max()
round(), isgn(), floor(), ceil(), rebin(), spectralindex(), pa(),
iif(), indexin(), replace(), ...
For a full description of the allowed syntax see the
Lattice Expression Language (LEL) documentation on the at:
http://aips2.nrao.edu/docs/notes/223/223.html
NOTE: where indexing and axis numbering are used in the above
functions they are 1-based, ie. numbering starts at 1.
Examples:
# Double all values in an image.
immath( imagesname='myimage.im', expr='IM0*2', outfile='double.im' )
# or with an explicit notation,
immath( expr='"myimage.im"*2', outfile='double.im' )
# Taking the sin of an image and adding it to another
# Note that the images need to be the same size
immath(images=['image1.im', 'image2.im'], expr='sin(IM1)+IM0;',outfile='newImage.im')
# Adding only the plane associated with the 'V' stokes value and
# the 1st channel together in two images
immath(imagename=[image1', 'image2'], expr='IM0+IM1',chans='1',stokes='V')
# Selecting a single plane (5th channel), of the 3-D cube and
# adding it to the original image. In this example the 2-D plane
# gets expanded out and the values are applied to each plane in the
# 3-D cube.
default('immath')
imagename='ngc7538.image'
outfile='chanFive.im'
expr='IM0'
chans='5'
go
default('immath')
imagename=['ngc7538.image', chanFive.im']
outfile='ngc7538_chanFive.im'
expr='IM0+IM1'
go
# Selecting and saving the inner 3/4 of an image for channels 40,42,44
# as well as channels less than 10
default('immath')
imagename='my_image.im'
expr='IM0'
box='25,25,123,123'
chans='<10;40,42,44'
outfile='my_image_inner.im' )
go
# Dividing an image by another, making sure we aren't dividing by zero
default('immath')
imagename=['orion.image', 'my.image']
expr='IM0/iif(IM1==0,1.0,IM1)'
outfile='my_orion.image'
go
# Applying a mask to all of the images in the expression
default('immath')
imagename=['ngc7538.image','ngc7538_clean.image']
expr='(IM0*10)+IM1'
mask='"ngc7538.mask"'
outfile='really_noisy_ngc7538.image'
go
# Applying a pixel mask contained in the image information
default('immath')
imagename='ngc5921.image'
expr='IM0*10'
mask='mask("ngc5921.mask")'
outfile='ngc5921.masked.image'
go
# Creating a Polarization Itensity image from an [Q, U, V] image.
default('immath')
outfile='Q.im'; imagename='3C138_pcal'; expr='IM0'; stokes='Q'; go();
outfile='U.im'; imagename='3C138_pcal'; expr='IM0'; stokes='U'; go();
outfile='V.im'; imagename='3C138_pcal'; expr='IM0'; stokes='V'; go();
#
outfile='pol_intensity'
stokes=''
imagename=['Q.im', 'U.im', 'V.im']
expr='sqrt(IM0*IM0 + IM1*IM1 + IM2*IM2)'
go
# or using pre-defined expression
default('immath')
outfile='pol_intensity'
mode='poli'
imagename=['Q.im','U.im','V.im']
go
# Creating a polarization position angle image
default('immath')
outfile='pol_angle.im'
mode='pola'
imagename=['Q.im','U.im'] # order of the two Stokes images does not matter
go
# Creating a spectral index image from the images at two different observing frequencies
default('immath')
outfile='mySource_sp.im'
mode='spix'
imagename=['mySource_5GHz.im','mySource_8GHz.im']
go
