Extracting data from an image (imval)

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6.8 Extracting data from an image (imval)

The imval task will extract the values of the data and mask from a specified region of an image and place in the task return value as a Python dictionary.

The inputs are:

#  imval :: Get the data value(s) and/or mask value in an image.
imagename  =      ’’   #  Name of the input image
region     =      ’’   #  Image Region.  Use viewer
box        =      ’’   #  Select one or more box regions
chans      =      ’’   #  Select the channel(spectral) range
stokes     =      ’’   #  Stokes params to image (I,IV,IQU,IQUV)
async      =   False

Area selection using box and region is detailed in § 6.1.1 and § 6.1.5 respectively. By default, box=’’ will extract the image information at the reference pixel on the direction axes.

Plane selection is controlled by chans and stokes. See § 6.1.2 for details on plane selection. By default, chans=’’ and stokes=’’ will extract the image information in all channels and Stokes planes.

For instance,

xval = imval(’myimage’, box=’144,144’, stokes=’I’ )

will extract the Stokes I value or spectrum at pixel 144,144, while

xval = imval(’myimage’, box=’134,134.154,154’, stokes=’I’ )

will extract a 21 by 21 pixel region.

Extractions are returned in NumPy arrays in the return value dictionary, plus some extra elements describing the axes and selection:

CASA <2>: xval = imval(’ngc5921.demo.moments.integrated’)

CASA <3>: xval
  Out[3]:
{’axes’: [[0, ’Right Ascension’],
          [1, ’Declination’],
          [3, ’Frequency’],
          [2, ’Stokes’]],
’blc’: [128, 128, 0, 0],
’data’: array([ 0.89667124]),
’mask’: array([ True], dtype=bool),
’trc’: [128, 128, 0, 0],
’unit’: ’Jy/beam.km/s’}

extracts the reference pixel value in this 1-plane image. Note that the ’data’ and ’mask’ elements are NumPy arrays, not Python lists.

To extract a spectrum from a cube:

CASA <8>: xval = imval(’ngc5921.demo.clean.image’,box=’125,125’)

CASA <9>: xval
  Out[9]:
{’axes’: [[0, ’Right Ascension’],
          [1, ’Declination’],
          [3, ’Frequency’],
          [2, ’Stokes’]],
’blc’: [125, 125, 0, 0],
’data’: array([  8.45717848e-04,   1.93370355e-03,   1.53750915e-03,
         2.88399984e-03,   2.38683447e-03,   2.89159478e-04,
         3.16268904e-03,   9.93389636e-03,   1.88773088e-02,
         3.01138610e-02,   3.14478502e-02,   4.03211266e-02,
         3.82498614e-02,   3.06552909e-02,   2.80734301e-02,
         1.72479432e-02,   1.20884273e-02,   6.13593217e-03,
         9.04005766e-03,   1.71429547e-03,   5.22095338e-03,
         2.49114982e-03,   5.30831399e-04,   4.80734324e-03,
         1.19265869e-05,   1.29435991e-03,   3.75700940e-04,
         2.34788167e-03,   2.72604497e-03,   1.78467855e-03,
         9.74952069e-04,   2.24676146e-03,   1.82263291e-04,
         1.98463408e-06,   2.02975096e-03,   9.65532148e-04,
         1.68218743e-03,   2.92119570e-03,   1.29359076e-03,
        -5.11484570e-04,   1.54162932e-03,   4.68662125e-04,
        -8.50282842e-04,  -7.91683051e-05,   2.95954203e-04,
        -1.30133145e-03]),
’mask’: array([ True,  True,  True,  True,  True,  True,  True,  True,  True,
        True,  True,  True,  True,  True,  True,  True,  True,  True,
        True,  True,  True,  True,  True,  True,  True,  True,  True,
        True,  True,  True,  True,  True,  True,  True,  True,  True,
        True,  True,  True,  True,  True,  True,  True,  True,  True,  True], dtype=bool),
’trc’: [125, 125, 0, 45],
’unit’: ’Jy/beam’}

To extract a region from the plane of a cube:

CASA <13>: xval = imval(’ngc5921.demo.clean.image’,box=’126,128,130,129’,chans=’23’)

CASA <14>: xval
  Out[14]:
{’axes’: [[0, ’Right Ascension’],
          [1, ’Declination’],
          [3, ’Frequency’],
          [2, ’Stokes’]],
’blc’: [126, 128, 0, 23],
’data’: array([[ 0.00938627,  0.01487772],
       [ 0.00955847,  0.01688832],
       [ 0.00696965,  0.01501907],
       [ 0.00460964,  0.01220793],
       [ 0.00358087,  0.00990202]]),
’mask’: array([[ True,  True],
       [ True,  True],
       [ True,  True],
       [ True,  True],
       [ True,  True]], dtype=bool),
’trc’: [130, 129, 0, 23],
’unit’: ’Jy/beam’}

CASA <15>: print xval[’data’][0][1]
0.0148777160794

In this example, a rectangular box was extracted, and you can see the order in the array and how to address specific elements.

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