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Basic Information on imom
Task: imom
Purpose: Compute intensity-weighted moment of an image
Categories: image analysis
IMOM finds the 'centroid' of a Miriad image, and the 'spread'
about that centroid, by evaluating the expressions
mom1 = SUM{ I(x)*x } / SUM{ I(x) }
mom2 = sqrt( SUM{ I(x)*(x-mom1)**2 } / SUM{ I(x) } )
along the first two axes of an image.
Notes:
* The first moment should always be in the range 1..naxis(n).
* mom2 is equivalent to the standard deviation, not the FWHM,
for a gaussian intensity distribution.
* mom2 shows the spread along the directions parallel to the
*image axes*. So if your object of interest is elongated along
some random angle, the mom2 values returned will be a combination
of the major and minor-axis sizes, projected onto the x- and y-axes.
* If all pixels in the region are blank, moments of -1.0 are returned
and a warning is printed.
* In case of a rounding error etc, moments of 0.0 are returned
and a warning is printed.
Key: in
Input image name. No default.
Key: options
The following options can be given (minimum match applies):
skew
Compute the 'third moment'. Since the formal third moment
has the rather useless dimensions of pixel**3, what
is actually returned is the dimensionless quantity
SUM{ I(x)*(x - mom1)**3 } / SUM{ I(x) } / mom2**3.
A negative value means the distribution is skewed towards
values less than the first moment; i.e. the pixels at
lower pixel number than mom1 are generally brighter than
those at higher pixel number. A positive value
means the pixels at higher pixel number than the first
moment are brighter.
clipmean
When you use this program to estimate the peak of a source,
better accuracy can be had by ignoring low-intensity values.
Setting this option forms the moments using only pixels that
are brighter than the (local) mean.
If a min or max value is specified (see below), the mean is
computed using only the values within the selected range.
clip1sigma
Forms moments using just the pixels brighter than
local_mean + local_rms.
If a min or max value is specified, the mean and rms are computed
using only the values within the selected range.
Key: region
The region of interest. See the help on region for more
information. This gives the region of pixels to calculate the
moments within. Only rectangular regions are supported.
The default is the entire image.
If a cube is given as the input image, the program computes
moments for each plane along axis 3 (within the selected range).
Key: min
The minimum pixel value for inclusion in the moment calculation.
The default is the image minimum.
Key: max
The maximum pixel value for inclusion in the moment calculation
The default is the image maximum.
Key: log
The output log file. The default is the terminal. Output to
the terminal is paged.