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. Revision: 1.6, 2012/02/29 23:54:44 UTC

Generated by miriad@atnf.csiro.au on 21 Jun 2016