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diff --git a/pkg/images/imutil/doc/imstat.hlp b/pkg/images/imutil/doc/imstat.hlp new file mode 100644 index 00000000..ed5183d9 --- /dev/null +++ b/pkg/images/imutil/doc/imstat.hlp @@ -0,0 +1,121 @@ +.help imstatistics Feb01 images.imutil +.ih +NAME +imstatistics -- compute and print image pixel statistics +.ih +USAGE +imstatistics images +.ih +PARAMETERS +.ls images +The input images or image sections for which pixel statistics are to be +computed. +.le +.ls fields = "image,npix,mean,stddev,min,max" +The statistical quantities to be computed and printed. +.le +.ls lower = INDEF +The minimum good data limit. All pixels are above the default value of INDEF. +.le +.ls upper = INDEF +The maximum good data limit. All pixels are above the default value of INDEF. +.le +.ls nclip = 0 +The maximum number of iterative clipping cycles. By default no clipping is +performed. +.le +.ls lsigma = 3.0 +The low side clipping factor in sigma. +.le +.ls usigma = 3.0 +The high side clipping factor in sigma. +.le +.ls binwidth = 0.1 +The width of the histogram bins used for computing the midpoint (estimate +of the median) and the mode. +The units are in sigma. +.le +.ls format = yes +Label the output columns and print the result in fixed format. If format +is "no" no column labels are printed and the output is in free format. +.le +.ls cache = no +Cache the image data in memory ? This can increase the efficiency of the +task if nclip > 0 or either of the midpt and mode statistics are computed. +.le +.ih +DESCRIPTION +The statistical quantities specified by the parameter \fIfields\fR are +computed and printed for each image in the list specified by \fIimages\fR. +The results are printed in tabular form with the fields listed in the order +they are specified in the fields parameter. The available fields are the +following. + +.nf + image - the image name + npix - the number of pixels used to do the statistics + mean - the mean of the pixel distribution + midpt - estimate of the median of the pixel distribution + mode - the mode of the pixel distribution + stddev - the standard deviation of the pixel distribution + skew - the skew of the pixel distribution + kurtosis - the kurtosis of the pixel distribution + min - the minimum pixel value + max - the maximum pixel value +.fi + +The mean, standard deviation, skew, kurtosis, min and max are computed in a +single pass through the image using the expressions listed below. +Only the quantities selected by the fields parameter are actually computed. + +.nf + mean = sum (x1,...,xN) / N + y = x - mean + variance = sum (y1 ** 2,...,yN ** 2) / (N-1) + stddev = sqrt (variance) + skew = sum ((y1 / stddev) ** 3,...,(yN / stddev) ** 3) / (N-1) + kurtosis = sum ((y1 / stddev) ** 4,...,(yN / stddev) ** 4) / (N-1) - 3 +.fi + +The midpoint and mode are computed in two passes through the image. In the +first pass the standard deviation of the pixels is calculated and used +with the \fIbinwidth\fR parameter to compute the resolution of the data +histogram. The midpoint is estimated by integrating the histogram and +computing by interpolation the data value at which exactly half the +pixels are below that data value and half are above it. The mode is +computed by locating the maximum of the data histogram and fitting the +peak by parabolic interpolation. + +.ih +EXAMPLES +1. To find the number of pixels, mean, standard deviation and the minimum +and maximum pixel value of a bias region in an image. + +.nf + cl> imstat flat*[*,1] + # IMAGE NPIX MEAN STDDEV MIN MAX + flat1[*,1] 800 999.5 14.09 941. 1062. + flat2[*,1] 800 999.4 28.87 918. 1413. +.fi + +The string "flat*" uses a wildcard to select all images beginning with the +word flat. The string "[*,1]" is an image section selecting row 1. + +2. Compute the mean, midpoint, mode and standard deviation of a pixel +distribution. + +.nf + cl> imstat m51 fields="image,mean,midpt,mode,stddev" + # IMAGE PIXELS MEAN MIDPT MODE STDDEV + M51 262144 108.3 88.75 49.4 131.3 +.fi + +.ih +BUGS +When using a very large number of pixels the accumulation of the sums +of the pixel values to the various powers may +encounter roundoff error. This is significant when the true standard +deviation is small compared to the mean. +.ih +SEE ALSO +.endhelp |