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AS - Average images - find & save variance

(08/15/96)

PURPOSE

Adds 2 or more (up to 20000) pictures together, computes several measures of variance (per point, total, etc.). The average and the variance image are stored in separate output files. The command works for 2D as well as 3D files. WARNING: See Note 3 below if you want a better variance measure! Use of this command for variances is no longer recommended!

SEE ALSO

AS AD [Average images - add to the existing average - find variance.]
AS DC [Average images - find & save variance, doc. file]
AS F [Average images - find statistics & Q factor map]
AS R [Average images - find & save variance, better]

USAGE

.OPERATION: AS X10
[The optional register is to receive the value of the average offset.]

FOR IMAGES TO BE AVERAGED
.ENTER 3 LETTER FILE PREFIX OR TEMPLATE (EG. PIC****): PIC
[Enter prefix of images to be added.]

.ENTER FILE NUMBERS: 1-10,12,15-17
[Enter numbers of files within a series to be added.]

.ALL, ODD-EVEN (A/O): O
['A '- all images will be added, 'O' - two subaverages will be calculated: one for odd-numbered images and one for even numbered images. For this option the next two questions will appear twice.]

.AVERAGE FILE: AVG001
[Enter name of output file receiving average.]

.VARIANCE FILE: VAR001
[Enter name of output file receiving variance map.]

[Questions for average and variance files are repeated if option 'O' was chosen.]

The following values are then printed out:

 
          Number of images 
          Number of elements 
          Total variance             Total std. deviation 
          Variance per point         Std. deviation per point 
          Average offset 
          Variance of average image 

NOTES

  1. The average file will be normalized so that its mean pixel value (X5) is equal to 0. To "restore" MIN and MAX values comparable to those of the input files, use 'AR'. For example, if the average offset was 0.00131, you would have ((P1+0.00131)*1.), with P1 as the input average file. This is important if you intend to contour the average using 'FC', since negative values in the file will give a nonsense result.

  2. If the optional register (e.g. X10) is used, 'AR' can be used with the formula ((P1+X10)*1.). If you calculate an average in more than one step, each average-offset is independent. To get the correct average-offset of the complete image, the average of the subaverage average-offsets has to be used.

  3. This version of 'AS' does subtracts the average from each image during the calculation of global average and variance. Thus, Operation 'AS' results in the variance estimator which is usually far 'too optimistic' - the variance obtained is too small and cannot be used for statistical tests. Use 'AS R'. instead.

SUBROUTINES

ADDS

CALLER

UTIL2

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