SKYSCLIM

Name

skysclim

Purpose

Compute stretch range for a hard stretch on the background in an image.

Description

Calling Sequence

skysclim,image,lowval,hival,meanval,sigma

Inputs

image - 2-d image to compute stretch range for.

Optional Input Parameters

Keyword Input Parameters

NPTS - Maximum number of points to grab at random from image for scaling
information. Default=601.
LOWCLIP - fraction of random sample to clip at the low end of the signal.
If lowclip=.1 and npts=100, then the 10 lowest values in the
random sample are excluded BEFORE the robust mean is computed
for the stretch range. This option will probably be just a bit
slower if invoked. This option will likely be more robust against
extreme values in the image. Default=0.0 (no clipping)
HICLIP - fraction of random sample to clip at the high end of the signal.
If hiclip=.9 and npts=100, then the 10 highest values in the
random sample are excluded BEFORE the robust mean is computed
for the stretch range. This option will probably be just a bit
slower if invoked. This option will likely be more robust against
extreme values in the image. Default=1.0 (no clipping)
THRESH - sigma threshold for excluding points (default=3.0)
SEED - value for the seed used by the random call. Default is undefined
causing random to use the system time as the seed

Outputs

lowval - Low DN value for sky stretch (meanval-3.0*sigma)
hival - High DN value for sky stretch (meanval+5.0*sigma)
meanval - Mean of random sample
sigma - standard deviation of random sample

Keyword Output Parameters

SAMPLE - Optional return of the sample of points from the sky that was
considered to be representative of the sky. The length of
this array will be less than or equal to NPTS.
WARNING! To preserve a faster execution option if you don't
want this sample returned a non-existant variable for this
keyword is treated the same as no keyword at all. If you do
pass an undefined variable you will get the sample back but
it will contain the entire sky sample, not the sample cleaned
of non-sky values. The input variable can be just about
anything to ensure getting back a cleaned sample.
STDMEAN - Return for the standard deviation of the mean of the returned
sky value.

Common Blocks

Side Effects

Restrictions

Procedure

Modification History

96/01/07 - Marc W. Buie
2000/02/29, MWB, added NPTS keyword
2002/11/17, MWB, added LOWCLIP, HICLIP keywords
2002/12/18, MWB, fixed bug causing crash when image is smaller than npts.
2005/11/22, MWB, added SAMPLE and THRESH keywords
2010/03/12, Florian Scheulen, added SEED keyword
2013/03/29, MWB, added STDMEAN output keyword
2014/02/11, MWB, added more data type options for SEED