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ASINHSCL

ASINHSCL

## Purpose

This is a utility routine to perform an inverse hyperbolic sine
function intensity transformation on an image. I think of this
as a sort of "tuned" gamma or power-law function. The algorithm,
and notion of "asinh magnitudes", comes from a paper by Lupton,
et. al, in The Astronomical Journal, 118:1406-1410, 1999 September.
I've relied on the implementation of Erin Sheldon, found here:
http://cheops1.uchicago.edu/idlhelp/sdssidl/plotting/tvasinh.html
I'm also grateful of discussions with Marshall Perrin on the IDL
newsgroup with respect to the meaning of the "softening parameter", beta,
and for finding (and fixing!) small problems with the code.
Essentially this transformation allow linear scaling of noise values,
and logarithmic scaling of signal values, since there is a small
linear portion of the curve and a much large logarithmic portion of
the curve. (See the EXAMPLE section for some tips on how to view this
transformation curve.)

## Author

FANNING SOFTWARE CONSULTING
David Fanning, Ph.D.
1645 Sheely Drive
Fort Collins, CO 80526 USA
Phone: 970-221-0438
E-mail: david@idlcoyote.com
Coyote's Guide to IDL Programming: http://www.idlcoyote.com

Utilities

## Calling Sequence

outputImage = ASINHSCL(image)

## Arguments

image: The image or signal to be scaled. Written for 2D images, but arrays
of any size are treated alike.

## Keywords

BETA: This keyword corresponds to the "softening parameter" in the Lupon et. al paper.
This factor determines the input level at which linear behavior sets in. Beta
should be set approximately equal to the amount of "noise" in the input signal.
IF BETA=0 there is a very small linear portion of the curve; if BETA=200 the
curve is essentially all linear. The default value of BETA is set to 3, which
is appropriate for a small amount of noise in your signal. The value is always
positive.
NEGATIVE: If set, the "negative" of the result is returned.
MAX: Any value in the input image greater than this value is
set to this value before scaling.
MIN: Any value in the input image less than this value is
set to this value before scaling.
OMAX: The output image is scaled between OMIN and OMAX. The
default value is 255.
OMIN: The output image is scaled between OMIN and OMAX. The
default value is 0.

## Return Value

outputImage: The output, scaled into the range OMIN to OMAX. A byte array.

None.

## Examples

Plot, ASinhScl(Indgen(256), Beta=0.0), LineStyle=0
OPlot, ASinhScl(Indgen(256), Beta=0.1), LineStyle=1
OPlot, ASinhScl(Indgen(256), Beta=1.0), LineStyle=2
OPlot, ASinhScl(Indgen(256), Beta=10.), LineStyle=3
OPlot, ASinhScl(Indgen(256), Beta=100), LineStyle=4

## Restrictions

Requires SCALE_VECTOR from the Coyote Library:
http://www.idlcoyote.com/programs/scale_vector.pro
Incorporates ASINH from the NASA Astronomy Library and renamed ASINHSCL_ASINH.
http://idlastro.gsfc.nasa.gov/homepage.html

## Modification History

Written by: David W. Fanning, 24 February 2006.
Removed ALPHA keyword and redefined the BETA keyword to correspond
to the "softening parameter" of Lupton et. al., following the
suggestions of Marshall Perrin. 25 April 2006. DWF.

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