# coyote: CGASINHSCL

- coyote/Image Processing
- [Source code]

Description

      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.)

Categories

      Image Processing

Examples

      Plot various values of beta::
          cgPlot,  cgASinhScl(Indgen(256), Beta=0.0), LineStyle=0
          cgOPlot, cgASinhScl(Indgen(256), Beta=0.1), LineStyle=1
          cgOPlot, cgASinhScl(Indgen(256), Beta=1.0), LineStyle=2
          cgOPlot, cgASinhScl(Indgen(256), Beta=10.), LineStyle=3
          cgOPlot, cgASinhScl(Indgen(256), Beta=100), LineStyle=4

Author

      FANNING SOFTWARE CONSULTING::
          David W. Fanning
          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

History

      Change 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.
        Renamed cgASinhScl from ASinhScl. 27 March 2015. DWF.
        Yikes! Two instances of naming problems from 2015! Fixed. 8 July 2016. DWF.

Copyright

      Copyright (c) 2008-2016, Fanning Software Consulting, Inc.
      rn the inverse hyperbolic sine of the argument. Taken from the NASA
      Astronomy Library and renamed for use in this program. The inverse
      rbolic sine is used for the calculation of asinh magnitudes, see
      on et al. (1999, AJ, 118, 1406). Expression given in  Numerical Recipes,
      s et al. (1992), eq. 5.6.7. Note that asinh(-x) = -asinh(x) and that
      h(0) = 0. and that if y = asinh(x) then x = sinh(y).

Returns

      A byte scaled image is returned.

Params

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

Keywords

      beta: in, optional, type=float, default=3.0
          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.
      max: in, optional
           Any value in the input image greater than this value is set to this value
           before scaling.
      min: in, optional
           Any value in the input image less than this value is set to this value
           before scaling.
      negative, in, optional, type=boolean, default=0
           If set, the "negative" of the result is returned.
      omax: in, optional, type=byte, default=255
           The output image is scaled between OMIN and OMAX.
      omin: in, optional, type=byte, default=0
           The output image is scaled between OMIN and OMAX.