Help for COLORT2

PURPOSE:
COLORT2 is the latest version of the color transformation program. Its
predecessors include COLOR (not the present IBIS COLOR program), COLOR2,
COLOR6, and COLORASM. Its purpose is to convert color images from the
blue-green-red color domain into one of several other color spaces, or
to perform the inverse transformation back into the blue-green-red
coordinate system.

COLORT2 is an upgrade of COLORT to run on halfword and fullword integer
and real*4 data.  Currently, this is only implemented for three of the
seven spaces supported by COLORT:  HSI, HSR and SPH.
EXECUTION:

colort2 (BL,GR,RD) (HUE,SAT,INT) TO=HSI       This maps from blue, green,
                                              red space into hue, saturation,
                                              and intensity.
colort2 (HUE,SAT,INT) (BL,GR,RD) FROM=HSI      This performs the inverse
                                              transformation of the previous
                                              example. Hue, saturation, and
                                              intensity are mapped into blue,
                                              green, and red.
OPERATION:
     COLORT2 performs color coordinate space transformations between the
standard blue-green-red Cartesian space any of seven other color spaces.
The definitions of these eight color spaces are:

BGR or DN   This is a Cartesian coordinate system, with the three axes
            being the blue, green, and red vectors. Precisely, these should
            be the reflectance from monochromatic illumination at 435.8 nm
            for blue, 546.1 nm for green, and 700.0 nm for red.

RGB         This is the same as BGR, just a different order of images.

TRISTIMULUS This is a Cartesian coordinate system, with the three axes
            being green, red, and intensity. Here, green and red refer to
            the fraction of total radiance contributed by that color.
            Intensity is the sum of all radiance. Scaling is such that
            255 DN is the maximum possible value of the coordinate.
            ** NOT CURRENTLY SUPPORTED IN COLORT2 **
CIE         This refers to the 1931 CIE-X, Y, Z system of chromaticity 
            coordinates. CIE is the "Commission Internationale de l'Eclairage".
            The system is Cartesian, with X, Y, and intensity axes.
            It is related to BGR by the equations
                            0.49000r + 0.31000g + 0.20000b
                        x = ------------------------------
                            0.66697r + 1.13240g + 1.20063b

                            0.17697r + 0.81240g + 0.01063b
                        y = ------------------------------
                            0.66697r + 1.13240g + 1.20063b

                intensity = 0.66697r + 1.13240g + 1.20063b
             Scaling is such that 255 DN means x=1, y=1, and intensity
             is at maximum value. 0 DN means x=0, y=0, intensity=0.
            ** NOT CURRENTLY SUPPORTED IN COLORT2 **
UCS          UCS refers to the coordinate system used in the 1960 CIE-UCS
             (Uniform Chromaticity-Scale) diagram, developed by D. L.
             MacAdam. It is related to the CIE system by the equations

                               4x
                       u = ---------
                           -2x+12y+3

                               6y
                       v = ---------
                           -2x+12y+3

               intensity = intensity

             Scaling is such that u and v = 0 at 0 DN; u and v = 1 at 400 DN.
            ** NOT CURRENTLY SUPPORTED IN COLORT2 **
SPHERICAL    This is the spherical coordinate system, with unit coordinates
             of longitude, colatitude, and radiance. In this system, the polar
             axis is defined as the achromatic line (blue=green=red). Radiance
             is the vector sum of blue, green and red, scaled so that all
             colors are at 0 DN for radiance = 0 DN, and all colors are at 
             255 DN for radiance = 255 DN. Longitude is scaled so that blue
             is at 43 DN, green is at 128 DN, red is at 213 DN. Colatitude
             (the polar angle) is scaled so that arctan(sqrt(2)) is at 255 DN.
HSR          This is similar to the spherical system, with unit coordinates
             of hue, saturation, and radiance. Radiance has the same meaning
             as before, and hue is synonymous with longitude. Saturation is
             like colatitude, but scaled so that 255 DN is the maximum
             permissible angle at that particular hue.

HSI          This is the same as the hue-saturation-radiance space, except
             that intensity is substituted for radiance. Intensity is scaled
             so that 255 DN is the maximum radiance value that a pixel with
             this hue and saturation could acquire in blue-green-red space.

CUBEROOT     This is a Cartesian space that approximates the Munsell space.
             The Munsell space is a space that is linear in perceived
             differences in both color and brightness. Its coordinates are
             L*, a*, and b*. The equations that define this transformation
             are:

                 L* = 25.29*cuberoot(green) - 18.38
                 a* = Ka(cuberoot(red)-cuberoot(green))
                 b* = Kb(cuberoot(green)-cuberoot(blue))
                         Ka =105 for red<green, =125 otherwise
                         Kb =30.5 for blue<green, =53.6 otherwise
                         red is defined as 1.02*X of CIE space
                         green is defined as Y of CIE space
                         blue is defined as 0.847z of CIE space
             L*, a*, and b* are scaled so that one unit in the above
             equations corresponds to 4 DN; 0 DN implies L*=-18.36;
             a* = 0 at 100 DN; b*=0 at 150 DN.
            ** NOT CURRENTLY SUPPORTED IN COLORT2 **
Most of the equations used in this program were lifted from COLOR SCIENCE,
by Wyszecki and Stiles. The reader is urged to consult this text for a more
detailed explanation of the concepts involved.
  
WRITTEN BY:  Alan R. Gillespie, September, 1976
COGNIZANT PROGRAMMER:  L. W. Kamp
REVISION:  8; May 1, 1984; (Conversion to VAX by S. Pohorsky)
REVISION:     Feb 3, 1986; (Conversion to VICAR2 I/O)
2012-Dec -lwk- converted to run on all data types.