/////////////////////////////////////////////////////////////////////////////
// desatdist.gsl
//
// Description:
//    Volume shader to desaturate distant objects
//
// Parameters:
//   extinctiondist - the distance at which the saturation falls to 1/e of
//                    its original value.
//   mindist - the saturation only starts to decrease at distances
//          greater than this.
//   maxdist - the saturation does not decrease at distances greater
//          than this.
//   units - units in which the distance parameters are expressed.
//
/////////////////////////////////////////////////////////////////////////////
// Copyright 2005 NVIDIA Corporation.  All Rights Reserved.
// 
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
// 
// * Redistributions of source code must retain the above copyright
//   notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright
//   notice, this list of conditions and the following disclaimer in the
//   documentation and/or other materials provided with the distribution.
// * Neither the name of NVIDIA nor the names of its contributors
//   may be used to endorse or promote products derived from this software
//   without specific prior written permission.
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// (This is the Modified BSD License)
/////////////////////////////////////////////////////////////////////////////



volume
desatdist
    [[ string description = "Desaturate the appearance of distant objects" ]]
(
    float extinctiondist = 100
        [[  string description = "Distance at which saturation decreases by 1/e",
            float UImin = 0 ]],
    float mindist = 0
        [[ string description = "Minimum distance at which desaturation starts",
           float UImin = 0 ]],
    float maxdist = 1e6
        [[ string description = "Maximum distance at which desaturation ends",
           float UImin = 0 ]],
    string units = "common"
        [[ string description = "Units used for distances",
           string UItype = "enum:common,km,m,cm,mm,mi,ft,in"
        ]]
  )
{
    //float Ilen = transformu (units, length(I));
    float Ilen = length(I);
    Ilen = clamp (Ilen, mindist, maxdist);
    float d = (Ilen - mindist) / extinctiondist;
    C = mix (color(luminance(C)), C, exp(-d));
}
