/*!
   \file lib/ogsf/gsdiff.c

   \brief OGSF library - manipulating surfaces (lower level functions)

   GRASS OpenGL gsurf OGSF Library 

   Routines to set up automatic on-the-fly recalculation
   of surface elevations, doing a "scaled difference" using another
   surface for reference

   Note that we're using a true difference here, between data set values,
   no translations, etc.

   \todo generalize this concept to allow transform functions which are
   dependent on surfaces that are dependent on other surfaces, etc., as long
   as the dependency doesn't loop back.

   (C) 1999-2008 by the GRASS Development Team

   This program is free software under the 
   GNU General Public License (>=v2). 
   Read the file COPYING that comes with GRASS
   for details.

   \author Bill Brown USACERL, GMSL/University of Illinois (November 1994)
   \author Doxygenized by Martin Landa <landa.martin gmail.com> (May 2008)
 */

#include <grass/ogsf.h>
#include "gsget.h"

static geosurf *Refsurf = NULL;
static typbuff *Refbuff = NULL;
static float Refscale = 1.0;

/*!
   \brief Set scale

   \param scale value
 */
void gsdiff_set_SDscale(float scale)
{
    Refscale = scale;

    return;
}

/*!
   \brief Get scale

   \return scale value
 */
float gsdiff_get_SDscale(void)
{
    return (Refscale);
}

/*!
   \brief ADD

   \param gsref
 */
void gsdiff_set_SDref(geosurf * gsref)
{
    Refsurf = gsref;
    Refbuff = gs_get_att_typbuff(gsref, ATT_TOPO, 0);

    return;
}

/*!
   \brief ADD

   \return pointer to geosurf struct
 */
geosurf *gsdiff_get_SDref(void)
{
    if (Refsurf && Refbuff) {
	return (Refsurf);
    }

    return (NULL);
}

/*!
   \brief ADD

   \param val
   \param offset

   \return value
 */
float gsdiff_do_SD(float val, int offset)
{
    float ref;

    if (Refbuff) {
	GET_MAPATT(Refbuff, offset, ref);
	return (ref + (val - ref) * Refscale);
    }

    return (val);
}