/******************************************************************************
 *
 * Project:  Viewshed Generation
 * Purpose:  Core algorithm implementation for viewshed generation.
 * Author:   Tamas Szekeres, szekerest@gmail.com
 *
 ******************************************************************************
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 ****************************************************************************/

#include "cpl_port.h"
#include "gdal_alg.h"

#include <cmath>
#include <cstring>
#include <array>
#include <limits>
#include <algorithm>

#include "cpl_conv.h"
#include "cpl_error.h"
#include "cpl_progress.h"
#include "cpl_vsi.h"
#include "gdal.h"
#include "gdal_priv.h"
#include "gdal_priv_templates.hpp"
#include "ogr_api.h"
#include "ogr_spatialref.h"
#include "ogr_core.h"
#include "commonutils.h"

CPL_CVSID("$Id: viewshed.cpp 2049f70c4d6d999c5acbb5e539debc54c872d258 2021-08-30 20:49:53 +0200 Tamas Szekeres $")


inline static void SetVisibility(int iPixel, double dfZ, double dfZTarget, double* padfZVal,
    std::vector<GByte>& vResult, GByte byVisibleVal, GByte byInvisibleVal)
{
    if (padfZVal[iPixel] + dfZTarget < dfZ)
        vResult[iPixel] = byInvisibleVal;
    else
        vResult[iPixel] = byVisibleVal;

    if (padfZVal[iPixel] < dfZ)
        padfZVal[iPixel] = dfZ;
}

inline static bool AdjustHeightInRange(const double* adfGeoTransform, int iPixel, int iLine, double& dfHeight, double dfDistance2, double dfCurvCoeff, double dfSphereDiameter)
{
    if (dfDistance2 <= 0 && dfCurvCoeff == 0)
        return true;

    double dfX = adfGeoTransform[1] * iPixel + adfGeoTransform[2] * iLine;
    double dfY = adfGeoTransform[4] * iPixel + adfGeoTransform[5] * iLine;
    double dfR2 = dfX * dfX + dfY * dfY;

    /* calc adjustment */
    if (dfCurvCoeff != 0 && dfSphereDiameter != std::numeric_limits<double>::infinity())
        dfHeight -= dfCurvCoeff * dfR2 / dfSphereDiameter;

    if (dfDistance2 > 0 && dfR2 > dfDistance2)
        return false;

    return true;
}

inline static double CalcHeightLine(int i, double Za, double Zo)
{
    if (i == 1)
        return Za;
    else
        return (Za - Zo) / (i - 1) + Za;
}

inline static double CalcHeightDiagonal(int i, int j, double Za, double Zb, double Zo)
{
    return ((Za - Zo) * i + (Zb - Zo) * j) / (i + j - 1) + Zo;
}

inline static double CalcHeightEdge(int i, int j, double Za, double Zb, double Zo)
{
    if (i == j)
        return CalcHeightLine(i, Za, Zo);
    else
        return ((Za - Zo) * i + (Zb - Zo) * (j - i)) / (j - 1) + Zo;
}

inline static double CalcHeight(double dfZ, double dfZ2, GDALViewshedMode eMode)
{
    if (eMode == GVM_Edge)
        return dfZ2;
    else if (eMode == GVM_Max)
        return (std::max)(dfZ, dfZ2);
    else if (eMode == GVM_Min)
        return (std::min)(dfZ, dfZ2);
    else
        return dfZ;
}


/************************************************************************/
/*                        GDALViewshedGenerate()                         */
/************************************************************************/

/**
 * Create viewshed from raster DEM.
 *
 * This algorithm will generate a viewshed raster from an input DEM raster
 * by using a modified algorithm of "Generating Viewsheds without Using Sightlines"
 * published at https://www.asprs.org/wp-content/uploads/pers/2000journal/january/2000_jan_87-90.pdf
 * This appoach provides a relatively fast calculation, since the output raster is
 * generated in a single scan.
 * The gdal/apps/gdal_viewshed.cpp mainline can be used as an example of
 * how to use this function.
 * The output raster will be of type Byte or Float64.
 *
 * \note The algorithm as implemented currently will only output meaningful results
 * if the georeferencing is in a projected coordinate reference system.
 *
 * @param hBand The band to read the DEM data from. Only the part of the raster
 * within the specified maxdistance around the observer point is processed.
 *
 * @param pszDriverName Driver name (GTiff if set to NULL)
 *
 * @param pszTargetRasterName The name of the target raster to be generated. Must not be NULL
 *
 * @param papszCreationOptions creation options.
 *
 * @param dfObserverX observer X value (in SRS units)
 *
 * @param dfObserverY observer Y value (in SRS units)
 *
 * @param dfObserverHeight The height of the observer above the DEM surface.
 *
 * @param dfTargetHeight The height of the target above the DEM surface. (default 0)
 *
 * @param dfVisibleVal pixel value for visibility (default 255)
 *
 * @param dfInvisibleVal pixel value for invisibility (default 0)
 *
 * @param dfOutOfRangeVal The value to be set for the cells that fall outside of the
 * range specified by dfMaxDistance.
 *
 * @param dfNoDataVal The value to be set for the cells that have no data.
 *                    If set to a negative value, nodata is not set.
 *                    Note: currently, no special processing of input cells at a nodata
 *                    value is done (which may result in erroneous results).
 *
 * @param dfCurvCoeff Coefficient to consider the effect of the curvature and refraction.
 * The height of the DEM is corrected according to the following formula:
 * [Height] -= dfCurvCoeff * [Target Distance]^2 / [Earth Diameter]
 * For the effect of the atmospheric refraction we can use 0.85714‬.
 *
 * @param eMode The mode of the viewshed calculation.
 * Possible values GVM_Diagonal = 1, GVM_Edge = 2 (default), GVM_Max = 3, GVM_Min = 4.
 *
 * @param dfMaxDistance maximum distance range to compute viewshed.
 *                      It is also used to clamp the extent of the output raster.
 *                      If set to 0, then unlimited range is assumed, that is to say the
 *                      computation is performed on the extent of the whole raster.
 *
 * @param pfnProgress A GDALProgressFunc that may be used to report progress
 * to the user, or to interrupt the algorithm.  May be NULL if not required.
 *
 * @param pProgressArg The callback data for the pfnProgress function.
 *
 * @param heightMode Type of information contained in output raster. Possible values
 *                   GVOT_NORMAL = 1 (default), GVOT_MIN_TARGET_HEIGHT_FROM_DEM = 2,
 *                   GVOT_MIN_TARGET_HEIGHT_FROM_GROUND = 3
 *
 *                   GVOT_NORMAL returns a raster of type Byte containing visible locations.
 *
 *                   GVOT_MIN_TARGET_HEIGHT_FROM_DEM and GVOT_MIN_TARGET_HEIGHT_FROM_GROUND
 *                   will return a raster of type Float64 containing the minimum target height
 *                   for target to be visible from the DEM surface or ground level respectively.
 *                   Parameters dfTargetHeight, dfVisibleVal and dfInvisibleVal will be ignored.
 *
 *
 * @param papszExtraOptions Future extra options. Must be set to NULL currently.
 *
 * @return not NULL output dataset on success (to be closed with GDALClose()) or NULL if an error occurs.
 *
 * @since GDAL 3.1
 */

GDALDatasetH GDALViewshedGenerate(GDALRasterBandH hBand,
                            const char* pszDriverName,
                            const char* pszTargetRasterName,
                            CSLConstList papszCreationOptions,
                    double dfObserverX, double dfObserverY, double dfObserverHeight,
                    double dfTargetHeight, double dfVisibleVal, double dfInvisibleVal,
                    double dfOutOfRangeVal, double dfNoDataVal, double dfCurvCoeff,
                    GDALViewshedMode eMode, double dfMaxDistance,
                    GDALProgressFunc pfnProgress, void *pProgressArg,
                    GDALViewshedOutputType heightMode, CSLConstList papszExtraOptions)

{
    VALIDATE_POINTER1( hBand, "GDALViewshedGenerate", nullptr );
    VALIDATE_POINTER1( pszTargetRasterName, "GDALViewshedGenerate", nullptr );

    CPL_IGNORE_RET_VAL(papszExtraOptions);

    if( pfnProgress == nullptr )
        pfnProgress = GDALDummyProgress;

    if( !pfnProgress( 0.0, "", pProgressArg ) )
    {
        CPLError( CE_Failure, CPLE_UserInterrupt, "User terminated" );
        return nullptr;
    }

    const GByte byNoDataVal = dfNoDataVal >= 0 && dfNoDataVal <= 255 ? static_cast<GByte>(dfNoDataVal) : 0;
    const GByte byVisibleVal = dfVisibleVal >= 0 && dfVisibleVal <= 255 ? static_cast<GByte>(dfVisibleVal) : 255;
    const GByte byInvisibleVal = dfInvisibleVal >= 0 && dfInvisibleVal <= 255 ? static_cast<GByte>(dfInvisibleVal) : 0;
    const GByte byOutOfRangeVal = dfOutOfRangeVal >= 0 && dfOutOfRangeVal <= 255 ? static_cast<GByte>(dfOutOfRangeVal) : 0;

    if(heightMode != GVOT_MIN_TARGET_HEIGHT_FROM_DEM && heightMode != GVOT_MIN_TARGET_HEIGHT_FROM_GROUND)
        heightMode = GVOT_NORMAL;

    /* set up geotransformation */
    std::array<double, 6> adfGeoTransform {{0.0, 1.0, 0.0, 0.0, 0.0, 1.0}};
    GDALDatasetH hSrcDS = GDALGetBandDataset( hBand );
    if( hSrcDS != nullptr )
        GDALGetGeoTransform( hSrcDS, adfGeoTransform.data());

    double adfInvGeoTransform[6];
    if (!GDALInvGeoTransform(adfGeoTransform.data(), adfInvGeoTransform))
    {
        CPLError(CE_Failure, CPLE_AppDefined, "Cannot invert geotransform");
        return nullptr;
    }

    /* calculate observer position */
    double dfX, dfY;
    GDALApplyGeoTransform(adfInvGeoTransform, dfObserverX, dfObserverY, &dfX, &dfY);
    int nX = static_cast<int>(dfX);
    int nY = static_cast<int>(dfY);

    int nXSize = GDALGetRasterBandXSize( hBand );
    int nYSize = GDALGetRasterBandYSize( hBand );

    if (nX < 0 ||
        nX > nXSize ||
        nY < 0 ||
        nY > nYSize)
    {
        CPLError(CE_Failure, CPLE_AppDefined, "The observer location falls outside of the DEM area");
        return nullptr;
    }

    /* calculate the area of interest */
    int nXStart = dfMaxDistance > 0? (std::max)(0, static_cast<int>(std::floor(nX - adfInvGeoTransform[1] * dfMaxDistance))) : 0;
    int nXStop = dfMaxDistance > 0? (std::min)(nXSize, static_cast<int>(std::ceil(nX + adfInvGeoTransform[1] * dfMaxDistance) + 1)) : nXSize;
    int nYStart = dfMaxDistance > 0? (std::max)(0, static_cast<int>(std::floor(nY + adfInvGeoTransform[5] * dfMaxDistance))) : 0;
    int nYStop = dfMaxDistance > 0? (std::min)(nYSize, static_cast<int>(std::ceil(nY - adfInvGeoTransform[5] * dfMaxDistance) + 1)) : nYSize;

    /* normalize horizontal index (0 - nXSize) */
    nXSize = nXStop - nXStart;
    nX -= nXStart;

    nYSize = nYStop - nYStart;

    if (nXSize == 0 || nYSize == 0)
    {
        CPLError(CE_Failure, CPLE_AppDefined, "Invalid target raster size");
        return nullptr;
    }

    std::vector<double> vFirstLineVal;
    std::vector<double> vLastLineVal;
    std::vector<double> vThisLineVal;
    std::vector<GByte> vResult;
    std::vector<double> vHeightResult;

    try
    {
        vFirstLineVal.resize(nXSize);
        vLastLineVal.resize(nXSize);
        vThisLineVal.resize(nXSize);
        vResult.resize(nXSize);

        if(heightMode != GVOT_NORMAL)
            vHeightResult.resize(nXSize);
    } catch (...)
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Cannot allocate vectors for viewshed");
        return nullptr;
    }

    double *padfFirstLineVal = vFirstLineVal.data();
    double *padfLastLineVal = vLastLineVal.data();
    double *padfThisLineVal = vThisLineVal.data();
    GByte *pabyResult = vResult.data();
    double *dfHeightResult = vHeightResult.data();

    GDALDriverManager *hMgr = GetGDALDriverManager();
    GDALDriver *hDriver = hMgr->GetDriverByName(pszDriverName ? pszDriverName : "GTiff");
    if (!hDriver)
    {
        CPLError(CE_Failure, CPLE_AppDefined, "Cannot get driver");
        return nullptr;
    }

    /* create output raster */
    auto poDstDS = std::unique_ptr<GDALDataset>(hDriver->Create(pszTargetRasterName, nXSize, nYStop - nYStart, 1, heightMode != GVOT_NORMAL ? GDT_Float64 : GDT_Byte,
                                                const_cast<char**>(papszCreationOptions)));
    if (!poDstDS)
    {
        CPLError(CE_Failure, CPLE_AppDefined,
            "Cannot create dataset for %s", pszTargetRasterName);
        return nullptr;
    }
    /* copy srs */
    if (hSrcDS)
        poDstDS->SetSpatialRef(GDALDataset::FromHandle(hSrcDS)->GetSpatialRef());

    std::array<double, 6> adfDstGeoTransform;
    adfDstGeoTransform[0] = adfGeoTransform[0] + adfGeoTransform[1] * nXStart + adfGeoTransform[2] * nYStart;
    adfDstGeoTransform[1] = adfGeoTransform[1];
    adfDstGeoTransform[2] = adfGeoTransform[2];
    adfDstGeoTransform[3] = adfGeoTransform[3] + adfGeoTransform[4] * nXStart + adfGeoTransform[5] * nYStart;
    adfDstGeoTransform[4] = adfGeoTransform[4];
    adfDstGeoTransform[5] = adfGeoTransform[5];
    poDstDS->SetGeoTransform(adfDstGeoTransform.data());

    auto hTargetBand = poDstDS->GetRasterBand(1);
    if (hTargetBand == nullptr)
    {
        CPLError(CE_Failure, CPLE_AppDefined,
            "Cannot get band for %s", pszTargetRasterName);
        return nullptr;
    }

    if (dfNoDataVal >= 0)
        GDALSetRasterNoDataValue(hTargetBand, heightMode != GVOT_NORMAL ? dfNoDataVal : byNoDataVal);

    /* process first line */
    if (GDALRasterIO(hBand, GF_Read, nXStart, nY, nXSize, 1,
        padfFirstLineVal, nXSize, 1, GDT_Float64, 0, 0))
    {
        CPLError(CE_Failure, CPLE_AppDefined,
            "RasterIO error when reading DEM at position(%d, %d), size(%d, %d)", nXStart, nY, nXSize, 1);
        return nullptr;
    }

    const double dfZObserver = dfObserverHeight + padfFirstLineVal[nX];
    double dfZ = 0.0;
    const double dfDistance2 = dfMaxDistance * dfMaxDistance;

    /* If we can't get a SemiMajor axis from the SRS, it will be
     * SRS_WGS84_SEMIMAJOR
    */
    double dfSphereDiameter(std::numeric_limits<double>::infinity());
    const OGRSpatialReference* poDstSRS = poDstDS->GetSpatialRef();
    if (poDstSRS)
    {
        OGRErr eSRSerr;
        double dfSemiMajor = poDstSRS->GetSemiMajor(&eSRSerr);

        /* If we fetched the axis from the SRS, use it */
        if (eSRSerr != OGRERR_FAILURE)
            dfSphereDiameter = dfSemiMajor * 2.0;
        else
            CPLDebug( "GDALViewshedGenerate", "Unable to fetch SemiMajor axis from spatial reference");

    }

    /* mark the observer point as visible */
    double dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfFirstLineVal[nX] : 0.0;
    pabyResult[nX] = byVisibleVal;
    if(heightMode != GVOT_NORMAL)
        dfHeightResult[nX] = dfGroundLevel;

    if (nX > 0)
    {
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfFirstLineVal[nX - 1] : 0.0;
        CPL_IGNORE_RET_VAL(
            AdjustHeightInRange(adfGeoTransform.data(),
                            1,
                            0,
                            padfFirstLineVal[nX - 1],
                            dfDistance2,
                            dfCurvCoeff,
                            dfSphereDiameter));
        pabyResult[nX - 1] = byVisibleVal;
        if(heightMode != GVOT_NORMAL)
            dfHeightResult[nX - 1] = dfGroundLevel;
    }
    if (nX < nXSize - 1)
    {
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfFirstLineVal[nX + 1] : 0.0;
        CPL_IGNORE_RET_VAL(
            AdjustHeightInRange(adfGeoTransform.data(),
                            1,
                            0,
                            padfFirstLineVal[nX + 1],
                            dfDistance2,
                            dfCurvCoeff,
                            dfSphereDiameter));
        pabyResult[nX + 1] = byVisibleVal;
        if(heightMode != GVOT_NORMAL)
            dfHeightResult[nX + 1] = dfGroundLevel;
    }

    /* process left direction */
    for (int iPixel = nX - 2; iPixel >= 0; iPixel--)
    {
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfFirstLineVal[iPixel] : 0.0;
        bool adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                            nX - iPixel,
                                            0,
                                            padfFirstLineVal[iPixel],
                                            dfDistance2,
                                            dfCurvCoeff,
                                            dfSphereDiameter);
        if (adjusted)
        {
            dfZ = CalcHeightLine(nX - iPixel,
                                 padfFirstLineVal[iPixel + 1],
                                 dfZObserver);

            if(heightMode != GVOT_NORMAL)
                dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfFirstLineVal[iPixel] + dfGroundLevel));

            SetVisibility(  iPixel,
                            dfZ,
                            dfTargetHeight,
                            padfFirstLineVal,
                            vResult,
                            byVisibleVal,
                            byInvisibleVal);
        }
        else
        {
            for (; iPixel >= 0; iPixel--)
            {
                pabyResult[iPixel] = byOutOfRangeVal;
                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = dfOutOfRangeVal;
            }
        }
    }
    /* process right direction */
    for (int iPixel = nX + 2; iPixel < nXSize; iPixel++)
    {
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfFirstLineVal[iPixel] : 0.0;
        bool adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                            iPixel - nX,
                                            0,
                                            padfFirstLineVal[iPixel],
                                            dfDistance2,
                                            dfCurvCoeff,
                                            dfSphereDiameter);
        if (adjusted)
        {
            dfZ = CalcHeightLine(iPixel - nX,
                                 padfFirstLineVal[iPixel - 1],
                                 dfZObserver);

            if(heightMode != GVOT_NORMAL)
                dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfFirstLineVal[iPixel] + dfGroundLevel));

            SetVisibility(iPixel,
                          dfZ,
                          dfTargetHeight,
                          padfFirstLineVal,
                          vResult,
                          byVisibleVal,
                          byInvisibleVal);
        }
        else
        {
            for (; iPixel < nXSize; iPixel++)
            {
                pabyResult[iPixel] = byOutOfRangeVal;
                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = dfOutOfRangeVal;
            }
        }
    }
    /* write result line */

    if (GDALRasterIO(hTargetBand, GF_Write, 0, nY - nYStart, nXSize, 1,
        heightMode != GVOT_NORMAL ? static_cast<void*>(dfHeightResult) : static_cast<void*>(pabyResult), nXSize, 1, heightMode != GVOT_NORMAL ? GDT_Float64 : GDT_Byte, 0, 0))
    {
        CPLError(CE_Failure, CPLE_AppDefined,
            "RasterIO error when writing target raster at position (%d,%d), size (%d,%d)", 0, nY - nYStart, nXSize, 1);
        return nullptr;
    }

    /* scan upwards */
    std::copy(vFirstLineVal.begin(),
              vFirstLineVal.end(),
              vLastLineVal.begin());
    for (int iLine = nY - 1; iLine >= nYStart; iLine--)
    {
        if (GDALRasterIO(hBand, GF_Read, nXStart, iLine, nXSize, 1,
            padfThisLineVal, nXSize, 1, GDT_Float64, 0, 0))
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                "RasterIO error when reading DEM at position (%d,%d), size (%d,%d)", nXStart, iLine, nXSize, 1);
            return nullptr;
        }

        /* set up initial point on the scanline */
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[nX] : 0.0;
        bool adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                            0,
                                            nY - iLine,
                                            padfThisLineVal[nX],
                                            dfDistance2,
                                            dfCurvCoeff,
                                            dfSphereDiameter);
        if (adjusted)
        {
            dfZ = CalcHeightLine(nY - iLine,
                                 padfLastLineVal[nX],
                                 dfZObserver);

            if(heightMode != GVOT_NORMAL)
                dfHeightResult[nX] = std::max(0.0, (dfZ - padfThisLineVal[nX] + dfGroundLevel));

            SetVisibility(nX,
                          dfZ,
                          dfTargetHeight,
                          padfThisLineVal,
                          vResult,
                          byVisibleVal,
                          byInvisibleVal);
        }
        else
        {
            pabyResult[nX] = byOutOfRangeVal;
            if(heightMode != GVOT_NORMAL)
                dfHeightResult[nX] = dfOutOfRangeVal;
        }

        /* process left direction */
        for (int iPixel = nX - 1; iPixel >= 0; iPixel--)
        {
            dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[iPixel] : 0.0;
            bool left_adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                                     nX - iPixel,
                                                     nY - iLine,
                                                     padfThisLineVal[iPixel],
                                                     dfDistance2,
                                                     dfCurvCoeff,
                                                     dfSphereDiameter);
            if (left_adjusted)
            {
                if (eMode != GVM_Edge)
                    dfZ = CalcHeightDiagonal(nX - iPixel,
                                             nY - iLine,
                                             padfThisLineVal[iPixel + 1],
                                             padfLastLineVal[iPixel],
                                             dfZObserver);

                if (eMode != GVM_Diagonal)
                {
                    double dfZ2 = nX - iPixel >= nY - iLine ?
                        CalcHeightEdge(nY - iLine,
                                       nX - iPixel,
                                       padfLastLineVal[iPixel + 1],
                                       padfThisLineVal[iPixel + 1],
                                       dfZObserver) :
                        CalcHeightEdge(nX - iPixel,
                                       nY - iLine,
                                       padfLastLineVal[iPixel + 1],
                                       padfLastLineVal[iPixel],
                                       dfZObserver);
                    dfZ = CalcHeight(dfZ, dfZ2, eMode);
                }

                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfThisLineVal[iPixel] + dfGroundLevel));

                SetVisibility(iPixel,
                              dfZ,
                              dfTargetHeight,
                              padfThisLineVal,
                              vResult,
                              byVisibleVal,
                              byInvisibleVal);
            }
            else
            {
                for (; iPixel >= 0; iPixel--)
                {
                    pabyResult[iPixel] = byOutOfRangeVal;
                    if(heightMode != GVOT_NORMAL)
                        dfHeightResult[iPixel] = dfOutOfRangeVal;
                }
            }
        }
        /* process right direction */
        for (int iPixel = nX + 1; iPixel < nXSize; iPixel++)
        {
            dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[iPixel] : 0.0;
            bool right_adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                                      iPixel - nX,
                                                      nY - iLine,
                                                      padfThisLineVal[iPixel],
                                                      dfDistance2,
                                                      dfCurvCoeff,
                                                      dfSphereDiameter);
            if (right_adjusted)
            {
                if (eMode != GVM_Edge)
                    dfZ = CalcHeightDiagonal(iPixel - nX,
                                             nY - iLine,
                                             padfThisLineVal[iPixel - 1],
                                             padfLastLineVal[iPixel],
                                             dfZObserver);

                if (eMode != GVM_Diagonal)
                {
                    double dfZ2 = iPixel - nX >= nY - iLine ?
                        CalcHeightEdge(nY - iLine,
                                       iPixel - nX,
                                       padfLastLineVal[iPixel - 1],
                                       padfThisLineVal[iPixel - 1],
                                       dfZObserver) :
                        CalcHeightEdge(iPixel - nX,
                                       nY - iLine,
                                       padfLastLineVal[iPixel - 1],
                                       padfLastLineVal[iPixel],
                                       dfZObserver);
                    dfZ = CalcHeight(dfZ, dfZ2, eMode);
                }

                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfThisLineVal[iPixel] + dfGroundLevel));

                SetVisibility(iPixel,
                              dfZ,
                              dfTargetHeight,
                              padfThisLineVal,
                              vResult,
                              byVisibleVal,
                              byInvisibleVal);
            }
            else
            {
                for (; iPixel < nXSize; iPixel++)
                {
                    pabyResult[iPixel] = byOutOfRangeVal;
                    if(heightMode != GVOT_NORMAL)
                        dfHeightResult[iPixel] = dfOutOfRangeVal;
                }
            }
        }

        /* write result line */
        if (GDALRasterIO(hTargetBand, GF_Write, 0, iLine - nYStart, nXSize, 1,
            heightMode != GVOT_NORMAL ? static_cast<void*>(dfHeightResult) : static_cast<void*>(pabyResult), nXSize, 1, heightMode != GVOT_NORMAL ? GDT_Float64 : GDT_Byte, 0, 0))
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                "RasterIO error when writing target raster at position (%d,%d), size (%d,%d)", 0, iLine - nYStart, nXSize, 1);
            return nullptr;
        }

        std::swap(padfLastLineVal, padfThisLineVal);

        if (!pfnProgress((nY - iLine) / static_cast<double>(nYSize),
                "", pProgressArg))
        {
            CPLError(CE_Failure, CPLE_UserInterrupt, "User terminated");
            return nullptr;
        }
    }
    /* scan downwards */
    memcpy(padfLastLineVal, padfFirstLineVal, nXSize * sizeof(double));
    for(int iLine = nY + 1; iLine < nYStop; iLine++ )
    {
        if (GDALRasterIO( hBand, GF_Read, nXStart, iLine, nXStop - nXStart, 1,
            padfThisLineVal, nXStop - nXStart, 1, GDT_Float64, 0, 0 ))
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                "RasterIO error when reading DEM at position (%d,%d), size (%d,%d)", nXStart, iLine, nXStop - nXStart, 1);
            return nullptr;
        }

        /* set up initial point on the scanline */
        dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[nX] : 0.0;
        bool adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                            0,
                                            iLine - nY,
                                            padfThisLineVal[nX],
                                            dfDistance2,
                                            dfCurvCoeff,
                                            dfSphereDiameter);
        if (adjusted)
        {
            dfZ = CalcHeightLine(iLine - nY,
                                 padfLastLineVal[nX],
                                 dfZObserver);

            if(heightMode != GVOT_NORMAL)
                dfHeightResult[nX] = std::max(0.0, (dfZ - padfThisLineVal[nX] + dfGroundLevel));

            SetVisibility(nX,
                          dfZ,
                          dfTargetHeight,
                          padfThisLineVal,
                          vResult,
                          byVisibleVal,
                          byInvisibleVal);
        }
        else
        {
            pabyResult[nX] = byOutOfRangeVal;
            if(heightMode != GVOT_NORMAL)
                dfHeightResult[nX] = dfOutOfRangeVal;
        }

        /* process left direction */
        for (int iPixel = nX - 1; iPixel >= 0; iPixel--)
        {
            dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[iPixel] : 0.0;
            bool left_adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                                     nX - iPixel,
                                                     iLine - nY,
                                                     padfThisLineVal[iPixel],
                                                     dfDistance2,
                                                     dfCurvCoeff,
                                                     dfSphereDiameter);
            if (left_adjusted)
            {
                if (eMode != GVM_Edge)
                    dfZ = CalcHeightDiagonal(nX - iPixel, iLine - nY,
                        padfThisLineVal[iPixel + 1], padfLastLineVal[iPixel], dfZObserver);

                if (eMode != GVM_Diagonal)
                {
                    double dfZ2 = nX - iPixel >= iLine - nY ?
                        CalcHeightEdge(iLine - nY,
                                       nX - iPixel,
                                       padfLastLineVal[iPixel + 1],
                                       padfThisLineVal[iPixel + 1],
                                       dfZObserver) :
                        CalcHeightEdge(nX - iPixel,
                                       iLine - nY,
                                       padfLastLineVal[iPixel + 1],
                                       padfLastLineVal[iPixel],
                                       dfZObserver);
                    dfZ = CalcHeight(dfZ, dfZ2, eMode);
                }

                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfThisLineVal[iPixel] + dfGroundLevel));

                SetVisibility(iPixel,
                              dfZ,
                              dfTargetHeight,
                              padfThisLineVal,
                              vResult,
                              byVisibleVal,
                              byInvisibleVal);
            }
            else
            {
                for (; iPixel >= 0; iPixel--)
                {
                    pabyResult[iPixel] = byOutOfRangeVal;
                    if(heightMode != GVOT_NORMAL)
                        dfHeightResult[iPixel] = dfOutOfRangeVal;
                }
            }
        }
        /* process right direction */
        for (int iPixel = nX + 1; iPixel < nXSize; iPixel++)
        {
            dfGroundLevel = heightMode == GVOT_MIN_TARGET_HEIGHT_FROM_DEM ? padfThisLineVal[iPixel] : 0.0;
            bool right_adjusted = AdjustHeightInRange(adfGeoTransform.data(),
                                                      iPixel - nX,
                                                      iLine - nY,
                                                      padfThisLineVal[iPixel],
                                                      dfDistance2,
                                                      dfCurvCoeff,
                                                      dfSphereDiameter);
            if (right_adjusted)
            {
                if (eMode != GVM_Edge)
                    dfZ = CalcHeightDiagonal(iPixel - nX,
                                             iLine - nY,
                                             padfThisLineVal[iPixel - 1],
                                             padfLastLineVal[iPixel],
                                             dfZObserver);

                if (eMode != GVM_Diagonal)
                {
                    double dfZ2 = iPixel - nX >= iLine - nY ?
                        CalcHeightEdge(iLine - nY,
                                       iPixel - nX,
                                       padfLastLineVal[iPixel - 1],
                                       padfThisLineVal[iPixel - 1],
                                       dfZObserver) :
                        CalcHeightEdge(iPixel - nX,
                                       iLine - nY,
                                       padfLastLineVal[iPixel - 1],
                                       padfLastLineVal[iPixel],
                                       dfZObserver);
                    dfZ = CalcHeight(dfZ, dfZ2, eMode);
                }

                if(heightMode != GVOT_NORMAL)
                    dfHeightResult[iPixel] = std::max(0.0, (dfZ - padfThisLineVal[iPixel] + dfGroundLevel));

                SetVisibility(iPixel,
                              dfZ,
                              dfTargetHeight,
                              padfThisLineVal,
                              vResult,
                              byVisibleVal,
                              byInvisibleVal);
            }
            else
            {
                for (; iPixel < nXSize; iPixel++)
                {
                    pabyResult[iPixel] = byOutOfRangeVal;
                    if(heightMode != GVOT_NORMAL)
                        dfHeightResult[iPixel] = dfOutOfRangeVal;
                }
            }
        }

        /* write result line */
        if (GDALRasterIO(hTargetBand, GF_Write, 0, iLine - nYStart, nXSize, 1,
            heightMode != GVOT_NORMAL ? static_cast<void*>(dfHeightResult) : static_cast<void*>(pabyResult), nXSize, 1, heightMode != GVOT_NORMAL ? GDT_Float64 : GDT_Byte, 0, 0))
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                "RasterIO error when writing target raster at position (%d,%d), size (%d,%d)", 0, iLine - nYStart, nXSize, 1);
            return nullptr;
        }

        std::swap(padfLastLineVal, padfThisLineVal);

        if (!pfnProgress((iLine - nYStart) / static_cast<double>(nYSize),
            "", pProgressArg))
        {
            CPLError( CE_Failure, CPLE_UserInterrupt, "User terminated" );
            return nullptr;
        }
    }

    if (!pfnProgress(1.0, "", pProgressArg))
    {
        CPLError(CE_Failure, CPLE_UserInterrupt, "User terminated");
        return nullptr;
    }

    return GDALDataset::FromHandle(poDstDS.release());
}
