raster/rt_core/rt_warp.c

/*
 *
 * WKTRaster - Raster Types for PostGIS
 * http://trac.osgeo.org/postgis/wiki/WKTRaster
 *
 * Copyright (C) 2013 Bborie Park <dustymugs@gmail.com>
 * Copyright (C) 2011-2013 Regents of the University of California
 *   <bkpark@ucdavis.edu>
 * Copyright (C) 2010-2011 Jorge Arevalo <jorge.arevalo@deimos-space.com>
 * Copyright (C) 2010-2011 David Zwarg <dzwarg@azavea.com>
 * Copyright (C) 2009-2011 Pierre Racine <pierre.racine@sbf.ulaval.ca>
 * Copyright (C) 2009-2011 Mateusz Loskot <mateusz@loskot.net>
 * Copyright (C) 2008-2009 Sandro Santilli <strk@kbt.io>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 */

#include "../../postgis_config.h"
/* #define POSTGIS_DEBUG_LEVEL 4 */

#include "librtcore.h"
#include "librtcore_internal.h"

/******************************************************************************
* rt_raster_gdal_warp()
******************************************************************************/

typedef struct _rti_warp_arg_t* _rti_warp_arg;
struct _rti_warp_arg_t {

	struct {
		GDALDriverH drv;
		GDALDatasetH ds;
		char *srs;
		int destroy_drv;
	} src, dst;

	GDALWarpOptions *wopts;

	struct {
		struct {
			char **item;
			int len;
		} option;

		struct {
			void *transform;
			void *imgproj;
			void *approx;
		} arg;

		GDALTransformerFunc func;
	} transform;

};

static _rti_warp_arg
_rti_warp_arg_init() {
	_rti_warp_arg arg = NULL;

	arg = rtalloc(sizeof(struct _rti_warp_arg_t));
	if (arg == NULL) {
		rterror("_rti_warp_arg_init: Could not allocate memory for _rti_warp_arg");
		return NULL;
	}

	arg->src.drv = NULL;
	arg->src.destroy_drv = 0;
	arg->src.ds = NULL;
	arg->src.srs = NULL;

	arg->dst.drv = NULL;
	arg->dst.destroy_drv = 0;
	arg->dst.ds = NULL;
	arg->dst.srs = NULL;

	arg->wopts = NULL;

	arg->transform.option.item = NULL;
	arg->transform.option.len = 0;

	arg->transform.arg.transform = NULL;
	arg->transform.arg.imgproj = NULL;
	arg->transform.arg.approx = NULL;

	arg->transform.func = NULL;

	return arg;
}

static void
_rti_warp_arg_destroy(_rti_warp_arg arg) {
	int i = 0;

	if (arg->dst.ds != NULL)
		GDALClose(arg->dst.ds);
	if (arg->dst.srs != NULL)
		CPLFree(arg->dst.srs);

	if (arg->dst.drv != NULL && arg->dst.destroy_drv) {
		GDALDeregisterDriver(arg->dst.drv);
		GDALDestroyDriver(arg->dst.drv);
	}

	if (arg->src.ds != NULL)
		GDALClose(arg->src.ds);
	if (arg->src.srs != NULL)
		CPLFree(arg->src.srs);

	if (arg->src.drv != NULL && arg->src.destroy_drv) {
		GDALDeregisterDriver(arg->src.drv);
		GDALDestroyDriver(arg->src.drv);
	}

	if (arg->transform.func == GDALApproxTransform) {
		if (arg->transform.arg.imgproj != NULL)
			GDALDestroyGenImgProjTransformer(arg->transform.arg.imgproj);
	}

	if (arg->wopts != NULL)
		GDALDestroyWarpOptions(arg->wopts);

	if (arg->transform.option.len > 0 && arg->transform.option.item != NULL) {
		for (i = 0; i < arg->transform.option.len; i++) {
			if (arg->transform.option.item[i] != NULL)
				rtdealloc(arg->transform.option.item[i]);
		}
		rtdealloc(arg->transform.option.item);
	}

	rtdealloc(arg);
	arg = NULL;
}

/**
 * Return a warped raster using GDAL Warp API
 *
 * @param raster : raster to transform
 * @param src_srs : the raster's coordinate system in OGC WKT
 * @param dst_srs : the warped raster's coordinate system in OGC WKT
 * @param scale_x : the x size of pixels of the warped raster's pixels in
 *   units of dst_srs
 * @param scale_y : the y size of pixels of the warped raster's pixels in
 *   units of dst_srs
 * @param width : the number of columns of the warped raster.  note that
 *   width/height CANNOT be used with scale_x/scale_y
 * @param height : the number of rows of the warped raster.  note that
 *   width/height CANNOT be used with scale_x/scale_y
 * @param ul_xw : the X value of upper-left corner of the warped raster in
 *   units of dst_srs
 * @param ul_yw : the Y value of upper-left corner of the warped raster in
 *   units of dst_srs
 * @param grid_xw : the X value of point on a grid to align warped raster
 *   to in units of dst_srs
 * @param grid_yw : the Y value of point on a grid to align warped raster
 *   to in units of dst_srs
 * @param skew_x : the X skew of the warped raster in units of dst_srs
 * @param skew_y : the Y skew of the warped raster in units of dst_srs
 * @param resample_alg : the resampling algorithm
 * @param max_err : maximum error measured in input pixels permitted
 *   (0.0 for exact calculations)
 *
 * @return the warped raster or NULL
 */
rt_raster rt_raster_gdal_warp(
	rt_raster raster,
	const char *src_srs, const char *dst_srs,
	double *scale_x, double *scale_y,
	int *width, int *height,
	double *ul_xw, double *ul_yw,
	double *grid_xw, double *grid_yw,
	double *skew_x, double *skew_y,
	GDALResampleAlg resample_alg, double max_err
) {
	CPLErr cplerr;
	char *dst_options[] = {"SUBCLASS=VRTWarpedDataset", NULL};
	_rti_warp_arg arg = NULL;

	int hasnodata = 0;

	GDALRasterBandH band;
	rt_band rtband = NULL;
	rt_pixtype pt = PT_END;
	GDALDataType gdal_pt = GDT_Unknown;
	double nodata = 0.0;

	double _gt[6] = {0};
	double dst_extent[4];
	rt_envelope extent;

	int _dim[2] = {0};
	double _skew[2] = {0};
	double _scale[2] = {0};
	int ul_user = 0;

	rt_raster rast = NULL;
	int i = 0;
	int numBands = 0;

	/* flag indicating that the spatial info is being substituted */
	int subspatial = 0;

	RASTER_DEBUG(3, "starting");

	assert(NULL != raster);

	/* internal variables */
	arg = _rti_warp_arg_init();
	if (arg == NULL) {
		rterror("rt_raster_gdal_warp: Could not initialize internal variables");
		return NULL;
	}

	/*
		max_err must be gte zero

		the value 0.125 is the default used in gdalwarp.cpp on line 283
	*/
	if (max_err < 0.) max_err = 0.125;
	RASTER_DEBUGF(4, "max_err = %f", max_err);

	/* handle srs */
	if (src_srs != NULL) {
		/* reprojection taking place */
		if (dst_srs != NULL && strcmp(src_srs, dst_srs) != 0) {
			RASTER_DEBUG(4, "Warp operation does include a reprojection");
			arg->src.srs = rt_util_gdal_convert_sr(src_srs, 0);
			arg->dst.srs = rt_util_gdal_convert_sr(dst_srs, 0);

			if (arg->src.srs == NULL || arg->dst.srs == NULL) {
				rterror("rt_raster_gdal_warp: Could not convert srs values to GDAL accepted format");
				_rti_warp_arg_destroy(arg);
				return NULL;
			}
		}
		/* no reprojection, a stub just for clarity */
		else {
			RASTER_DEBUG(4, "Warp operation does NOT include reprojection");
		}
	}
	else if (dst_srs != NULL) {
		/* dst_srs provided but not src_srs */
		rterror("rt_raster_gdal_warp: SRS required for input raster if SRS provided for warped raster");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* load raster into a GDAL MEM dataset */
	arg->src.ds = rt_raster_to_gdal_mem(raster, arg->src.srs, NULL, NULL, 0, &(arg->src.drv), &(arg->src.destroy_drv));
	if (NULL == arg->src.ds) {
		rterror("rt_raster_gdal_warp: Could not convert raster to GDAL MEM format");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}
	RASTER_DEBUG(3, "raster loaded into GDAL MEM dataset");

	/* special case when src_srs and dst_srs is NULL and raster's geotransform matrix is default */
	if (
		src_srs == NULL && dst_srs == NULL &&
		rt_raster_get_srid(raster) == SRID_UNKNOWN
	) {
		double gt[6];

#if POSTGIS_DEBUG_LEVEL > 3
		GDALGetGeoTransform(arg->src.ds, gt);
		RASTER_DEBUGF(3, "GDAL MEM geotransform: %f, %f, %f, %f, %f, %f",
			gt[0], gt[1], gt[2], gt[3], gt[4], gt[5]);
#endif

		/* default geotransform */
		rt_raster_get_geotransform_matrix(raster, gt);
		RASTER_DEBUGF(3, "raster geotransform: %f, %f, %f, %f, %f, %f",
			gt[0], gt[1], gt[2], gt[3], gt[4], gt[5]);

		/* substitute spatial info (lack of) with a real one EPSG:32731 (WGS84/UTM zone 31s) */
		if (FLT_EQ(gt[0], 0.0) && FLT_EQ(gt[3], 0.0) && FLT_EQ(gt[1], 1.0) && FLT_EQ(gt[5], -1.0) &&
		    FLT_EQ(gt[2], 0.0) && FLT_EQ(gt[4], 0.0))
		{
			double ngt[6] = {166021.4431, 0.1, 0, 10000000.0000, 0, -0.1};

			rtwarn("Raster has default geotransform. Adjusting metadata for use of GDAL Warp API");

			subspatial = 1;

			GDALSetGeoTransform(arg->src.ds, ngt);
			GDALFlushCache(arg->src.ds);

			/* EPSG:32731 */
			arg->src.srs = rt_util_gdal_convert_sr("EPSG:32731", 0);
			arg->dst.srs = rt_util_gdal_convert_sr("EPSG:32731", 0);

#if POSTGIS_DEBUG_LEVEL > 3
			GDALGetGeoTransform(arg->src.ds, gt);
			RASTER_DEBUGF(3, "GDAL MEM geotransform: %f, %f, %f, %f, %f, %f",
				gt[0], gt[1], gt[2], gt[3], gt[4], gt[5]);
#endif
		}
	}

	/* set transform options */
	if (arg->src.srs != NULL || arg->dst.srs != NULL) {
		arg->transform.option.len = 2;
		arg->transform.option.item = rtalloc(sizeof(char *) * (arg->transform.option.len + 1));
		if (NULL == arg->transform.option.item) {
			rterror("rt_raster_gdal_warp: Could not allocation memory for transform options");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}
		memset(arg->transform.option.item, 0, sizeof(char *) * (arg->transform.option.len + 1));

		for (i = 0; i < arg->transform.option.len; i++) {
			const char *srs = i ? arg->dst.srs : arg->src.srs;
			const char *lbl = i ? "DST_SRS=" : "SRC_SRS=";
			size_t sz = sizeof(char) * (strlen(lbl) + 1);
			if ( srs ) sz += strlen(srs);
			arg->transform.option.item[i] = (char *) rtalloc(sz);
			if (NULL == arg->transform.option.item[i]) {
				rterror("rt_raster_gdal_warp: Could not allocation memory for transform options");
				_rti_warp_arg_destroy(arg);
				return NULL;
			}
			sprintf(arg->transform.option.item[i], "%s%s", lbl, srs ? srs : "");
			RASTER_DEBUGF(4, "arg->transform.option.item[%d] = %s", i, arg->transform.option.item[i]);
		}
	}
	else
		arg->transform.option.len = 0;

	/* transformation object for building dst dataset */
	arg->transform.arg.transform = GDALCreateGenImgProjTransformer2(arg->src.ds, NULL, arg->transform.option.item);
	if (NULL == arg->transform.arg.transform) {
		rterror("rt_raster_gdal_warp: Could not create GDAL transformation object for output dataset creation");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* get approximate output georeferenced bounds and resolution */
	cplerr = GDALSuggestedWarpOutput2(
		arg->src.ds, GDALGenImgProjTransform,
		arg->transform.arg.transform, _gt, &(_dim[0]), &(_dim[1]), dst_extent, 0);
	if (cplerr != CE_None) {
		rterror("rt_raster_gdal_warp: Could not get GDAL suggested warp output for output dataset creation");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}
	GDALDestroyGenImgProjTransformer(arg->transform.arg.transform);
	arg->transform.arg.transform = NULL;

	/*
		don't use suggested dimensions as use of suggested scales
		on suggested extent will result in suggested dimensions
	*/
	_dim[0] = 0;
	_dim[1] = 0;

	RASTER_DEBUGF(3, "Suggested geotransform: %f, %f, %f, %f, %f, %f",
		_gt[0], _gt[1], _gt[2], _gt[3], _gt[4], _gt[5]);

	/* store extent in easier-to-use object */
	extent.MinX = dst_extent[0];
	extent.MinY = dst_extent[1];
	extent.MaxX = dst_extent[2];
	extent.MaxY = dst_extent[3];

	extent.UpperLeftX = dst_extent[0];
	extent.UpperLeftY = dst_extent[3];

	RASTER_DEBUGF(3, "Suggested extent: %f, %f, %f, %f",
		extent.MinX, extent.MinY, extent.MaxX, extent.MaxY);

	/* scale and width/height are mutually exclusive */
	if (
		((NULL != scale_x) || (NULL != scale_y)) &&
		((NULL != width) || (NULL != height))
	) {
		rterror("rt_raster_gdal_warp: Scale X/Y and width/height are mutually exclusive.  Only provide one");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* user-defined width */
	if (NULL != width) {
		_dim[0] = abs(*width);
		_scale[0] = fabs((extent.MaxX - extent.MinX) / ((double) _dim[0]));
	}
	/* user-defined height */
	if (NULL != height) {
		_dim[1] = abs(*height);
		_scale[1] = fabs((extent.MaxY - extent.MinY) / ((double) _dim[1]));
	}

	/* user-defined scale */
	if (
		((NULL != scale_x) && (FLT_NEQ(*scale_x, 0.0))) &&
		((NULL != scale_y) && (FLT_NEQ(*scale_y, 0.0)))
	) {
		_scale[0] = fabs(*scale_x);
		_scale[1] = fabs(*scale_y);

		/* special override since we changed the original GT scales */
		if (subspatial) {
			/*
			_scale[0] *= 10;
			_scale[1] *= 10;
			*/
			_scale[0] /= 10;
			_scale[1] /= 10;
		}
	}
	else if (
		((NULL != scale_x) && (NULL == scale_y)) ||
		((NULL == scale_x) && (NULL != scale_y))
	) {
		rterror("rt_raster_gdal_warp: Both X and Y scale values must be provided for scale");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* scale not defined, use suggested */
	if (FLT_EQ(_scale[0], 0.0) && FLT_EQ(_scale[1], 0.0))
	{
		_scale[0] = fabs(_gt[1]);
		_scale[1] = fabs(_gt[5]);
	}

	RASTER_DEBUGF(4, "Using scale: %f x %f", _scale[0], -1 * _scale[1]);

	/* user-defined skew */
	if (NULL != skew_x) {
		_skew[0] = *skew_x;

		/*
			negative scale-x affects skew
			for now, force skew to be in left-right, top-down orientation
		*/
		if (
			NULL != scale_x &&
			*scale_x < 0.
		) {
			_skew[0] *= -1;
		}
	}
	if (NULL != skew_y) {
		_skew[1] = *skew_y;

		/*
			positive scale-y affects skew
			for now, force skew to be in left-right, top-down orientation
		*/
		if (
			NULL != scale_y &&
			*scale_y > 0.
		) {
			_skew[1] *= -1;
		}
	}

	RASTER_DEBUGF(4, "Using skew: %f x %f", _skew[0], _skew[1]);

	/* reprocess extent if skewed */
	if (FLT_NEQ(_skew[0], 0.0) || FLT_NEQ(_skew[1], 0.0))
	{
		rt_raster skewedrast;

		RASTER_DEBUG(3, "Computing skewed extent's envelope");

		skewedrast = rt_raster_compute_skewed_raster(
			extent,
			_skew,
			_scale,
			0.01
		);
		if (skewedrast == NULL) {
			rterror("rt_raster_gdal_warp: Could not compute skewed raster");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		if (_dim[0] == 0)
			_dim[0] = skewedrast->width;
		if (_dim[1] == 0)
			_dim[1] = skewedrast->height;

		extent.UpperLeftX = skewedrast->ipX;
		extent.UpperLeftY = skewedrast->ipY;

		rt_raster_destroy(skewedrast);
	}

	/* dimensions not defined, compute */
	if (!_dim[0])
		_dim[0] = (int) fmax((fabs(extent.MaxX - extent.MinX) + (_scale[0] / 2.)) / _scale[0], 1);
	if (!_dim[1])
		_dim[1] = (int) fmax((fabs(extent.MaxY - extent.MinY) + (_scale[1] / 2.)) / _scale[1], 1);

	/* temporary raster */
	rast = rt_raster_new(_dim[0], _dim[1]);
	if (rast == NULL) {
		rterror("rt_raster_gdal_warp: Out of memory allocating temporary raster");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* set raster's spatial attributes */
	rt_raster_set_offsets(rast, extent.UpperLeftX, extent.UpperLeftY);
	rt_raster_set_scale(rast, _scale[0], -1 * _scale[1]);
	rt_raster_set_skews(rast, _skew[0], _skew[1]);

	rt_raster_get_geotransform_matrix(rast, _gt);
	RASTER_DEBUGF(3, "Temp raster's geotransform: %f, %f, %f, %f, %f, %f",
		_gt[0], _gt[1], _gt[2], _gt[3], _gt[4], _gt[5]);
	RASTER_DEBUGF(3, "Temp raster's dimensions (width x height): %d x %d",
		_dim[0], _dim[1]);

	/* user-defined upper-left corner */
	if (
		NULL != ul_xw &&
		NULL != ul_yw
	) {
		ul_user = 1;

		RASTER_DEBUGF(4, "Using user-specified upper-left corner: %f, %f", *ul_xw, *ul_yw);

		/* set upper-left corner */
		rt_raster_set_offsets(rast, *ul_xw, *ul_yw);
		extent.UpperLeftX = *ul_xw;
		extent.UpperLeftY = *ul_yw;
	}
	else if (
		((NULL != ul_xw) && (NULL == ul_yw)) ||
		((NULL == ul_xw) && (NULL != ul_yw))
	) {
		rterror("rt_raster_gdal_warp: Both X and Y upper-left corner values must be provided");
		rt_raster_destroy(rast);
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* alignment only considered if upper-left corner not provided */
	if (
		!ul_user && (
			(NULL != grid_xw) || (NULL != grid_yw)
		)
	) {

		if (
			((NULL != grid_xw) && (NULL == grid_yw)) ||
			((NULL == grid_xw) && (NULL != grid_yw))
		) {
			rterror("rt_raster_gdal_warp: Both X and Y alignment values must be provided");
			rt_raster_destroy(rast);
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		RASTER_DEBUGF(4, "Aligning extent to user-specified grid: %f, %f", *grid_xw, *grid_yw);

		do {
			double _r[2] = {0};
			double _w[2] = {0};

			/* raster is already aligned */
			if (FLT_EQ(*grid_xw, extent.UpperLeftX) && FLT_EQ(*grid_yw, extent.UpperLeftY)) {
				RASTER_DEBUG(3, "Skipping raster alignment as it is already aligned to grid");
				break;
			}

			extent.UpperLeftX = rast->ipX;
			extent.UpperLeftY = rast->ipY;
			rt_raster_set_offsets(rast, *grid_xw, *grid_yw);

			/* process upper-left corner */
			if (rt_raster_geopoint_to_cell(
				rast,
				extent.UpperLeftX, extent.UpperLeftY,
				&(_r[0]), &(_r[1]),
				NULL
			) != ES_NONE) {
				rterror("rt_raster_gdal_warp: Could not compute raster pixel for spatial coordinates");
				rt_raster_destroy(rast);
				_rti_warp_arg_destroy(arg);
				return NULL;
			}

			if (rt_raster_cell_to_geopoint(
				rast,
				_r[0], _r[1],
				&(_w[0]), &(_w[1]),
				NULL
			) != ES_NONE) {
				rterror("rt_raster_gdal_warp: Could not compute spatial coordinates for raster pixel");

				rt_raster_destroy(rast);
				_rti_warp_arg_destroy(arg);
				return NULL;
			}

			/* shift occurred */
			if (FLT_NEQ(_w[0], extent.UpperLeftX)) {
				if (NULL == width)
					rast->width++;
				else if (NULL == scale_x) {
					double _c[2] = {0};

					rt_raster_set_offsets(rast, extent.UpperLeftX, extent.UpperLeftY);

					/* get upper-right corner */
					if (rt_raster_cell_to_geopoint(
						rast,
						rast->width, 0,
						&(_c[0]), &(_c[1]),
						NULL
					) != ES_NONE) {
						rterror("rt_raster_gdal_warp: Could not compute spatial coordinates for raster pixel");
						rt_raster_destroy(rast);
						_rti_warp_arg_destroy(arg);
						return NULL;
					}

					rast->scaleX = fabs((_c[0] - _w[0]) / ((double) rast->width));
				}
			}
			if (FLT_NEQ(_w[1], extent.UpperLeftY)) {
				if (NULL == height)
					rast->height++;
				else if (NULL == scale_y) {
					double _c[2] = {0};

					rt_raster_set_offsets(rast, extent.UpperLeftX, extent.UpperLeftY);

					/* get upper-right corner */
					if (rt_raster_cell_to_geopoint(
						rast,
						0, rast->height,
						&(_c[0]), &(_c[1]),
						NULL
					) != ES_NONE) {
						rterror("rt_raster_gdal_warp: Could not compute spatial coordinates for raster pixel");

						rt_raster_destroy(rast);
						_rti_warp_arg_destroy(arg);
						return NULL;
					}

					rast->scaleY = -1 * fabs((_c[1] - _w[1]) / ((double) rast->height));
				}
			}

			rt_raster_set_offsets(rast, _w[0], _w[1]);
			RASTER_DEBUGF(4, "aligned offsets: %f x %f", _w[0], _w[1]);
		}
		while (0);
	}

	/*
		after this point, rt_envelope extent is no longer used
	*/

	/* get key attributes from rast */
	_dim[0] = rast->width;
	_dim[1] = rast->height;
	rt_raster_get_geotransform_matrix(rast, _gt);

	/* scale-x is negative or scale-y is positive */
	if ((
		(NULL != scale_x) && (*scale_x < 0.)
	) || (
		(NULL != scale_y) && (*scale_y > 0)
	)) {
		double _w[2] = {0};

		/* negative scale-x */
		if (
			(NULL != scale_x) &&
			(*scale_x < 0.)
		) {
			if (rt_raster_cell_to_geopoint(
				rast,
				rast->width, 0,
				&(_w[0]), &(_w[1]),
				NULL
			) != ES_NONE) {
				rterror("rt_raster_gdal_warp: Could not compute spatial coordinates for raster pixel");
				rt_raster_destroy(rast);
				_rti_warp_arg_destroy(arg);
				return NULL;
			}

			_gt[0] = _w[0];
			_gt[1] = *scale_x;

			/* check for skew */
			if (NULL != skew_x && FLT_NEQ(*skew_x, 0.0))
				_gt[2] = *skew_x;
		}
		/* positive scale-y */
		if (
			(NULL != scale_y) &&
			(*scale_y > 0)
		) {
			if (rt_raster_cell_to_geopoint(
				rast,
				0, rast->height,
				&(_w[0]), &(_w[1]),
				NULL
			) != ES_NONE) {
				rterror("rt_raster_gdal_warp: Could not compute spatial coordinates for raster pixel");
				rt_raster_destroy(rast);
				_rti_warp_arg_destroy(arg);
				return NULL;
			}

			_gt[3] = _w[1];
			_gt[5] = *scale_y;

			/* check for skew */
			if (NULL != skew_y && FLT_NEQ(*skew_y, 0.0))
				_gt[4] = *skew_y;
		}
	}

	rt_raster_destroy(rast);
	rast = NULL;

	RASTER_DEBUGF(3, "Applied geotransform: %f, %f, %f, %f, %f, %f",
		_gt[0], _gt[1], _gt[2], _gt[3], _gt[4], _gt[5]);
	RASTER_DEBUGF(3, "Raster dimensions (width x height): %d x %d",
		_dim[0], _dim[1]);

	if ( _dim[0] == 0 || _dim[1] == 0 ) {
		rterror("rt_raster_gdal_warp: The width (%d) or height (%d) of the warped raster is zero", _dim[0], _dim[1]);
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* load VRT driver */
	if (!rt_util_gdal_driver_registered("VRT")) {
		RASTER_DEBUG(3, "Registering VRT driver");
		GDALRegister_VRT();
		arg->dst.destroy_drv = 1;
	}
	arg->dst.drv = GDALGetDriverByName("VRT");
	if (NULL == arg->dst.drv) {
		rterror("rt_raster_gdal_warp: Could not load the output GDAL VRT driver");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* create dst dataset */
	arg->dst.ds = GDALCreate(arg->dst.drv, "", _dim[0], _dim[1], 0, GDT_Byte, dst_options);
	if (NULL == arg->dst.ds) {
		rterror("rt_raster_gdal_warp: Could not create GDAL VRT dataset");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* set dst srs */
	if (arg->dst.srs != NULL) {
		cplerr = GDALSetProjection(arg->dst.ds, arg->dst.srs);
		if (cplerr != CE_None) {
			rterror("rt_raster_gdal_warp: Could not set projection");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}
		RASTER_DEBUGF(3, "Applied SRS: %s", GDALGetProjectionRef(arg->dst.ds));
	}

	/* set dst geotransform */
	cplerr = GDALSetGeoTransform(arg->dst.ds, _gt);
	if (cplerr != CE_None) {
		rterror("rt_raster_gdal_warp: Could not set geotransform");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* add bands to dst dataset */
	numBands = rt_raster_get_num_bands(raster);
	for (i = 0; i < numBands; i++) {
		rtband = rt_raster_get_band(raster, i);
		if (NULL == rtband) {
			rterror("rt_raster_gdal_warp: Could not get band %d for adding to VRT dataset", i);
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		pt = rt_band_get_pixtype(rtband);
		gdal_pt = rt_util_pixtype_to_gdal_datatype(pt);
		if (gdal_pt == GDT_Unknown)
			rtwarn("rt_raster_gdal_warp: Unknown pixel type for band %d", i);

		cplerr = GDALAddBand(arg->dst.ds, gdal_pt, NULL);
		if (cplerr != CE_None) {
			rterror("rt_raster_gdal_warp: Could not add band to VRT dataset");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		/* get band to write data to */
		band = NULL;
		band = GDALGetRasterBand(arg->dst.ds, i + 1);
		if (NULL == band) {
			rterror("rt_raster_gdal_warp: Could not get GDAL band for additional processing");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		/* set nodata */
		if (rt_band_get_hasnodata_flag(rtband) != FALSE) {
			hasnodata = 1;
			rt_band_get_nodata(rtband, &nodata);
			if (GDALSetRasterNoDataValue(band, nodata) != CE_None)
				rtwarn("rt_raster_gdal_warp: Could not set nodata value for band %d", i);
			RASTER_DEBUGF(3, "nodata value set to %f", GDALGetRasterNoDataValue(band, NULL));
		}
	}

	/* create transformation object */
	arg->transform.arg.transform = arg->transform.arg.imgproj = GDALCreateGenImgProjTransformer2(
		arg->src.ds, arg->dst.ds,
		arg->transform.option.item
	);
	if (NULL == arg->transform.arg.transform) {
		rterror("rt_raster_gdal_warp: Could not create GDAL transformation object");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}
	arg->transform.func = GDALGenImgProjTransform;

	/* use approximate transformation object */
	if (max_err > 0.0) {
		arg->transform.arg.transform = arg->transform.arg.approx = GDALCreateApproxTransformer(
			GDALGenImgProjTransform,
			arg->transform.arg.imgproj, max_err
		);
		if (NULL == arg->transform.arg.transform) {
			rterror("rt_raster_gdal_warp: Could not create GDAL approximate transformation object");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}

		arg->transform.func = GDALApproxTransform;
	}

	/* warp options */
	arg->wopts = GDALCreateWarpOptions();
	if (NULL == arg->wopts) {
		rterror("rt_raster_gdal_warp: Could not create GDAL warp options object");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}

	/* set options */
	arg->wopts->eResampleAlg = resample_alg;
	arg->wopts->hSrcDS = arg->src.ds;
	arg->wopts->hDstDS = arg->dst.ds;
	arg->wopts->pfnTransformer = arg->transform.func;
	arg->wopts->pTransformerArg = arg->transform.arg.transform;
	arg->wopts->papszWarpOptions = (char **) CPLMalloc(sizeof(char *) * 2);
	arg->wopts->papszWarpOptions[0] = (char *) CPLMalloc(sizeof(char) * (strlen("INIT_DEST=NO_DATA") + 1));
	strcpy(arg->wopts->papszWarpOptions[0], "INIT_DEST=NO_DATA");
	arg->wopts->papszWarpOptions[1] = NULL;

	/* set band mapping */
	arg->wopts->nBandCount = numBands;
	arg->wopts->panSrcBands = (int *) CPLMalloc(sizeof(int) * arg->wopts->nBandCount);
	arg->wopts->panDstBands = (int *) CPLMalloc(sizeof(int) * arg->wopts->nBandCount);
	for (i = 0; i < arg->wopts->nBandCount; i++)
		arg->wopts->panDstBands[i] = arg->wopts->panSrcBands[i] = i + 1;

	/* set nodata mapping */
	if (hasnodata) {
		RASTER_DEBUG(3, "Setting nodata mapping");
		arg->wopts->padfSrcNoDataReal = (double *) CPLMalloc(numBands * sizeof(double));
		arg->wopts->padfDstNoDataReal = (double *) CPLMalloc(numBands * sizeof(double));
		arg->wopts->padfSrcNoDataImag = (double *) CPLMalloc(numBands * sizeof(double));
		arg->wopts->padfDstNoDataImag = (double *) CPLMalloc(numBands * sizeof(double));
		if (
			NULL == arg->wopts->padfSrcNoDataReal ||
			NULL == arg->wopts->padfDstNoDataReal ||
			NULL == arg->wopts->padfSrcNoDataImag ||
			NULL == arg->wopts->padfDstNoDataImag
		) {
			rterror("rt_raster_gdal_warp: Out of memory allocating nodata mapping");
			_rti_warp_arg_destroy(arg);
			return NULL;
		}
		for (i = 0; i < numBands; i++) {
			band = rt_raster_get_band(raster, i);
			if (!band) {
				rterror("rt_raster_gdal_warp: Could not process bands for nodata values");
				_rti_warp_arg_destroy(arg);
				return NULL;
			}

			if (!rt_band_get_hasnodata_flag(band)) {
				/*
					based on line 1004 of gdalwarp.cpp
					the problem is that there is a chance that this number is a legitimate value
				*/
				arg->wopts->padfSrcNoDataReal[i] = -123456.789;
			}
			else {
				rt_band_get_nodata(band, &(arg->wopts->padfSrcNoDataReal[i]));
			}

			arg->wopts->padfDstNoDataReal[i] = arg->wopts->padfSrcNoDataReal[i];
			arg->wopts->padfDstNoDataImag[i] = arg->wopts->padfSrcNoDataImag[i] = 0.0;
			RASTER_DEBUGF(4, "Mapped nodata value for band %d: %f (%f) => %f (%f)",
				i,
				arg->wopts->padfSrcNoDataReal[i], arg->wopts->padfSrcNoDataImag[i],
				arg->wopts->padfDstNoDataReal[i], arg->wopts->padfDstNoDataImag[i]
			);
		}
	}

	/* warp raster */
	RASTER_DEBUG(3, "Warping raster");
	cplerr = GDALInitializeWarpedVRT(arg->dst.ds, arg->wopts);
	if (cplerr != CE_None) {
		rterror("rt_raster_gdal_warp: Could not warp raster");
		_rti_warp_arg_destroy(arg);
		return NULL;
	}
	/*
	GDALSetDescription(arg->dst.ds, "/tmp/warped.vrt");
	*/
	GDALFlushCache(arg->dst.ds);
	RASTER_DEBUG(3, "Raster warped");

	/* convert gdal dataset to raster */
	RASTER_DEBUG(3, "Converting GDAL dataset to raster");
	rast = rt_raster_from_gdal_dataset(arg->dst.ds);

	_rti_warp_arg_destroy(arg);

	if (NULL == rast) {
		rterror("rt_raster_gdal_warp: Could not warp raster");
		return NULL;
	}

	/* substitute spatial, reset back to default */
	if (subspatial) {
		double gt[6] = {0, 1, 0, 0, 0, -1};
		/* See http://trac.osgeo.org/postgis/ticket/2911 */
		/* We should proably also tweak rotation here */
		/* NOTE: the times 10 is because it was divided by 10 in a section above,
		 *       I'm not sure the above division was needed */
		gt[1] = _scale[0] * 10;
		gt[5] = -1 * _scale[1] * 10;

		rt_raster_set_geotransform_matrix(rast, gt);
		rt_raster_set_srid(rast, SRID_UNKNOWN);
	}

	RASTER_DEBUG(3, "done");

	return rast;
}