| 232 | | * the prototype of an optimized version, trying to set those large nodata areas as well as areas where only one raster is present (this is the case when unioning two contiguous non-overlapping rasters) as a block (not pixel by pixel) is still in development. See http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_mapalgebra_optimized.sql The idea is to set blocks of the resulting raster using ST_SetValues() (described in [http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking02 Objective 2.0.05]) instead of processing one pixel at a time. This is somewhat similar to setting a large block of memory with memcpy() or memset() rather than setting a buffer one value at a time. The resulting raster is divided into rectangular block with the _MapAlgebraParts() function. |
| 233 | | |
| 234 | | * Both rasters must have the same SRID. If not, ST_MapAlgebra() should return an error: "ERROR: Operation on two geometries with different SRIDs" |
| 235 | | |
| 236 | | * Both raster must be aligned. This is determined using the ST_SameAlignment() function described below. If a resampling is necessary they use the planned ST_Resample() function to resample the second raster to the first one before processing (or the first to the second if an optional parameter is provided). If ST_Resample() is not yet implemented when these functions are implemented, just return an error message: "ERROR: MapAlgebra on rasters with different alignment not yet implemented." |
| | 232 | * The prototype of an optimized version, trying to set those large areas of nodata values as well as areas where only one raster is present (this is the case when unioning two contiguous non-overlapping rasters) as a block (not pixel by pixel) is still in development. See http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_mapalgebra_optimized.sql The idea is to set blocks of the resulting raster using ST_SetValues() (described in [http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking02 Objective 2.0.05]) instead of processing one pixel at a time. This is somewhat similar to setting a large block of memory with memcpy() or memset() rather than setting a buffer one value at a time. The resulting raster is divided into rectangular block with the _MapAlgebraParts() function. |
| | 233 | |
| | 234 | * Both rasters must have the same SRID. If not, ST_MapAlgebra() should return an error: "ERROR: Operation on two geometries with different SRIDs". |
| | 235 | |
| | 236 | * An optional option could be added to allow specifying which raster, between the first and the second, is the MASTER raster. The MASTER raster is used to determine the pixeltype, the alignment and the nodatavalue of the resulting raster. If the MASTER raster do not have a nodata value defined and it is necessary to set one in the resulting raster then the minimal possible value for the pixeltype should be used as nodata value. In case this option would not be added or in case it is not passed to the function, the first raster should be used to determine those informations. |
| | 237 | |
| | 238 | * Both raster must be aligned. This is determined using the ST_SameAlignment() function described below. If a resampling is necessary they use the planned ST_Resample() function to resample the second raster to the first one before processing (or the first to the second if a MASTER parameter is provided). If ST_Resample() is not yet implemented when these functions are implemented, just return an error message: "ERROR: MapAlgebra on rasters with different alignment not yet implemented." |
| 257 | | * Alternate expressions like nodata1expr and nodata2expr are used to simplify complex decision expression trying to deal with the presence of nodata value pixels. Having three short expressions like this:[[BR]][[BR]] 'rast2', 'rast2', 'rast1'[[BR]][[BR]]is simpler to understand than a single complex expression dealing with nodata like this:[[BR]][[BR]] ‘CASE WHEN rast2 IS NULL THEN rast1 ELSE rast2 END’[[BR]][[BR]]This is a simple case. In more complex cases, expressions can quickly get incomprehensible and alternate expressions greatly simplify the task of writing expressions, even if having three extra parameters may seams cumbersome. |
| | 259 | All expressions default to NULL. When an expression is NULL every pixel filling the condition of this expression is set to NULL (nodata). |
| | 260 | |
| | 261 | * Alternate expressions like nodata1expr and nodata2expr are used to simplify complex decision expression trying to deal with the presence of nodata value pixels. Having three short expressions like this:[[BR]][[BR]] 'rast2', 'rast2', 'rast1'[[BR]][[BR]]is simpler to understand than a single complex expression dealing with nodata like this:[[BR]][[BR]] ‘CASE WHEN rast2 IS NULL THEN rast1 ELSE rast2 END’[[BR]][[BR]]This is a simple case. In more complex cases, expressions can quickly get incomprehensible and alternate expressions greatly simplify the task of writing expressions, even if having three extra parameters may seams cumbersome. |
| | 262 | |
