| 85 | | |
| 86 | | ---- |
| 87 | | == '''Objective 2.0.03 - Being able to create a raster as the expression of two rasters''' == |
| 88 | | |
| 89 | | '''ST_SameAlignment(raster, raster)''' |
| 90 | | |
| 91 | | This function returns true if both rasters' grids are aligned. |
| 92 | | |
| 93 | | Two rasters grid are aligned if: |
| 94 | | |
| 95 | | -They share the same pixel scales and skews |
| 96 | | |
| 97 | | -At least one of any of the four corner of any pixel of one raster fall on any corner of the grid of the other raster. |
| 98 | | |
| 99 | | Alignment is not the same concept as georeference. Two rasters can be aligned but not have the same georeference. |
| 100 | | |
| 101 | | Rotation is important here since two rasters grid might look perfectly aligned but are not if their rotated are different. |
| 102 | | |
| 103 | | Knowing if two rasters share the same alignment is useful when it is time to decide if one of them must be resampled before doing other operations (like ST_MapAlgebra on two rasters). |
| 104 | | |
| 105 | | '''Variants''' |
| 106 | | |
| 107 | | 1) ST_SameAlignment(ulx1, uly1, scalex1, scaley1, skewx1, skewy1, ulx2, uly2, scalex2, scaley2, skewx2, skewy2) |
| 108 | | |
| 109 | | 2) ST_SameAlignment(rast1 raster, rast2 raster) |
| 110 | | |
| 111 | | The first variant is useful to PL/pgSQL function which have already get the values of the parameters. |
| 112 | | |
| 113 | | '''Implementation Details''' |
| 114 | | |
| 115 | | Only the first variant should be implemented in C. The second one is a PL/pgSQL variants. The C implementation should follow the PL/pgSQL version implemented in http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_mapalgebra.sql |
| 116 | | |
| 117 | | It is not clear if this PL/pgSQL implementation works when raster are rotated. To verify. |
| 118 | | |
| 119 | | See discussion in http://trac.osgeo.org/postgis/ticket/589 |
| 120 | | |
| 121 | | |
| 122 | | '''ST_MapAlgebra on two rasters''' |
| 123 | | |
| 124 | | Specifications of ST_MapAlgebra on two rasters are still in progress as a new optimised version, filling large areas of same value more quickly, is planned. See http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_mapalgebra_optimized.sql |
| 125 | | |
| 126 | | See discussion in http://trac.osgeo.org/postgis/ticket/590 |
