Changes between Version 83 and Version 84 of WKTRaster/SpecificationWorking01

Timestamp:

Dec 18, 2009, 2:04:00 PM (14 years ago)

Author:

pracine

Comment:

—

WKTRaster/SpecificationWorking01

@@ -237 +237 @@
  This function could be roughly implemented as a SQL function looking like 'SELECT ST_Collect((ST_AsPolygon(raster)).geom)'. For sure a more specialised version could be faster than ST_AsPolygon.
 
-'''ST_AsPolygon(raster) -> geomval set''' - Returns a set of "geomval" value, one for each group of pixel having the same value.
-
- This is a set-returning function (SRF). A "geomval" value is a complex type composed of a polygon geometry (one for each group of contiguous pixel of sharing same value) and the value associated with these pixels. The value associated with the group of pixel are always returned as a double precision value. The shape of each polygon follow pixels edges.
-
+'''ST_AsPolygon(raster) -> geomval set''' - Returns a set of "geomval" value, one for each contiguous group of pixel sharing the same value.
+
+ This is a set-returning function (SRF). A "geomval" value is a complex type composed of a polygon geometry (one for each contiguous group of pixel sharing the same value) and the value associated with this geometry. These values are always returned as a value of type double precision. The shape of each polygon follow pixels edges.
+
+ This function should be used with precaution on raster with float pixel type as it could return one "geomval" (or polygon) per pixel. This kind of raster should be reclassified (using the planned ST_Reclassify(raster,...) function) before using ST_AsPolygon().
+
+ '''Implementation details'''
+ 
  A ST_AsPolygon(raster, 'REGROUP') variant should regroup every resulting polygon having the same value into complex (or multipart) polygons.
- 
- This function should be used with precaution on raster with float pixel type as it could return one "geomval" (or polygon) per pixel. This kind of raster should be reclassified (using the planned ST_Reclassify(raster,...) function) before using ST_AsPolygon().
-
- '''Implementation details'''
  
  This function is at the base of the first version of ST_Intersection which compute the intersection between a raster and a geometry.
@@ -267 +267 @@
 
 
-'''ST_AsWKTPolygon(raster) -> text set''' - Returns a set of text representation of geometry, one for each group of pixel having the same value.
+'''ST_AsWKTPolygon(raster) -> wktgeomval set''' - Returns a set of "geomval" value, one for each contiguous group of pixel sharing the same value.
+
+ This is a set-returning function (SRF). A "wktgeomval " value is a complex type composed of a the wkt representation of a geometry (one for each contiguous group of pixel sharing the same value) and the value associated with this geometry. These values are always returned as a value of type double precision. The shape of each polygon follow pixels edges.
+
+ This function should be used with precaution on raster with float pixel type as it could return one "geomval" (or polygon) per pixel. This kind of raster should be reclassified (using the planned ST_Reclassify(raster,...) function) before using ST_AsPolygon().
 
  '''Implementation details'''
@@ -273 +277 @@
  This function construct the WKT strings representing the geometries grouping pixels of a raster having the same value. It does not directly construct geometries and therefore prevent WKT Raster from having to link with liblwgeom.a (see "Why avoid to link with PostGIS?" below) It should also return the value associated with this WKT polygon.
 
- Ways for a pg/plSQL or SQL function to return many columns:
-
- Functions with Output Parameters (PgSQL doc 34.4.3):
-
- CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT product int)[[BR]]
- AS 'SELECT $1 + $2, $1 * $2'[[BR]]
- LANGUAGE SQL;
-
- The function must be called in the FROM clause, otherwise it returns a 
- 
- SELECT * FROM sum_n_product(11,42);
-
-
- Ways for a pg/plSQL or SQL function to return many rows:
-
- It should be implemented by passing the raster to GDAL and convert each polygon produced by the [http://www.gdal.org/gdal__alg_8h.html GDALPolygonize function] to a WKT string.
+ It should be implemented as a C function passing the raster to GDAL and converting each polygon produced by the [http://www.gdal.org/gdal__alg_8h.html GDALPolygonize function] to a WKT string accompagned by its corresponding value in a wktgeomval type. See http://www.postgresql.org/docs/8.4/interactive/xfunc-c.html#AEN44970 on how to return rows and sets.