Add another example to st_value demonstrating sampling and fix other example

git-svn-id: http://svn.osgeo.org/postgis/trunk@5568 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/doc/reference_wktraster.xml b/doc/reference_wktraster.xml
index 73637a14ceb4102a4f69bfd7cf2ada87184ad6cf..bf39d368e81d7bd4999fffb0b3ba845247ddecbb 100644 (file)
--- a/doc/reference_wktraster.xml
+++ b/doc/reference_wktraster.xml
@@ -1213,7 +1213,7 @@ FROM (SELECT ST_Translate(ST_MakeEnvelope(
                        ST_UpperLeftX(rast) + ST_PixelSizeX(rast),
                        ST_UpperLeftY(rast) + ST_PixelSizeY(rast), 0
                        ), ST_PixelSizeX(rast)*x, ST_PixelSizeY(rast)*y
-               ) As pixpolyg, ST_Value(rast, 1, x, y) As b1val
+               ) As pixpolyg, ST_Value(rast, 2, x, y) As b2val
        FROM dummy_rast CROSS JOIN
 generate_series(1,1000) As x CROSS JOIN generate_series(1,1000) As y
 WHERE rid =  2 
@@ -1221,21 +1221,54 @@ WHERE rid =  2
 WHERE  
        ST_Intersects(
                pixpolyg, 
-               ST_GeomFromText('POLYGON ((3427935.6902608695 5793250.889478261, 
-                       3427932.0770434784 5793251.682173912, 
-                       3427931.9295652173 5793250.400956522, 
-                       3427933.0576086957 5793248.985, 3427933.518478261 5793249.192391304, 
-                       3427934.611826087 5793248.999913043, 3427935.6902608695 5793250.889478261))',0)
-               ) AND b1val != 254;
+               ST_GeomFromText('POLYGON((3427928 5793244,3427927.75 5793243.75,3427928 5793243.75,3427928 5793244))',0)
+               ) AND b2val != 254;
+
+
+               shadow
+------------------------------------------------------------------------------------
+ MULTIPOLYGON(((3427928 5793243.9,3427928 5793243.85,3427927.95 5793243.85,3427927.95 5793243.9,
+ 3427927.95 5793243.95,3427928 5793243.95,3427928.05 5793243.95,3427928.05 5793243.9,3427928 5793243.9)),((3427927.95 5793243.9,3427927.95 579324
+3.85,3427927.9 5793243.85,3427927.85 5793243.85,3427927.85 5793243.9,3427927.9 5793243.9,3427927.9 5793243.95,
+3427927.95 5793243.95,3427927.95 5793243.9)),((3427927.85 5793243.75,3427927.85 5793243.7,3427927.8 5793243.7,3427927.8 5793243.75
+,3427927.8 5793243.8,3427927.8 5793243.85,3427927.85 5793243.85,3427927.85 5793243.8,3427927.85 5793243.75)),
+((3427928.05 5793243.75,3427928.05 5793243.7,3427928 5793243.7,3427927.95 5793243.7,3427927.95 5793243.75,3427927.95 5793243.8,3427
+927.95 5793243.85,3427928 5793243.85,3427928 5793243.8,3427928.05 5793243.8,
+3427928.05 5793243.75)),((3427927.95 5793243.75,3427927.95 5793243.7,3427927.9 5793243.7,3427927.85 5793243.7,
+3427927.85 5793243.75,3427927.85 5793243.8,3427927.85 5793243.85,3427927.9 5793243.85,
+3427927.95 5793243.85,3427927.95 5793243.8,3427927.95 5793243.75)))
+                               </programlisting>
+                               
+                               <programlisting>
+--- Checking all the pixels of a large raster tile can take a long time.
+--- You can dramatically improve speed at some lose of precision by orders of magnitude 
+--  by sampling pixels using the step optional parameter of generate_series.  
+--  This next example does the same as previous but by checking 1 for every 4 (2x2) pixels and putting in the last checked
+--  putting in the checked pixel as the value for subsequent 4
+       
+SELECT ST_AsText(ST_Union(pixpolyg)) As shadow
+FROM (SELECT ST_Translate(ST_MakeEnvelope(
+               ST_UpperLeftX(rast), ST_UpperLeftY(rast), 
+                       ST_UpperLeftX(rast) + ST_PixelSizeX(rast)*2,
+                       ST_UpperLeftY(rast) + ST_PixelSizeY(rast)*2, 0
+                       ), ST_PixelSizeX(rast)*x, ST_PixelSizeY(rast)*y
+               ) As pixpolyg, ST_Value(rast, 2, x, y) As b2val
+       FROM dummy_rast CROSS JOIN
+generate_series(1,1000,2) As x CROSS JOIN generate_series(1,1000,2) As y
+WHERE rid =  2 
+       AND x &lt;= ST_Width(rast)  AND y &lt;= ST_Height(rast)  ) As foo
+WHERE  
+       ST_Intersects(
+               pixpolyg, 
+               ST_GeomFromText('POLYGON((3427928 5793244,3427927.75 5793243.75,3427928 5793243.75,3427928 5793244))',0)
+               ) AND b2val != 254;
 
                shadow
 ------------------------------------------------------------------------------------
- MULTIPOLYGON(((3427933.75 5793248.5,3427933.75 5793250,3427935.25 5793250,
- 3427935.25 5793248.5,3427933.75 5793248.5)),
- ((3427930.75 5793250,3427930.75 5793251.5,3427930.75 5793253,3427932.25 5793253,
-3427932.25 5793251.5,3427932.25 5793250,3427930.75 5793250)),
-((3427935.25 5793250,3427935.25 5793251.5,3427936.75 5793251.5,
-3427936.75 5793250,3427935.25 5793250)))
+ MULTIPOLYGON(((3427927.9 5793243.85,3427927.8 5793243.85,3427927.8 5793243.95,
+ 3427927.9 5793243.95,3427928 5793243.95,3427928.1 5793243.95,3427928.1 5793243.85,3427928 5793243.85,3427927.9 5793243.85)),
+ ((3427927.9 5793243.65,3427927.8 5793243.65,3427927.8 5793243.75,3427927.8 5793243.85,3427927.9 5793243.85,
+ 3427928 5793243.85,3427928 5793243.75,3427928.1 5793243.75,3427928.1 5793243.65,3427928 5793243.65,3427927.9 5793243.65)))
                                </programlisting>
                        </refsection>