[postgis-devel] [PostGIS] #502: [raster] Optimize ST_Intersection to return ST_Value when intersecting with points

[PostGIS]

#502: [raster] Optimize ST_Intersection to return ST_Value when intersecting with
points
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  Reporter:  pracine         |       Owner:  pracine      
      Type:  enhancement     |      Status:  assigned     
  Priority:  low             |   Milestone:  PostGIS 2.0.0
 Component:  postgis raster  |     Version:  trunk        
Resolution:                  |    Keywords:               
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Comment(by pracine):

 For this we need ST_Value to behave like ST_Intersection by returning a
 SET OF 2 or 4 values when the requested coordinated fall between two
 pixels or four pixels.

 Actually it is more the opposite: The ST_Value variant taking a point
 geometry should be "downtimized" (only 3 occurrence in Google !!) as a
 wrapper around ST_Intersection so that when a point located the
 intersection of 2 or 4 pixels is passed, the function returns 2 or 4
 values since the right behavior is the one of ST_Intersection in this
 case. For sure this would make this variant of ST_Value much slower since
 ST_Intersection has to polygonize the tile first which ST_Value do not do
 right now (it converts the point to a x and a y raster coordinate and
 returns the value at this coordinate).

 To make it to be fast, we must then first optimize ST_Intersection by
 being able to polygonize a subregion of the tile (the one intersecting
 with the geometry). This would generally optimize ST_Intersection for any
 geometry. In the case of ST_Value this region would reduce to a maximum of
 4 pixels and hence would be quite fast.

 It is already possible to pass a mask to the GDAL polygonize function.
 That mean we could construct a mask with the area we want to polygonize.
 Would this be fast? For sure it would be more efficient to be able to just
 pass x1,y2, x2,y2 to the function to delimitate this area. To see with
 GDAL people.

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Ticket URL: <http://trac.osgeo.org/postgis/ticket/502#comment:8>
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