[postgis-devel] [PostGIS] #1058: [raster] Two-raster spatial operations framework proposal

[PostGIS]

#1058: [raster] Two-raster spatial operations framework proposal
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 Reporter:  bnordgren       |       Owner:  pracine      
     Type:  enhancement     |      Status:  new          
 Priority:  medium          |   Milestone:  PostGIS 2.0.0
Component:  postgis raster  |     Version:  1.5.X        
 Keywords:                  |  
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 Note: this patch requires #1050 and #1053, and introduces a dependency
 from rt_pg to postgis-2.0.so. The patch applies against #7453.

 I doubt this is usable yet, but there's enough here to start people
 talking (e.g., draw fire). This is a C based framework for two-raster
 operations returning a raster result. The two "major phases" of such a
 task are: precalculate the expected extent of the result; and iterate over
 the result raster to produce one value per cell. Both major phases have
 been encapsulated in their own function, and both use ''function
 callbacks'' as a mechanism to implement extension points. The framework
 takes differing alignments, cell sizes, and projections into account, such
 that the callbacks need only to worry about their fundamental task.


 ----


 The grid extent function has one extension point defined: calculate extent
 of result. The remainder of the function retrieves the extents of the two
 input rasters and projects them into the result srid, creates a raster of
 the appropriate size, and sets various metadata fields. The four current
 implementations of this extension point calculate extents for
 "intersection", "difference", "union" and "symdifference".


 ----


 The result-raster-iteration function has two extension points defined:
 determine whether a cell is included in the result; and provide a value
 for the cell.

 There are four implementations of "determine whether a cell is included in
 the result", corresponding to the four extent calculations:
 "intersection", "difference", "union" and "symdifference".

 There are two implementations of "provide a value for the cell": one
 generates a mask; the other copies image data from one of the input
 rasters (r1 if possible, r2 if not). The implementation which copies image
 data sets "not present" cells to nodata.


 ----


 Provided one can access Postgresql's expression parser from C, it should
 be possible to implement a 2 raster !MapAlgebra by implementing a "provide
 a value for the cell" extension. The interface may have to be augmented
 slightly to allow "user" parameters to be passed (such as the expression
 itself, as well as the bindings from symbol to band)

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Ticket URL: <http://trac.osgeo.org/postgis/ticket/1058>
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