[postgis-devel] [PostGIS] #1174: [raster] C-level index-to-geocoordinate functions

[PostGIS]

#1174: [raster] C-level index-to-geocoordinate functions
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  Reporter:  bnordgren       |       Owner:  pracine      
      Type:  enhancement     |      Status:  reopened     
  Priority:  medium          |   Milestone:  PostGIS 2.0.0
 Component:  postgis raster  |     Version:  1.5.X        
Resolution:                  |    Keywords:               
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Comment(by dustymugs):

 Replying to [comment:3 bnordgren]:
 > With respect, r7884 does two potentially important things differently:
 >
 >  1. It recomputes the matrix inverse of the affine transform on every
 single call (as opposed to only when the transform changes.) As this is a
 per-pixel operation, whatever small penalty this may incur could well be
 multiplied by a factor of several hundred million.

 Only the commented-out validation code generates the matrix inverse.  At
 minimum, the active code only calculates the denominator of the rearranged
 equation for division by zero.  That denominator could be cached...

 >  1. There's a strange mix of delegating the operation to GDAL for the
 cell-to-geopoint direction, but implementing it ourselves for the
 geopoint-to-cell operation. Worse, neither the comments nor the error
 messages indicate that this is a matrix inverse operation and there's no
 mention of where the equation comes from. (Or that "d" is the matrix
 determinant instead of the scale.) If the commented-out-test-code ever
 gets deleted, someone could well spend a very long time just trying to
 figure out what this code is intending to do when investigating some
 random future problem.
 >

 Neither geopoint_to_cell or cell_to_geopoint use GDAL.  "d" is just the
 denominator and processed prior to everything else just to test for
 divisiion by zero.

 > Can we at least add a comment with a link to here:
 http://mathworld.wolfram.com/MatrixInverse.html ?

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/1174#comment:4>
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