[postgis-tickets] [PostGIS] #2800: [raster] Coverage topology broken by ST_Rescale

[PostGIS]

#2800: [raster] Coverage topology broken by ST_Rescale
--------------------+-------------------------------------------------------
 Reporter:  strk    |       Owner:  dustymugs
     Type:  defect  |      Status:  new      
 Priority:  medium  |   Milestone:           
Component:  raster  |     Version:  trunk    
 Keywords:          |  
--------------------+-------------------------------------------------------

Comment(by strk):

 Adding some debugging lines I find that ST_Resize (the working one, being
 passed percent width/height) ends up calling rt_raster_gdal_warp with an
 integer width/height like this:
 {{{
 NOTICE:  rt_raster_gdal_warp user-defined width 3, computed xscale 0.6144
 NOTICE:  rt_raster_gdal_warp user-defined height 3, computed yscale 0.6144
 }}}

 While ST_Rescale ends up calling rt_raster_gdal_warp with a float scale
 like this:
 {{{
 NOTICE:  rt_raster_gdal_warp user-defined scale x=0.6, y=0.6
 NOTICE:  rt_raster_gdal_warp computed dims w=3, h=3
 }}}

 The source scale of the raster is: 0.00719999999999928 (both X and Y).
 The source size is 256 x 256.

 ST_Resize ends up being called with percent parameters 0.0119999999999988,
 being the result of
 source scale / target scale (0.6). The full query is:
 {{{
 SELECT ST_Resize("rast", CASE WHEN abs(ST_ScaleX("rast"))::float8 < 1 THEN
 abs(ST_ScaleX("rast"))::float8/1 ELSE 1.0 END, CASE WHEN
 abs(ST_ScaleY("rast"))::float8 < 1 THEN abs(ST_ScaleY("rast"))::float8/1
 ELSE 1.0 END) AS geom,"rid" FROM dataraster
 }}}

 ST_Rescale gets called with 0.6 instead.

 The result from ST_Resize retains the original extent while the result
 from ST_Rescale makes it slightly smaller:
 {{{
 with r as ( select rast from hyp_1250m limit 1 ) select
 box2d(st_envelope(rast)) as orig,
 box2d(st_envelope(st_resize(rast,.0119999999999988,.0119999999999988))) as
 resized, box2d(st_envelope(st_rescale(rast,0.6,0.6))) as rescaled from r;
 -[ RECORD 1 ]-------------------------------------
 orig     | BOX(-180 88.1568000000002,-178.1568 90)
 resized  | BOX(-180 88.1568000000002,-178.1568 90)
 rescaled | BOX(-180 88.2,-178.2 90)
 }}}

 I guess all of this is expected, which would make the confusion simply due
 to lack of documentation.
 It should be stated in the manual page when ST_Resize retains extent (I
 guess when passed a factor of original size, but not when passed fixed
 size) and when (if ever) ST_Rescale retains extent.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/2800#comment:4>
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