[GRASS-dev] [GRASS GIS] #2368: Python version of r.grow does not support shrinking

[GRASS GIS]

#2368: Python version of r.grow does not support shrinking
-------------------------+-------------------------------------------------
  Reporter:  wenzeslaus  |      Owner:  grass-dev@…
      Type:              |     Status:  new
  enhancement            |
  Priority:  minor       |  Milestone:  7.2.0
 Component:  Raster      |    Version:  svn-trunk
Resolution:              |   Keywords:  r.grow, r.grow.distance, r.buffer,
       CPU:              |  r.buffer.lowmem
  Unspecified            |   Platform:  Unspecified
-------------------------+-------------------------------------------------

Comment (by mmetz):

 Replying to [comment:9 neteler]:
 > Replying to [comment:7 neteler]:
 > > Replying to [comment:6 mmetz]:
 > > > Replying to [comment:3 glynn]:
 > > > > The easiest change would be to have an "invert" flag which caused
 the distance= map to contain the distance from the nearest null cell
 (currently, it's the distance from the nearest non-null cell, so the non-
 null cells themselves have a distance of 0).
 > > >
 > > > Attached is a minimally invasive patch to add a new flag to
 r.grow.distance, calculating the distance to the nearest null cell.
 > >
 > > Patch applied to trunk in r67620.
 >
 > Can I backport this to 7.2.0? Again I need a negative buffer...

 This is a new feature and as such does not qualify for backport.

 If you want to shrink you can use

 {{{
 r.mapcalc "input_inv = if(isnull(input), 1, null())"
 r.grow input=input_inv
 }}}

 Replying to [comment:10 neteler]:
 > Just tested r.grow with a negative radius and the following error
 occurs:
 > {{{
 > # NC sample dataset
 > v.in.region box
 > v.to.rast input=box output=box use=val val=1
 >
 > r.grow input=box output=box_shrink radius=-20.01
 > }}}

 The example is wrong because there are no NULL cells in the current region
 for the input raster. When growing, for each NULL cell the distance to the
 nearest non-NULL cell is calculated. When shrinking, for each non-NULL
 cell the distance to the nearest NULL cell is calculated. That means there
 must be both NULL and non-NULL cells in the input raster for the current
 region, otherwise nothing can be grown or shrunk.

 The example can not work because r.grow does not support negative
 distances. A test needs to be added to the script to call r.grow.distance
 with the new flag if the radius is negative, and a different r.mapcalc
 expression must be used to create the final output:

 {{{
 "$output = if($dist < $radius,null(),$old)"
 }}}


 > {{{
 > Reading raster map <box>...
 >  100%
 > Writing output raster maps...
 >  100%
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 >  100%
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 > ERROR 1: PredictorSetup:Horizontal differencing "Predictor" not
 supported with 64-bit samples
 > Color table for raster map <box_shrink> set to 'box'
 > }}}

 These look like GDAL errors, they are definitively not GRASS errors.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2368#comment:12>
GRASS GIS <https://grass.osgeo.org>