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Looking for an open source tool to grow/shrink raster edge by n pixels. I noticed Grass has r.grow but hoping there might be a way to do it with GDAL, PostGIS or python library.

More information: I want to shrink it around the edge where I have data not the entire rectangular region. Consider this raster: enter image description here

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  • Check the extents and pixel size of the original image with gdalinfo, calculate the desired xmin ymin xmax ymax values feed them to -te (target extents) parameter of gdalwarp gdal.org/gdalwarp.html. For making all that automatically you should write a script.
    – user30184
    Commented Mar 23, 2016 at 9:04
  • Thanks but I think that will warp all the data, I just want to remove pixels along the edge where data exists. I've added an example.
    – Ben
    Commented Mar 25, 2016 at 6:56
  • If input srs and output srs are the same the pixels are just copied. But our image clarifies your question and adjusting the extents does not help you.
    – user30184
    Commented Mar 25, 2016 at 13:09

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So I ended up making a GRASS script to mitigate edge effect from course imagery such as Landsat 8. Script is as follows. I know each step could be simplified but have it in steps helps understand what is going on.

Basically it does a neighbourhood count. It uses the output to discover the edge of the imagery. These gaps are given a null value. The null value is filled in with median value from 5x5 neighbourhood. The output is then masked to the original image.

See input & output side by side.

Input

Output

run_command("r.neighbors",
            overwrite = True,
            input = "in@PERMANENT",
            output = "count",
            method = "count",
            size = 5)

run_command("r.mapcalc",
            overwrite = True,
            expression = "step1=if(count@PERMANENT<25  &  count@PERMANENT>0,0,1)")

run_command("r.mapcalc",
            overwrite = True,
            expression = "step2=if( step1@PERMANENT==0,null(),1)")

run_command("r.mapcalc",
            overwrite = True,
            expression = "step3=if( count@PERMANENT ==0,0,1)* step2@PERMANENT")

run_command("r.mapcalc",
            overwrite = True,
            expression = "step4=if(isnull(in@PERMANENT )==1,0,in@PERMANENT)")

run_command("r.mapcalc",
            overwrite = True,
            expression = "step5=step4@PERMANENT * step3@PERMANENT")

run_command("r.neighbors",
            overwrite = True,
            input = "step5@PERMANENT",
            output = "step6",
            method = "median",
            size = 5)

run_command("r.mapcalc",
            overwrite = True,
            expression = "step7=if(isnull(step6@PERMANENT )==1, step6@PERMANENT, in@PERMANENT )")

run_command("r.mapcalc",
            overwrite = True,
            expression = "step8=if(isnull(in)==0,step7,null())")
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