[gdal-dev] Downsample with averaging?

[Norman Vine]

On Feb 21, 2011, at 3:26 PM, Jay Jennings wrote:

> Hello list,
> (using GDAL 1.8.0) I am trying to create a 32:1 down-sampled overview of a GeoTiff satellite image.  My first thought was gdal_translate, with args such as “-outsize 3.125% 3.125%”… which produces surprisingly high quality given the absence of a resampling option.   However I’m looking for a downsampling scheme that creates a result pixel by averaging all relevant source pixels (I know, for 32:1 downsample, that means 1024 source pixels for each result pixel !) with the hope of an output that is not “speckled” or “grainy” insofar as possible.
>  
> I also looked at gdaladdo, which does have the “-r average” resampling option… the guidance at gdal.org seems to suggest that it can produce a 32:1 GeoTIFF external overview with a command like this:
>  
> gdaladdo  -r  average  -ro  XXXXX.tif  32
>  
> But that produces a surprising error message, namely:
> ERROR 4: `XXXXX.tif.ovr' does not exist in the file system, and is not recognised as a supported dataset name.
>  
> Am I barking up the wrong tree with gdaladdo for this purpose ?  Anybody have any suggestions for highest-quality down-sampling ? Thanks in advance.
>  

Jay

I think you need to build your overlays incrementally in order to get the average
of the 'block' at each overview level as gdaladdo has a very small sampling window

gdaladdo -r average my.tif 2 4 8 16 32

better to use gauss sampling
see http://en.wikipedia.org/wiki/Pyramid_%28image_processing%29

gdaladdo -r gauss my.tiff 2 4 8 16 32

note you may want to convert your image to a floating point form first 
so as not to possibly lose precision due to the multiple passes

there is a gdal method to extract a single overview level from a tif but I can never 
remember it maybe someone will chime in if not you can use your gdal_translate
trick.

HTH

Norman


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