>> 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 so, that would be really helpful. Does anyone know how to extract a single overview from a pyramid'd image such as made from a command like: gdaladdo XXXX.tif 2 4 8 16 32 64 128 256 From: Norman Vine [mailto:n...@cape.com] Sent: Tuesday, February 22, 2011 4:09 AM To: Jay Jennings Cc: gdal-dev@lists.osgeo.org Subject: Re: [gdal-dev] Downsample with averaging? 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<http://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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