[gdal-dev] Feature request: gauss and other interpolations in gdalwarp

Jan Hartmann j.l.h.hartmann at uva.nl
Thu Mar 28 04:27:11 PDT 2013

Perhaps I should clarify a bit what I meant, I haven't had any reactions 
until now, positive or negative, and it is important for me.

I use Gdalwarp and gdaladdo extensively for goereferencing and tiling 
large historical maps serieses (raster scans). To display them 
efficiently, I need to create layers at different scale levels, e.g. if 
the original maps have a pixel size of 10 meters, I need to resample 
them to rasters with pixel sizes of 20, 40, 80 , 160, 320 and 640 
meters, and tile all those maps appropriately. So the original maps have 
to be resampled quite drastically. In Gdalwarp there is no adequeate 
resampling algorithm, and you end up with very grainy map at those lower 
resolutions. Gdaladdo has several more algorithms, with gauss in many 
cases the most efficient. However, even with the gauss filter, maps 
resampled at very low resolutions turn out too hazy. For an example see 
the 1930 map of the Netherlands:


If you zoom in to more detailed levels, you'll see the way the image 
sharpens. For the effect on black-white image choose the TMK-map (1850) 
with the top-center button. Filters like "unsharp mask" would perform 
much better in these cases.

So I would like to propose two enhancements to gdal:

- add additional filters to gdalwarp, gauss and the filters mentioned below.
- implement more filters for gdalwarp and gdaladdo, e.g. "unsharp mask", 
or the "mode" filter asked by Jack below. Perhaps even add the 
possibility to specify parameters, like in ImageMagick

I don't know how difficult is, and whether the gdal devs would find this 
really an improvement for gdal. I could do this with some ImageMagick or 
Gimp scripts, but it would be a kludge. As we are going to georeference 
the complete cadastral and topographical map base of the Netherlands 
from 1832 to 1994 the next few years (millions of map-scans), this 
exentsion of gdal would come in very handy. And funding it will really 
be no problem at all.

I would appreciate any kind of comment on this, positive or negative.



Dr. J. Hartmann
Department of Geography
University of Amsterdam

On 03/27/2013 10:13 PM, John Twilley wrote:
> I'm interested in this feature request as well.  Adding the mode 
> resampling algorithm to gdalwarp would be very beneficial to my 
> projects, right up there with being able to access the warp API from 
> Python.  Is this at all possible?  Should I submit a feature request 
> on Trac, or what?  Just let me know!
> Jack.
> -- 
> mathuin at gmail dot com
> On Mon, Feb 18, 2013 at 8:17 AM, Jan Hartmann <j.l.h.hartmann at uva.nl 
> <mailto:j.l.h.hartmann at uva.nl>> wrote:
>     Hi devs,
>     Would it be possible to add gauss and other interpolations to
>     gdalwarp? At the moment I georeference large scans to 2000*2000
>     tiles at the most detailed scale, and then create 2000*2000 tiles
>     at resolutions of 2, 4 6 etc  times the original scale, using
>     gdaladdo and gauss or other interpolations. It would help
>     immensely if I could do that directly with gdalwarp.
>     Funding would probably no problem. The question is: can and should
>     it be done?
>     Jan
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