Jan,
I can't speak the development request, but you may be able to do some
sharpening using the <Kernel> object in a VRT. I've used
<Coefs>-0.111 -0.111 -0.111 -0.111 2 -0.111 -0.111 -0.111 -0.111</Coefs>
to apply sharpening to satellite imagery.
And a few years ago I found that using wavelet compression on
scanned topographic maps when downsampling made the text more readable
(preserves edges more). I used ECW back then, but I suspect JPEG2000
may do it as well...
Best Regards,
Brent Fraser
On 3/28/2013 5:27 AM, Jan Hartmann wrote:
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:
http://mapserver.sara.nl/topo/triang/
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.
Regards,
Jan
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 <[email protected]
<mailto:[email protected]>> 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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