Hi Frank, I didn't know about that predefined palette, because that is
just what I was looking for! If it's not too much trouble to add the
non-dither option, you'll have solved again a problem of mine.
A late merry Christmas from a white Amsterdam, although the snow is
melting here.
Jan
On 26-12-2009 17:51, Frank Warmerdam wrote:
Jan Hartmann wrote:
Hi,
I put the following question on the GRASS user list. It isn't
possible with GRASS (or the programs mentioned by Glynn), but if
someone on this list can help me further, I would be very grateful. I
have been struggling for some time with this problem, and I'll have
to process lots of scanned old maps in the near future.
Jan,
The rgb2pct.py script can be used to downsample 24bit images to 8bit
paletted form. I see it now supports providing a predefined palette.
It might be best to pick a representative map, and use it for the first
downsampling - restricting the number of colors with the -n flag - and
then use the resulting palette for the other maps.
If the discoloring is not too severe, and if you restrict the palette
to a very distinct set of colors, you might find this reduces things to
a sufficiently similar set of images.
Doh! I just looked and rgb2pct.py always does dithering which means
it will end up using speckled pixels of the available colors to
try and approximate the original. This will be quite unsuitable for
your purposes.
If you would like, I'd be willing to expose a non-dithered option in
rgb2pct.py which might be useful.
Note that if the discoloring is too severe some additional sorts of
image processing might be required to get a good result.
Best regards,
> I am working with series of old topographical maps, ranging from
10 to > 600 map sheets per map. Each sheet is discolored a bit from
age. How can > I process a complete series in such a way that there
are no differences > visible any more between map sheets? Preferably
I would like to > downsample them to 8 bits color table, as the
originals only have a > small set of colors. In other words: how do
I calibrate set of maps to > the same optimal color table?
This is probably better done with general image-processing software,
e.g. pnmquant (from the Netpbm package) or "convert -colors ..." (from
ImageMagick).
-- Glynn Clements <gl...@gclements.plus.com>
------------------------------------------------------------------------
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
