[Gdal-dev] GeoQuadTree - an open format for storinggeoreferencedimages

Frank Warmerdam warmerdam at pobox.com
Wed Nov 15 10:07:44 EST 2006

Jordi Gilabert Vall wrote:
> I only wanted a lossless compression, so I chose PNG format for tiles, 
> but many people also wants a lossless compression, so I'm going to 
> support two types of compression, PNG and maybe JPEG or JPEG2000. I like 
> the PNG format because with 4-bands PNGs (RGBA), the alpha channel 
> allows me to store non-data pixels. With JPEG format, I would 
> "sacrifice" a RGB color to represent non-data values. There is also the 
> need of supporting multi-band images, this could be accomplished with as 
> many PNGs as bands. These are small changes, I will try to make it on 
> the next release.
> On the other side, I think storage space is not the big problem 
> nowadays. I prefer more efficient algorithms in terms of CPU usage, than 
> efficient in terms of storage space. If someone prefer the reduction of 
> storage space, the wavelet compression approach could be preferred 
> (LizardTech's MrSID, ER Mapper's...). I want the freedom to choose a 
> simpler, efficient an FREE approach.
> GeoQuadTree handles a pyramid of overviews. When you create a 
> GeoQuadTree image, you specifiy the number of levels in the pyramid. 
> Every time you import a image into a GeoQuadTree, the utility builds the 
> pyramid of overviews (only updates the tiles affected upside in the 
> pyramid).


I would be interested in incorporating your geoquadtree gdal driver into
the mainline GDAL source tree, but I see that it is licensed under the GPL.
In the interest of simplicity of licensing I prefer to keep all the core
GDAL source tree to the MIT/X license already used.  So, unless you are
interested in licensing the driver in a GDAL compatible form, I think it
will need to stay as a plugin.

I would like to see the XML metadata file for geoquadtree format include
a coordinate system element with the coordinate system in Well Known Text
format.  An alternative might be in anything supported for SRS names in the
OGC WxS specifications (such like EPSG:4326 or the OGC URN specifications).

I also would like to see some flexibility in the format of the tiles.  At
the very least png, and jpeg.  JPEG2000, and TIFF might also be desirable.
TIFF I mention because of the flexibility of pixel data types, and
multispectral support.  Of course TIFF is a bit heavy for very small tiles.

You mentioned above about having to sacrifice an RGB color for a nodata
marker with jpeg format, but this doesn't really work because jpeg is so
lossy.  So it is essentially impossible to use nodata values with jpeg.

Overall I would have preferred to see a GeoQuadTree format that was also
a valid instance of the url view of a "standard tile cache" per all the
"tile cache standard" work being done by various folks.  That would be
super-cool, but perhaps not a high priority for you.

PS. A very nice professional job on the geoquadtree.org web site.  But it
lacks in ... humanity.  Perhaps an FAQ on who did the work and why?

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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