On Sun, Oct 6, 2013 at 12:41 AM, Andrea Aime <[email protected]> wrote: > On Sat, Oct 5, 2013 at 4:21 PM, Frank Warmerdam <[email protected]> wrote: >> >> Great advice from Jukka, and I'd just note that GDAL >> *will* efficiently pull out reduced resolution images on >> the fly without overviews if you have a file organized >> with a one scanline block even in the year 2013. For >> map serving you are still much better off for several >> reasons with the tiles+overviews approach, but I do >> love the properly that a one-block-scanline file can be >> quickly subsamled by viewer applications like OpenEV >> and QGIS. > > > Hi Frank, > this is quite interesting. Can you elaborate a bit of what "one block > scanline" means?
Andrea, It means using a strip organized file with the strip one row long. This can be accomplished with the BLOCKYSIZE creation option as mentioned by Jukka. > E.g., what is the actual tiling structure of this file? > Does presence of compression affect the ability to do the subsampling? Note that the sampling advantage is still only in the vertical. A high resolution horizontal scanline is still read and subsampled conventionally. It is not clear that compression will make any more difference than normal. > How about quality of the output? (e.g., is GDAL reading one pixel every N to > generate > the reduced resolution version?) Well, the normal GDAL subsampling rules apply which is nearest neighbour for simple windowed subsamples. So building overviews gives the advantage that you can apply a better rule, like averaging. Best regards, -- ---------------------------------------+-------------------------------------- I set the clouds in motion - turn up | Frank Warmerdam, [email protected] light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush | Geospatial Software Developer _______________________________________________ mapserver-users mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/mapserver-users
