Jorge Arévalo wrote: > Hello, > > 2009/8/12 Mateusz Loskot <[email protected]>: >> Frank Warmerdam wrote: >>> 2009/8/9 Jorge Arévalo <[email protected]>: >>>> Then, my point could be, when creating the dataset (Open static >>>> method), perform a query to get all the blocks that cover the >>>> extent of the raster (the extent is in RASTER_COLUMNS table) and >>>> store them in any place (A new class attribute?, flush the data on >>>> a tmp file?). Then, override IRasterIO in Dataset to get the data >>>> fetched in the extent query, instead of perform a new call. And >>>> override the RasterBand::IReadBlock method too. >>>> >>>> It will supppose a change of perspective, but it will improve the >>>> performance, of course. >>> Jorge, >>> >>> I'm afraid I have not quite followed the discussion of the IRasterIO >>> implementation, but I find the above statement quite alarming. It is >>> imperative that we *not* end up reading back all blocks if only a few >>> are requested. Imagine a 100GB image split up into blocks in a >>> table. If an application using GDAL is run and only needs a small >>> area of the file it is important that the blocks not requested never >>> be pulled from the DB. >> I can only second Frank's comment. >> IOW, general idea is to query minimum (sub)set of tiles requested, >> where request is based on spatial extent (i.e. geographic coordiantes) >> or extent within pixel space of a coverage (specified as range of rows >> and columns of a raster). >> > > OK. Then, and correct me if I'm wrong, the point is to query only the > extent asked by the program using GDAL driver. Then, I should query > only the blocks requested in, for example, the IReadBlock method, just > as I did. But Tamas' suggestion was to minimize the number of server's > rounds implementing a kind of cache system at driver's level.
My comment was related to similar use case as gdal_translate and its -srcwin and -projwin parameters But, I may be missing some of Tamas' points as I've not been tracking the discussion closely enough. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
