Aha, cool, I was not aware of that. I am using SRTM DEM data, so subsampling is not an option, I need full res. I should check if they are inner tiled (although I don't assume so). Is that something that gdal can help me with, to perform the tiling? Any hint welcome.
Thanks for bearing with me! M. On 5/5/2010 10:16 AM, Andrea Aime wrote: > Martin Tomko ha scritto: >> Thanks Andrea, >> I really don't know enough how the mechanisms work behind the scenes, >> hence my question. >> I understand that if you have a tiled source of coverage, only the >> relevant tiles can be read. (btw, I only played with tiled tiffs a >> long time ago - would have been 2.4 or so, and had troubles. Is there >> a nice snipped of code available somewhere, showing how to start with >> the GeoTIFF reader and an index shapefile?) > > You're talking about mosaics there. TIFFs can be inner tiled. > >> But is it really so that there is no way to read in only the >> appropriate portion? >> Imagine a tiff+world fiel, or a geotiff - IFF we could read just the >> "metadata", we can reconstruct the Envelope (in the latter case even >> referenced envelope), and do an intersect operation with any other >> envelope, for instance coming from a geometry. The result of the >> intersection could then be transformed into the grid coordinates of >> the coverage (knowing the dimensions and the starting pixel), and >> only read these in, with the new envelope set. > > No, it's not possible. Coverages are read by blocks. > A PNG/JPEG is laid out so that you have to uncompress the whole to > read, so it's not possible to do any cropping while reading. > A inner tiled TIFF instead is structured so that the file is split into > smaller blocks, and you read those instead (but each tile, you read > fullh). > If it's not tiled it's usually striped, so you can read one row at a > time (and afaik, the row is the minimum unit that we can read). > That said, assuming it's not compressed, if it is, it's just like > a PNG/JPEG. > > When tiles/stripes are not compressed in theory it should be possible > to read only parts of it, not sure what happens in that case (e.g., > I know we can set subsampling, which will read one pixel out of > every n as a cheap way to reduce the resolution, but not sure > what happens about the ROI when reading). > > Cheers > Andrea > > ------------------------------------------------------------------------------ _______________________________________________ Geotools-gt2-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/geotools-gt2-users
