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 -- Andrea Aime OpenGeo - http://opengeo.org Expert service straight from the developers. ------------------------------------------------------------------------------ _______________________________________________ Geotools-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/geotools-devel
