Changes between Initial Version and Version 1 of UserDocs/ReadInZip

Timestamp:

Sep 8, 2008, 12:01:34 PM (16 years ago)

Author:

Even Rouault

Comment:

--

UserDocs/ReadInZip

@@ +1 @@
+= Reading a GDAL dataset in a .gz file or a .zip archive = 
+
+== Summary ==
+
+From GDAL 1.6.0, it is possible to access a GDAL dataset inside a compressed archive and read it on-the-fly.
+The archive formats that are handled are single gzip'ed file (ending with .gz) and ZIP archives (ending with .zip)
+
+This is implemented as 2 virtual file systems (like /vsimem for 
+example), /vsigzip and /vsizip. Only read-only access is supported.
+Note that performance will not be very impressive, as random access inside gzip data
+is slow by nature, although some optimizations have been made to make it 
+faster and generally usable ("snapshots" of the gzip state are taken from time to time, so
+that further access to a given offset inside the file just needs to restart decompression
+from the nearest snapshot)
+
+Note that .tar.gz archives are not supported, and there's no plan to support them
+as it is a very inappropriate format for seeking.
+
+== How to use that capability with a gzip file ? ==
+
+For example :
+{{{
+gdalinfo /vsigzip/path/to/the/file.gz
+}}}
+were path/to/the/file.gz is relative or absolute. If the path is absolute, it should begin with a /, so the line looks like /vsigzip//home/gdal/...
+
+The first time that a .gz file is read, a small .gz.properties file will be 
+generated (if possible) to capture the uncompressed data size. This will make 
+following opening of that dataset much faster.
+
+A VSIStatL("/vsigzip/...") call will return the uncompressed size of the file.
+
+== How to use that capability with a ZIP archive ? ==
+
+{{{
+gdalinfo /vsizip/path/to/the/file.zip/path/inside/the/zip/file
+}}}
+
+were path/to/the/file.zip is relative or absolute and path/inside/the/zip/file is 
+the relative path to the file inside the archive.
+
+For example gdalinfo /vsizip/myarchive.zip/subdir1/file1.tif
+
+The ReadDir() method is implemented for the .zip archives, so a driver will be 
+able to find files relative to the given file inside the archive. For 
+example, you can read a CADRG dataset from the zipped archive of the a.toc 
+file and all its NITF tiles.
+
+A VSIStatL("/vsizip/...") call will return the uncompressed size of the file.
+Directories inside the ZIP file can be distinguished from regular files with
+the VSI_ISDIR(stat.st_mode) macro as for regular file systems.
+
+Small syntaxic sugar : if the .zip file contains only one file located at its 
+root, just mentionning "/vsizip/path/to/the/file.zip" will work.
+
+== Drivers supporting that capability ==
+
+The fact that this new capability is implemented as virtual file systems imply 
+that it will only work for GDAL drivers supporting the "large file API". A 
+list of such drivers is : PNG, JPEG, ILWIS, GTiff, GIF, JP2KAK, NITF, ADRG, 
+DTED, SRTMHGT, BMP, LCP, HFA (Erdas Imagine), AAIGRID. Other drivers may work 
+too (I just looked for those advertizing the GDAL_DCAP_VIRTUALIO capability)