ECW -- ERMapper Compress Wavelets (.ecw)
GDAL supports .ecw format for read access and write. The current
implementation reads any number of bands but returns only as eight bit
image data. Georeferencing transform information is read, but projection
and datum definitions are ignored.
Caveats and Limitions
- Windows Only: Currently it appears that the ECW SDK is avalable
only for Windows, so ECW support in GDAL is only available on windows.
- Compilation Optional: Inclusion of ECW support in GDAL is a
build time option, and disabled by default. GDAL for windows will not
support ECW if it was not configured in during the build of the GDAL
librarys (controlled by uncommenting and adjusting the lines starting with
ECW in the gdal\nmake.opt file).
- Extra DLLs Required: To work, the ECW support requires that
the ECW SDK DLLs NCSEcw.dll, NCSEcwC.dll, NCSUtil.dll and NCScnet.dll be
somewhere in the path, for instance in the same directory as the GDAL DLL.
Creation Issues
The free ECW compression kit only supports compression of images up to
500MB. To compress very large images it is necessary to license ECW
technology from ERMapper. Files to be compressed into ECW format must also
be at least 128x128. Non-eight bit source data will be rescaled by
the ECW SDK in some manner that I don't understand ... the resulting image
is effectively eight bit.
While georeferencing information is written to the ECW file, the coordinate
system definition is currently not (this could be corrected).
Creation Options:
- TARGET=percent: Set the target size reduction as a percentage of
the original. If not provided defaults to 75 for an 75% reduction.
- BLOCKXSIZE=n: Width of blocks to use for compression. Must be
one of 64, 128, 256, 512, 1024, 2048. Default is 256.
- BLOCKYSIZE=n: Width of blocks to use for compression. Must be
one of 64, 128, 256, 512, 1024, 2048. Default is 256.
- BLOCKSIZE=n: Same as setting BLOCKXSIZE and BLOCKYSIZE.
ECW format does not support creation of overviews since the ECW format
is already considered to be optimized for "arbitrary overviews".
See Also: