[gdal-dev] ECW Image Web Server support

[Frank Warmerdam]

Tyler Mitchell wrote:

> Hi guys,
> I must say I am very impressed with the support that has been built into
> gdal for ecw. They have a "web" format of this imagery and I was curious
> to try out gdalinfo and gdal_translate on it. Sure enough, it worked like
> a charm. I know after reading through the gdal documentation that image
> sources are not constrained to files, but I was blown away by the fact that
> this was built in here. I assume you need the ecw sdk loaded - I'm on
> windows 2000 so it works for me! Thanks a bunch.
> 
> FYI - Here's a sample of my syntax:
> gdal_translate -outsize 5% 5%
> ecwp://www.earthetc.com/images/medical/xray/hand.ecw test_ecw.tif
> 
> My only question is why can't I do:
> gdal_translate -of JPEG -outsize 5% 5%
> ecwp://www.earthetc.com/images/medical/xray/hand.ecw test_ecw.jpg
> 
> I get told:
> Size is 12753, 8689
> 0.ERROR 6: GDALDriver::Create() ... no create method implemented for this
> format.
> GDALCreate() failed.
> 
> Any ideas? I assume I need some jpeg library installed - any
> recommendations to where to get a win32 version if this is the case?


Tyler,

This problem isn't particularly specific to JPEG. Some file formats
(those that use the CreateCopy() approach) require as input an well
formed GDALDataset to be written to the newly created file. gdal_translate
only works with these files if no change to the input image organization
(ie. subsampling, selecting only some bands, etc) is done.

Once I complete the work on "virtual" datasets, I will modif gdal_translate
to create a virtual dataset for situations like this.

In the meantime, you could convert to a TIFF file, and then convert to
JPEG.

PS. There are a number of weaknesses with the ECW support still. Most notably
it doesn't have good projection support, and some I/O strategies can be
pretty inefficient. I did a better job on an ECW reader for a client, but
that work is proprietary so I haven't been able to retro fit it into GDAL
without reinventing it from scratch.

Best regards,

-- 
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