[Qgis-user] Re: ecw problems

[Jukka Rahkonen]

Alister Hood <alister.hood at ...> writes:

> 
> > Date: Mon, 05 Dec 2011 10:40:32 +0000
> > From: Giovanni Manghi <giovanni.manghi at ...>
> > Subject: Re: [Qgis-developer] Re: [Qgis-user] Re: ecw problems
> > To: pieri70 <pietro.rossin at ...>
> > Cc: qgis-user at ...
> > Message-ID: <1323081632.2277.19.camel <at> sibirica>
> > Content-Type: text/plain; charset="UTF-8"
> > 
> > On Mon, 2011-12-05 at 01:15 -0800, pieri70 wrote:
> > > Hello  Jürgen
> > > which kind of compressed raster do you use instead of ECW (if you use
> > > any..??
> 
> Personally I'd try JPEG2000 (using openjpeg).  But you might need to build
openjpeg and gdal yourself :(
> 
> > try
> > 
> > GeoTIFF TILED=YES COMPRESS=JPEG QUALITY=75 and overviews.
> 
> Do you use JPEG compression for maximum compatibility?  I would have thought
you'd get much better results
> with something else.

JPEG-in-TIFF is by no means maximally compatible but it suits very well for
GDAL based systems, especially Mapserver. The result is not a JPEG image, 
it is a TIFF image but the image data inside TIFF is compressed with JPEG
algorithm. It
is not uncommon that tiff drivers cannot open such images but GDAL has no
problems.

It is essential to use JPEG compression with tiling. JPEG must be 
decompressed totally but if tiff file is split into tiles GDAL can 
pick just a correct set of tiles to be decompressed.

Jpeg compressed tiffs have about same file size as ECW and JPEG2000 with 
about the same visual quality but the artifacts are different. GDAL is 
handling jpeg-in-tiff images at least as fast as ECW images and much faster
than JPEG2000 images even with the fastest JPEG2000 driver which is usually
JP2KAK (Kakadu)or JP2ECW (ERDAS). They are not free. JP2MrSID a third
non-free driver but I have not used it. OpenJPEG driver JP2OpenJPEG is 
much better than Jasper driver JPEG2000 at least with big images.

For a desktop use I would guess that JPEG2000 with OpenJPEG driver is fast
enough. For a busy WMS server it is not so good choice.

I do not mean that JPEG2000 is slow by the nature. If used, for example, with
native Kakadu tools it is flaming fast. The problem is that JPEG2000 is very
complicated format and open source libraries are not yet as good as the best
commercial ones.

-Jukka Rahkonen-