[gdal-dev] Performance issues with gdal vrt

[Greg Coats]

Because JPEG2000 images inherently contain a pyramid of reduced  
resolution images, the Kakadu Software demo application kdu_expand,  
that is available for free for Win32, Mac OS X, Linux, Solaris,  
provides a very fast way to create reduced resolution overview images.  
For me, the kdu_expand -reduce operations typically take less than one  
second. Greg

kdu_compress -v
This is Kakadu's "kdu_compress" application.
     Compiled against the Kakadu core system, version v6.2.1.1
     Current core system version is v6.2.1.1
kdu_compress -i 18stj940125.tif -o 18stj940125.jp2 Creversible=yes

kdu_expand -v
This is Kakadu's "kdu_expand" application.
     Compiled against the Kakadu core system, version v6.2.1.1
     Current core system version is v6.2.1.1
kdu_expand   -i 18stj940125.jp2 -o 18stj940125_reduce1.tif -reduce 1
kdu_expand   -i 18stj940125.jp2 -o 18stj940125_reduce2.tif -reduce 2
kdu_expand   -i 18stj940125.jp2 -o 18stj940125_reduce3.tif -reduce 3
kdu_expand   -i 18stj940125.jp2 -o 18stj940125_reduce4.tif -reduce 4

On Sep 30, 2009, at 7:52 AM, Even Rouault wrote:

> As other drivers, the VRT driver is capable of using external  
> overviews
> (normally in GeoTIFF format, but that's not required...), so here's  
> a possible
> workaround, but that's probably not very practical to use unless you  
> use some
> scripting to do it.
>
> Let's suppose I've a file, test.tif, with a overview of factor 1/2
> 1) create the VRT : gdalbuildvrt test.vrt test.tif
> 2) copy test.vrt as test.vrt.ovr
> 3) edit test.vrt.ovr to divide by 2 the rasterXSize and rasterYSize  
> of the
> <VRTDataset> element, as well as the values of the attributes xOff,  
> yOff, xSize,
> ySize of each <DstRect> element
>
> If you need overview of factor 1/4, copy test.vrt.ovr as  
> test.vrt.ovr.ovr and
> repeat step 3)... A bit of cheating, but seems to work here.