[Gdal-dev] Re: single ECW output from multiple input tiles

[Frank Warmerdam]

On 5/14/05, Ken Boss <Ken.Boss at dnr.state.mn.us> wrote:
> Many thanks, this is very nearly exactly what I was looking for.  vrt
> datasets feed nicely into gdal_translate for outputting to ecw.  My only
> remaining obstacle is ensuring that valid image data doesn't get
> overwritten by nodata values from overlapping inputs.
> 
> Reading at
> http://xserve.flids.com/pipermail/gdal-dev/2004-September/007053.html,
> I got the idea that I could add a <NoDataValue>0</NoDataValue> tag to
> the VRTRasterBand sections of the .vrt file in order to address this
> issue, but alas, this seems to have no effect, at least when translating
> to ecw.  I was hoping that maybe a source nodata value could be passed
> as an ECW creation option, but no such luck.

Ken,

In VRT there are "SimpleSources" and "ComplexSources".  The
complex sources include support for nodata values that are not
copied, and for on the fly scaling.   I *think* you would setup
a complex source similarly to a simple source except that you
use the element <ComplexSource> instead of <SimpleSource> and
the complex source can have NODATA, ScaleOffset and ScaleRatio
tags for which the values are used during image processing.

If you can't get this to work let me know, and I will prepare an example.

I would warning that .vrt's have degrading performance as the number
of sources goes up, so they aren't a great way of managing a mosaic
of hundreds of images. 

Best regards,
-- 
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