[gdal-dev] Optimisation of ECW files

Frank Warmerdam warmerdam at pobox.com
Tue Nov 8 14:49:33 EST 2011

On Tue, Nov 8, 2011 at 2:40 AM, Yves Jacolin <yjacolin at free.fr> wrote:
> Here is my issues after merging all the tiles:
> * we lose extent in the final ECW file


You don't mention how you are merging the files, but generally speaking
GDAL should be able to capture the extents and coordinate system when
writing to ECW.  ECW coordinate system handling can be somewhat flaky
and it is possible that if the coordinate system mapping fails even the
extents are not written (or read) though I'm not sure about that.  You may
need to pick a specific ECW coordinate system and specify it with
creation options if this is the source of the issue.

> * we have some black background which is not what expected
> * we need to update this final tiles frequently, and so we need fast process :)
> ECW format doesn't manage nodata so do it mean that I can't have alphaband and
> so I have no solution to get another background color (white or transparent)?
> Does exist some flag to save extent in the final ECW ?
> Have you some advice in order to get what I want (alphaband, good perfomance)?
> I can change file format if needed. We though to use TIFF files with a merge of
> some of them to get bigger tiles. We can either merge all the tiles, and then
> merge new tile with this final files or create tileindex for the bigger tiles.

In theory you could maintain a separate alpha/mask file and associate
it using a wrapping VRT file around the ECW and alpha band.  But this
is a somewhat complicated approach and it may also result in ECW
reads being done in chunks for MapServer which could substantially
impact performance.

Also, in general, ECW is not updatable-in-place, so I don't see how
it could be suitable as a format for frequent updates.  You basically
would need to regenerate it from the source tiles for each update.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Software Developer

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