[gdal-dev] Optimisation of ECW files

Even Rouault even.rouault at mines-paris.org
Tue Nov 8 14:36:06 EST 2011

Le mardi 08 novembre 2011 11:40:33, Yves Jacolin a écrit :
> Hello,
> I have an issue with one of my project which need some frequent update and
> I need some advice.
> We have a dataset of small ECW files (5 300 files from 30 to 300 ko). This
> is not the way I want to manage my data to get good performance with
> MapServer. Too much file, too smal.

You didn't really say what was the problem with many small files... Why not 
putting them in a tileindex, possibly with an additionnal .qix spatial index ?

That will be the by far the easier solution to manage the update which is a 
big constraint in your process.

If you have specific performance problem with the ECW format you could probably 
translate those individual files into anothe format (uncompressed tiff, tiff with 
jpeg compression for example)

> All this tiles don't form a regular single tile. I mean some part are hole.
> Here is my issues after merging all the tiles:
> * we lose extent in the final ECW file
> * 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.
> Any hints, advices?
> Thanks!
> Y.

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