[gdal-dev] Optimisation of ECW files

[Yves Jacolin (free)]

Hello,

Le mardi 8 novembre 2011 20:49:33, Frank Warmerdam a écrit :
> 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
> 
> Yves,
> 
> 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.

I used gdal_merge.py from a vrt file. I will test to assign a ECW coordinate 
system.

> > * 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.
ok it was my conclusion also but I was not sure.

Thanks,

Y.
-- 
Yves Jacolin