[gdal-dev] [Qgis-developer] Large raster (ecw) identify very long

[Radim Blazek]

On Wed, Aug 22, 2012 at 11:40 AM, haubourg
<regis.haubourg at eau-adour-garonne.fr> wrote:
> Hi all,
> I'm ready to support developpement now, so let's undig a topic discussed
> previously here:
> http://hub.qgis.org/issues/4594 http://hub.qgis.org/issues/4594
>
> Identify for very large ecw (from 1 to 100 Go - 1 080 000 * 1 090 000
> pixels) is extremely long (more than 1min) and results to memory leak.  Very
> often, this only gives a useless RGB info because ecw are orthophotos or
> topographic scans.
>
> This is a problem if identify mode queries all layers under mouse clic.
>
> 1 - Memory leak can be solved by disabling Gdal cache for ecw says Nathan.*
> Do you confirm ? Can you give more details so that I write a contract for
> it? *

QGIS is using GDALRasterIO() which reads a single pixel on original
resolution. AFAIK, ECW is using tiles internally so it should be all
very fast. I can imagine 2 problems:

- the tiles in ECW file are too big - can you verify somehow how big
are the tiles?

- GDAL is reading bigger portion of data than necessary (cc to GDAL list)

Is it drawing of the raster also so slow (on raster resolution zoom)?

Radim

> 2 - *I would also like to add a behaviour to handle very long response time,
> I would really like to have your feedback on it:*
>      - Option 1 : Disable identify for very large raster layers in project
> properties when raster is loaded. Keep QGS properties if raster is loaded
> when opening a project file. Should we add options to configure that in
> general options (" Disable very large raster identify " + "raster size
> threshold")
>      - Option 2 : Offer a checkbox in identify mode options  that could be
> named "disable raster identify"
>      - Option 3 : Considering it's a very particular problem for huge ecw
> users and let's make a plugin that deactivates identify for big raster.   I
> must say I don't vote for here because most french public user I know are
> using big ecw, and I think it might be harder and more expensive to do this
> from python.. (and adds a python plugin to load, adds a thing more to handle
> on enterprise config.. )
>
> *I vote for Option 1*
>
> Thoughts guys?
>
> (And welcome to Victor by the way, congratulations!)
>
>
>
>
> --
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