[Qgis-developer] Slow raster loading

[Tim Sutton]

Hi

On Mon, Jan 30, 2012 at 5:07 PM, Giovanni Manghi
<giovanni.manghi at gmail.com> wrote:
> Hi all,
>
> On Mon, 2012-01-30 at 13:16 +0100, Paolo Cavallini wrote:
>> Hi all.
>> Currently, loading coloured rasters with stretching to minmax on is extremely slow
>> (almost unusable):
>> http://hub.qgis.org/issues/4421
>> E.g. I could load a large RasterLite in <1 sec instead of minutes (!).
>> The trick, from the user point of view, is to disable stretching by default, and
>> enable it only for rasters where the option makes sense (e.g. a monoband, greyscale).
>> Obviously we could do it better, disabling the stretch for the rasters where it does
>> not make sense.
>> In addition, Giovanni noticed several other small bugs.
>
>
> my notes are in the ticket.
>
> There a few bugs (see ticket) and a couple of things need to be changed:
>
> *) computing of stats takes a long time, it is easy to see that also
> with not that big rasters it takes a lot, especially if multi-band,
> hurting the user experience. The proposal is to make computing of stats
> optional.
>

Computing stats should be a once off event since I adapted the gdal
provider to use gdal native stats. Is your experience on first load or
on subsequent loads too?

I am not sure it is so easy to avoid collecting stats. We used to have
some logic that set stats to be only collected when they were needed,
I'll need to check if that is still the case or not (but I suspect
not).

> *) in the raster properties there are two properties ("contrast
> enhancement" and "standard deviation") that have a button to save the
> option in a way to allow the setting be persistent among qgis sessions.
> This obviously should be moved to the general options, allowing to
> override at a layer level.

That makes sense to me.

I would suggest that we wait until after Marco's resampling branch is
merged before looking at these things as many parts of raster
implementation are changing with it.

Regards

Tim

>
> cheers
>
> -- Giovanni --
>
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