[Qgis-developer] Slow raster loading

[Giovanni Manghi]

Hi Tim,


> > *) 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?


subsequent loads are fast, with the exception of GRASS rasters, probably
because stats are not written anywhere.

computing rasterlite stats is very slow if compared with all other
formats.

I agree with others that when computing stats should at least show a
progress bar, but (also) having them computed optionally would be even
better.

When stretching is active standard deviation is enabled is set to 2 by
default. In my experience this is causing to render rasters with
"strange" colormaps, while a smaller value (1.7) seems to give better
results.



> 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.


ok


cheers

-- Giovanni --