[Qgis-user] Re: GRASS raster reading to QGIS 1.0.0 Kore

[Micha Silver]

Patrick Giraudoux wrote:

> Martin Dobias a écrit :
>> 2009/1/4 Patrick Giraudoux <patrick.giraudoux at univ-fcomte.fr>:
>>   
>>> Seems that I can narrow the question now. Actually, QGIS Kore reads GRASS
>>> maps well except when the maps  have been created from raster ramp color
>>> have been optimized with i.landsat.rgb. For example in GRASS:
>>>
>>> r.composite r=etm7 g=etm5 b=etm2 out=south752brut
>>>
>>> makes a map 'south752brut' which is well read within QGIS
>>>
>>> Now if I do:
>>> i.landsat.rgb r=etm7 g=etm5 b=etm2
>>> r.composite r=etm7 g=etm5 b=etm2 out=south752opt
>>>
>>> it makes a map 'south752opt' which is extremly long to read (several
>>> minutes), and when read makes change (displacement, zoom, etc...) extremely
>>> long to be displayed at a speed that prevent any work
>>>
>>> However, 'south752opt' and 'south752brut' are quick/normally displayed both
>>> at the same speed when on work within GRASS (e.g. d.rast south752opt)
>>>
>>> Any hint ?
>>>     
>>
>> Hi Patrick,
>>
>> we're using GDAL library to access GRASS rasters, so the problem may
>> be there. I would suggest you to try reading the raster in some other
>> GDAL-based software [1] to see whether it also suffers slow
>> performance. If so, file a bug for GDAL, if the problem happens only
>> in QGIS file a ticket for QGIS with details.
>>
>> Martin
>>
>> [1] https://trac.osgeo.org/gdal/wiki/SoftwareUsingGdal
>>
>>   
> Ok. I will try and come back with the result ASAP.
>
> Additionally before your mail I have made some other trials that may 
> (?) help to understand what happens... Looks like corroborating your 
> suspicion about bad interactions between rgdal and qgis. When I export 
> the file using r.out.gdal format=GTiff and then try and read the tif 
> file externally from QGIS, I have the same (bad/slow) result as when 
> reading the grass file from QGIS within GRASS. Then, if I export the 
> GRASS file with r.out.tiff I get a file which is easily read from QGIS 
> and correctly (quickly) displayed as a three band tif object (as an 
> external raster, of course). Furthermore, what surprises me is that 
> when I read a raster GRASS file created with r.composite within QGIS, 
> I get a 1-grey band file  with "color palette" ("palette de couleur" 
> in French) activated in the "properties" box of the layer (however, I 
> was more expecting a three band rgb).
>
> Do those info put you a bit more clearly on the track ?
>
> Thanks for your ideas and advice to where to adress the trouble...
>
> Patrick
>
Some months ago there was discussion on this list pointing to two 
possible reasons for slow rendering of rasters:
http://www.mail-archive.com/qgis-user@lists.osgeo.org/msg00813.html
One was tiled rasters. For some reason it seemed like rasters with tiles 
(created with gdal_translate -co "TILED=YES") rendered somewhat slowly.
The second problem was incorrect CRS headers in the raster. Setting the 
correct projection header with gdalwarp caused tiffs to render faster.
I don't know if this is relevant to your case. (Does i.landsat.rgb set 
CRS headers??) But it's worth checking.
-- 
Micha
 
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