[GRASS-dev] Problems with r.out.gdal byte export: colors modified in RGB multilayer GeoTIFF

[Vincent Bain]

I recently experienced some buggy image color table management by Qgis,
did not dig much deeper. Sorry if my case may be the exact inverse of
yours, but I finally did it to obtain a more "faithful" color rendering:

- composited an initial 3-band image
	r.composite [...]
- then exported it with
	r.out.gdal [...] format=GTiff type=UInt16
- and expanded it again to a 3-band rgb image via :
	gdal_translate -expand rgb [...]


Anyway, since Qgis Lyon, we have some trouble wih image color
management.

Yours,
V.

Le mercredi 10 février 2016 à 10:05 +0100, Markus Neteler a écrit :
> Hi,
> 
> I am currently processing satellite data > 8bit and created a nice
> "natural color" RGB composite (channels separated).
> Using the #map operator of r.mapcalc I could successfully reduce each
> channel to 8 bit, after i.colors.enhance the respective RGB composite
> looks almost identical.
> The scope is to process the data in GRASS GIS, then export them for a
> performant OGC Web service.
> 
> So far so nice. Now, exporting it as a multilayer GeoTIFF (I made a
> group of the 3 channels using i.group, then gave that as input to
> r.out.gdal) leads to yellowish colors which are not the same any more.
> Adding the -c flag did not help.
> 
> GRASS 7.1.svn (utm48n): > r.out.gdal -c input=rgb_group
> output=rgb_8bit_flag_c.tif
> Checking GDAL data type and nodata value...
>  100%
> Using GDAL data type <Byte>
>  100%
> Using GDAL data type <Byte>
>  100%
> Using GDAL data type <Byte>
> Exporting raster data to GTiff format...
>  100%
>  100%
>  100%
> r.out.gdal complete. File <rgb_8bit_flag_c.tif> created.
> [Raster MASK present]
> 
> Screenshot attached (top left: original; top right: reduced to 8 bit;
> low: exported multilayer-RGB shown in QGIS).
> 
> I also tried the HFA driver, same issue.
> 
> What's the trick?
> 
> thanks
> Markus
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