[Qgis-user] Four band raster displaying funny

[Andrew Harfoot]

I think QGIS is innocent in this - if a band is set as an alpha channel 
then it should be handled as such by default in a viewer (so mark down 
Arc for not using the alpha information!).

GDAL is the culprit as it is adding the alpha interpretation without 
being prompted. I have just replicated this with some RGBI imagery 
myself: prior to passing through GDAL's hands the IR band is present, 
but isn't interpreted as an alpha channel. I can't get the -setci switch 
to do anything though :(

Cheers,

Andy

On 02/12/2013 16:00, Jonathan Moules wrote:
> Hi Andy,
> Yep, that was it. I didn't know QGIS could do that; another good 
> example of software trying to be "smart" and confusing the poor user. :-)
>
> ====
>
> I didn't know gdalwarp could do mosaicing too. I'll have to test it. 
> I'll ask on the gdal list if I want to try the -setci parameter.
>
> Many thanks!
> Jonathan
>
>
> On 2 December 2013 15:48, Andrew Harfoot <ajph at geodata.soton.ac.uk 
> <mailto:ajph at geodata.soton.ac.uk>> wrote:
>
>     PS. gdalwarp offers more flexibility when mosaicing rasters, and
>     is better at memory management. I have just noticed that in GDAL
>     1.10 and above there is an gdalwarp option -setci that 'Sets the
>     color interpretation of the bands of the target dataset from the
>     source dataset'. This could be used to remove the assignment of
>     the alpha channel to the IR band on merging. Sadly there isn't an
>     example of its usage!
>
>
>     Cheers,
>
>     Andy
>
>     On 02/12/2013 11:53, Jonathan Moules wrote:
>>     Hi List,
>>     I've got a 4 band raster aerial photography (RGBI) that comprises
>>     lots of tiles. I've merged some of the tiles together with:
>>
>>         gdal_merge -o 1.tif -of GTiff -co TILED=YES -co BIGTIFF=YES
>>         -co COMPRESS=JPEG -co JPEG_QUALITY=50 -co BLOCKXSIZE=512 -co
>>         BLOCKYSIZE=512 --optfile tiff_list.txt
>>
>>
>>     But the resultant file looks funny in QGIS.
>>     This is what the source file looks like (correct):
>>     Inline images 1
>>
>>     This is what the merged file looks like (wrong):
>>     Inline images 2
>>
>>     All the shadows are a whitey colour. This doesn't happen with
>>     3-band (RGB) images.
>>     I've tried comparing individual bands; they all look different in
>>     the 4-band.
>>
>>     However, if I open the four-band in ArcGIS, it looks fine (both
>>     source and original).
>>
>>     Anyone know what's going on? Is it a QGIS bug or is it doing
>>     something "smart"; I can't see anything odd going on with symbology.
>>
>>     Thanks,
>>     Jonathan
>>
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>
>     -- 
>     Andy Harfoot
>
>     GeoData Institute
>     University of Southampton
>     Southampton
>     SO17 1BJ
>
>     Tel:+44 (0)23 8059 2719  <tel:%2B44%20%280%2923%208059%202719>
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>
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>
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-- 
Andy Harfoot

GeoData Institute
University of Southampton
Southampton
SO17 1BJ

Tel:  +44 (0)23 8059 2719
Fax:  +44 (0)23 8059 2849

www.geodata.soton.ac.uk

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