[Qgis-user] Four band raster displaying funny

[Andrew Harfoot]

Hi Jonathan,

Is it possible that QGIS is using the infrared band as an alpha channel? 
This might explain the whitening effect, especially in non-vegetated 
areas. I would check the Layer properties Transparency tab to see 
whether anything is set as the transparency band. If it is, you could 
override QGIS by setting a default style.

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
Fax:  +44 (0)23 8059 2849

www.geodata.soton.ac.uk

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