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 <[email protected]
<mailto:[email protected]>> 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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