Interesting!
Just found that you can override the GDAL behaviour of adding alpha
interpretation (this is the default as described in the GTiff format
spec here <http://www.gdal.org/frmt_gtiff.html>) by adding the GDAL -co
command PHOTOMETRIC=RGB. Not sure how this tallys with the YCBCR colour
model used by JPEG though?
Cheers,
Andy
On 02/12/2013 16:30, Jonathan Moules wrote:
Hi Andy,
I guess that makes sense.
Relating to gdalwarp:
- Output files by default are larger than gdal_merge.
- But they can be much smaller. You have to set *both* -wm and
--config GDAL_CACHEMAX - if you only set -wm, then the file is
actually larger!
- gdal_merge seems to do something that results in some heavy blurring
when using -co PHOTOMETRIC=YCBCR - this doesn't happen with gdalwarp.
So the optimal filesize for an aerial photograph is rendered with
something like:
gdalwarp -of GTiff -wm 9999 --config GDAL_CACHEMAX 9999 -co
TILED=YES -co BIGTIFF=YES -co COMPRESS=JPEG -co JPEG_QUALITY=80
-co BLOCKXSIZE=512 -co BLOCKYSIZE=512 -co PHOTOMETRIC=YCBCR
input1.tif input2.tif output.tif
I've not tried the four-band stuff again; just trying to optimise by
3-band.
Thanks,
Jonathan
On 2 December 2013 16:10, Andrew Harfoot <[email protected]
<mailto:[email protected]>> wrote:
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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University of Southampton
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