<div dir="ltr"><div class="gmail_extra"><div>Hi Andy,</div><div>I guess that makes sense.</div><div><br></div><div>Relating to gdalwarp:</div><div>- Output files by default are larger than gdal_merge.</div><div>- 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!</div>
<div>- gdal_merge seems to do something that results in some heavy blurring when using -co PHOTOMETRIC=YCBCR - this doesn't happen with gdalwarp.</div><div><br></div><div>So the optimal filesize for an aerial photograph is rendered with something like:</div>
<div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">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</blockquote>
<div><br></div><div>I've not tried the four-band stuff again; just trying to optimise by 3-band.</div><div><br></div><div>Thanks,</div><div>Jonathan</div>
<br><br><div class="gmail_quote">On 2 December 2013 16:10, Andrew Harfoot <span dir="ltr"><<a href="mailto:ajph@geodata.soton.ac.uk" target="_blank">ajph@geodata.soton.ac.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div>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!).<br>
<br>
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 :(<br>
<br>
Cheers,<br>
<br>
Andy<div><div><br>
<br>
On 02/12/2013 16:00, Jonathan Moules wrote:<br>
</div></div></div><div><div>
<blockquote type="cite">
<div dir="ltr">Hi Andy,
<div>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. :-)<br>
<div class="gmail_extra">
<div><br>
</div>
<div>====</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Many thanks!<br>
Jonathan</div>
<br>
<br>
<div class="gmail_quote">On 2 December 2013 15:48, Andrew
Harfoot <span dir="ltr"><<a href="mailto:ajph@geodata.soton.ac.uk" target="_blank">ajph@geodata.soton.ac.uk</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div>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!
<div><br>
<br>
Cheers,<br>
<br>
Andy<br>
<br>
On 02/12/2013 11:53, Jonathan Moules wrote:<br>
</div>
</div>
<blockquote type="cite">
<div>
<div>
<div dir="ltr">Hi List,
<div>I've got a 4 band raster aerial
photography (RGBI) that comprises lots of
tiles. I've merged some of the tiles
together with:</div>
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">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</blockquote>
<div><br>
</div>
<div>But the resultant file looks funny in
QGIS.</div>
<div>This is what the source file looks like
(correct):</div>
<div><img alt="Inline
images 1"><br>
</div>
<div><br>
</div>
<div>This is what the merged file looks like
(wrong):</div>
<div><img alt="Inline
images 2"><br>
</div>
<div><br>
</div>
<div>All the shadows are a whitey colour. This
doesn't happen with 3-band (RGB) images.</div>
<div>I've tried comparing individual bands;
they all look different in the 4-band.</div>
<div><br>
</div>
<div>However, if I open the four-band in
ArcGIS, it looks fine (both source and
original).</div>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>Thanks,</div>
<div>Jonathan</div>
</div>
<br>
</div>
</div>
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