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This sounds more like a problem with a missing NODATA value rather
than transparency/alpha. As far as I can tell from the docs, there
is no means to specify the NODATA value in the gdal_retile command,
including the CO options. If the only place that true black occurs
is in the regions with no input, you could use gdal_edit.py to add a
NODATA value.<br>
<br>
On 5/3/2015 11:23 PM, Tom Wardrop wrote:
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Hi all,</div>
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color: rgb(0, 0, 0);">
<br>
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<div style="font-family: Calibri, sans-serif; font-size: 14px;
color: rgb(0, 0, 0);">
I retiled whole lot of imagery over the weekend using the
following gdal_retile command:</div>
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<div style="font-family: Calibri, sans-serif; font-size: 14px;">
<p style="margin: 0px; font-size: 12px; font-family: Courier;">gdal_retile.py
-v -r bilinear -levels 8 -ps 2048 2048 -s_srs EPSG:28355 -co
"TILED=YES" -co "ALPHA=YES" -targetDir ~/output ~/input/*.tif</p>
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<div style="font-family: Calibri, sans-serif; font-size: 14px;"><br>
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<div style="font-family: Calibri, sans-serif; font-size: 14px;">The
result was good until I realised all the areas that should be
transparent were instead black. It seems the problem is the
source imagery didn’t have an alpha channel; it didn’t need one
as there was no transparent data to represent. Because the
source imagery didn’t fill the entire extent, gdal_retile had to
create loads of transparent and partially-transparent tiles,
which of course wound up black.</div>
<div style="font-family: Calibri, sans-serif; font-size: 14px;"><br>
</div>
<div><font face="Calibri,sans-serif">I was under the impression
that "</font><font face="Courier"><span style="font-size:
12px;">-co ALPHA=YES</span></font><font
face="Calibri,sans-serif">” would instruct gdal_retile to
generate an alpha channel for all the generated tiles. Is this
a bug? If not, how do I otherwise instruct </font><font
face="Calibri,sans-serif">gdal_retile to generate an alpha
channel when required?</font></div>
<div><font face="Calibri,sans-serif"><br>
</font></div>
<div><font face="Calibri,sans-serif">I’m working around this by
using gdal_translate to re-save the source imagery with an
(empty) alpha channel, so gdal_retile now outputs
transparent tiles. </font></div>
<div><font face="Calibri,sans-serif"><br>
</font></div>
<div><font face="Calibri,sans-serif">Any feedback is appreciated.</font></div>
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<p class="MsoNormal" style="color: rgb(0, 0, 0); margin: 0cm 0cm
0.0001pt;">Cheers,</p>
<p class="MsoNormal" style="color: rgb(0, 0, 0); margin: 0cm 0cm
0.0001pt;">Tom</p>
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<o:p> </o:p></p>
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