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<div>It would be really nice if I could find a single pass solution for this. I need to retile 200GB of imagery and would prefer not to have to process it twice. I think my work-around using gdal_translate is a better solution than converting black pixels to
alpha, besides, I’m using GeoServer with the ImagePyramid plugin which I don’t believe supports “nodata” transparency. It needs an alpha channel.</div>
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<b><span style="font-size: 10pt; font-family: Arial, sans-serif;">Tom Wardrop</span></b><span style="font-size: 8pt; font-family: Arial, sans-serif;"><br>
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<span style="font-size: 10.5pt; font-family: Webdings; color: green;">ý</span><span style="font-size: 7pt; font-family: Arial, sans-serif; color: green;"> Go green, keep it on screen - think before you print<o:p></o:p></span></p>
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<span style="font-weight:bold">From: </span>David Strip <<a href="mailto:gdal@stripfamily.net">gdal@stripfamily.net</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, 5 May 2015 12:49 am<br>
<span style="font-weight:bold">To: </span>Tom Wardrop <<a href="mailto:tomw@msc.qld.gov.au">tomw@msc.qld.gov.au</a>>, "<a href="mailto:gdal-dev@lists.osgeo.org">gdal-dev@lists.osgeo.org</a>" <<a href="mailto:gdal-dev@lists.osgeo.org">gdal-dev@lists.osgeo.org</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [gdal-dev] gdal_retile not generating alpha band<br>
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<div bgcolor="#FFFFFF" text="#000000">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>
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On 5/3/2015 11:23 PM, Tom Wardrop wrote:
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Hi all,</div>
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I retiled whole lot of imagery over the weekend using the following gdal_retile command:</div>
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<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;">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>
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<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>
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<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>
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<div><font face="Calibri,sans-serif">Any feedback is appreciated.</font></div>
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Cheers,</p>
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Tom</p>
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