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<div class="moz-cite-prefix">Le 06/10/2022 à 07:55, thomas bonfort a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAOM3y2igs=64FUS5vZx-LcQQO2hy_CBoeHhTnK6EKy+XMdk+9Q@mail.gmail.com">
<div dir="auto">
<div>There are only very specific cases where it would be valid
to directly copy overview tiles, namely if both image sizes
are an exact power of 2. I'll let Even contradict me if I'm
wrong, but I highly doubt that what you are trying to do is
possible.</div>
</div>
</blockquote>
<p>If your input files have the same overview levels and resolution,
gdalbuildvrt should already build a VRT with the
<OverviewList> element listing those overview levels</p>
<p>Demo with a file from GDAL autotest suite<br>
</p>
<p>$ gdal_translate autotest/gdrivers/data/small_world.tif left.tif
-srcwin 0 0 200 200 -outsize 1024 0 -of COG<br>
$ gdal_translate autotest/gdrivers/data/small_world.tif right.tif
-srcwin 200 0 200 200 -outsize 1024 0 -of COG<br>
$ gdalbuildvrt out.vrt left.tif right.tif<br>
$ grep Overview out.vrt<br>
<OverviewList resampling="nearest">2</OverviewList><br>
$ gdalinfo out.vrt | grep Overview<br>
Overviews: 1024x512<br>
</p>
<p>and that should be useable by the COG driver with -co
OVERVIEWS=FORCE_USE_EXISTING</p>
<p>Note however that the overview exposed by the VRT is a virtual
overview: so in the general case, there's no guarantee that the
values of the pixel exactly match those of the source overview,
depending on the spatial registration of each VRT source. If you
have a nice mosaic of adjacent sources with power-of-2 dimensions,
then yes, you're in the good situation.<br>
</p>
<p>But, if you were to produce a JPEG-compressed COG, don't expect
*lossless* copy of overviews in the case of JPEG compression for
target overviews: GDAL will get the decompressed values of the
JPEG compressed overviews of the sources and would re-compress
them. In the above mentioned perfect case, that could potentially
be implemented (like "gdal_translate input.jpeg output.tif -co
COMPRESS=JPEG" does currently), but it isn't and it would be a
very specific non-trivial code to write.<br>
</p>
<p>Even<br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:CAOM3y2igs=64FUS5vZx-LcQQO2hy_CBoeHhTnK6EKy+XMdk+9Q@mail.gmail.com">
<div dir="auto">
<div dir="auto">regards,</div>
<div dir="auto">Thomas<br>
<br>
<div class="gmail_quote" dir="auto">
<div dir="ltr" class="gmail_attr">Le jeu. 6 oct. 2022,
05:58, Ian Reese <<a href="mailto:xycarto@gmail.com"
moz-do-not-send="true" class="moz-txt-link-freetext">xycarto@gmail.com</a>>
a écrit :<br>
</div>
<blockquote class="gmail_quote">
<div dir="ltr">
<div class="gmail_default">Hi GDAL,</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">Let's say I have two GTiff
files in COG format:</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">f1.tif</div>
<div class="gmail_default">f2.tif</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">f1.tif and f2.tif are created
like so:</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">
<table>
<tbody>
<tr>
<td id="m_-3397547538372092727gmail-LC15">gdal_translate
input.tif <span>f1.tif</span> \</td>
</tr>
<tr>
</tr>
</tbody>
</table>
<table>
<tbody>
<tr>
<td id="m_-3397547538372092727gmail-LC16">-of
COG \</td>
</tr>
<tr>
</tr>
</tbody>
</table>
<table>
<tbody>
<tr>
<td id="m_-3397547538372092727gmail-LC17"> -co
BLOCKSIZE=256 \</td>
</tr>
<tr>
</tr>
</tbody>
</table>
-co COMPRESS=JPEG \</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">I can easily create a vrt of
these two COGs and get the expected behaviour where
the VRT honours the overviews of the f1 and f2 COGs.</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">gdalbuildvrt cog.vrt f1.tif
f2.tif</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">This is my question, is it
possible to create a COG Tif from this VRT that copies
over the overviews from each COG in the VRT?</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">For example something like:<br>
<br>
</div>
<div class="gmail_default">gdal_translate cog.vrt
cog.tif -of COG -co OVERVIEWS=FORCE_USE_EXISTING</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">I suspect this may not be
possible. My guess is that translate is confused by
which overviews to use when creating the single
cog.tif? Or that when I created f1.tif and f2.tif I
missed a setting?</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">I have tried many
combinations to get this to work, but I think it might
be time to ask the experts. Any insight to this would
be greatly appreciated.</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">Cheer,</div>
<div class="gmail_default"><br>
</div>
<div class="gmail_default">Ianto<br>
</div>
<div class="gmail_default">
<div>
<div><span></span></div>
</div>
</div>
<br>
-- <br>
<div dir="ltr" data-smartmail="gmail_signature">
<div dir="ltr">
<div>Ian C Reese</div>
<div><a href="https://xycarto.com" target="_blank"
rel="noreferrer" moz-do-not-send="true"
class="moz-txt-link-freetext">https://xycarto.com</a><br>
</div>
</div>
</div>
</div>
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My software is free, but my time generally not.</pre>
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