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<p>Martin,</p>
<p>it would be interesting if you could identify which change caused
the slow down you notice. Nothing jumps to mind. git bisect could
help (assuming this is only GDAL related, and not due to changes
in PROJ versions)</p>
<p>The value of the error threshold is in pixels. So 0.125 means you
accept up to a 1/8th pixel error, so yes, that's already quite
high quality. An error threshold of 0 is rarely needed, unless you
use RPC ortho-rectification with a DEM, especially in mountainous
regions.</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 07/06/2021 à 18:19, Martin Chapman a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:024801d75bb8$f4f5d2a0$dee177e0$@hyperacktive.com">
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<p class="MsoNormal">Even, Frank or anyone in the know about
gdal warp API,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I have been using the <span>GDALCreateGenImgProjTransformer
function to create my transformer arg in the warp options
for the ChunkAndWarpImage function with an error threshold
set to 0.0 for a while now and everything works great. I
did this initially because I wanted the best possible warp
results. Recently, I noticed that the performance of that
operation had slowed down significantly in the newer
versions and I am assuming that’s because the process has
been improved to be more rigorous and produce better results
(?). </span></p>
<p class="MsoNormal"><span> </span></p>
<p class="MsoNormal"><span>Anyhow, I was looking at how I could
speed up the process without sacrificing too much accuracy
and I noticed that the default behavior in the gdalwarp
utility uses an error threshold of 0.125 and the
GDALCreateApproxTransformer function to create the
transformer arg. I read the documentation and I think I
understand the ramifications of doing so, but I was hoping
somebody could give me a more detailed explanation of how
much the warp results will be affected by using the
GDALCreateApproxTransformer function with a threshold of
0.125. I am assuming it must not be that bad because Frank
choose that value as the default value years ago. I noticed
it really improves the performance but I didn’t want to make
the conversion without knowing exactly what I am
sacrificing. </span></p>
<p class="MsoNormal"><span> </span></p>
<p class="MsoNormal"><span>I am assuming with an error threshold
of 0.0 it is a per pixel warp operation and if the threshold
is greater than 0.0 and the GDALCreateApproxTransformer
function is used it ends up being a grid warp. Is that
correct? Does the threshold mean that the warped pixel may
be off by one pixel in the results or could it be worse than
that? </span></p>
<p class="MsoNormal"><span> </span></p>
<p class="MsoNormal"><span>Thanks in advance for any light you
can shed on the subject!</span></p>
<p class="MsoNormal"><span> </span></p>
<p class="MsoNormal"><span>Best regards,</span></p>
<p class="MsoNormal"><span>Martin Chapman </span></p>
</div>
<br>
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