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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">Hi Even,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"> I implemented the bilinear weighting like we discussed and that took out the systematic pixel shift from 1 to 0.5 (sign depending on orientation).
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">You can see the changes here:
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"><a href="https://github.com/piyushrpt/gdal/commit/b199cbae9ea15e31c787c1480c342bb84debf774">https://github.com/piyushrpt/gdal/commit/b199cbae9ea15e31c787c1480c342bb84debf774</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">I also tried playing with the upsampling factor (1.3 to 4.0) and the discrepancy numbers were consistent after 1.5. Probably, the minimum needs to be sqrt(2) for square pixels.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">Using the weighting fixed the truncation / rounding error and the results seem consistent when I move the output extent around.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">The systematic 0.5 pixel shift suggests that there is still an issue. Is my interpretation that the GeoLocTransformer is supposed to operate on pixel center coordinates correct?
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">i.e – if my geoloc arrays were regular grid and I feed in the coordinates to the transformer, they return pixel centers – 0.5,0.5 etc.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">I’m going to try implementing some round trip checks to see if that shows something? <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri">Piyush<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:Calibri"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-family:Calibri;color:black">From: </span>
</b><span style="font-family:Calibri;color:black">Even Rouault <even.rouault@spatialys.com><br>
<b>Date: </b>Thursday, September 28, 2017 at 12:25 PM<br>
<b>To: </b>"Agram, Piyush S (334D)" <Piyush.Agram@jpl.nasa.gov><br>
<b>Cc: </b>"gdal-dev@lists.osgeo.org" <gdal-dev@lists.osgeo.org><br>
<b>Subject: </b>Re: [gdal-dev] Geolocation arrays - location interpretation<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">On jeudi 28 septembre 2017 17:26:52 CEST Agram, Piyush S (334D) wrote:<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> Hi Even,<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> I think I have figured out how the GeoLocTransformer works. Dumped out<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> the backmap and might have some changes.<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">>
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> 1) Rounding up to the nearest integer is definitely the issue here.<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">>
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> 2) I’m going to try this first – maybe a weighted approach will be<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> better. For now, if a pixel falls in the middle of square – I’m going to<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> add it to all of the 4 vertices. Then average all the data that went into a<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> pixel of the backmap.<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-paragraph-type:empty;-qt-block-indent:0">
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">Perhaps bilinear resampling so that the weights are proportionnal ?<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-paragraph-type:empty;-qt-block-indent:0">
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> 3) For the regular grid tests, if this shows improvement (hopefully) –<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> I can share the implementation and we can possibly discuss about adding a<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> weighting scheme. I don’t want to increase the runtime too much by adding<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">> complexity.<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-paragraph-type:empty;-qt-block-indent:0">
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">I think (sub-pixel) accuracy is more important than speed. Speed improvements can be investigated later if needed.<o:p></o:p></p>
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<o:p></o:p></p>
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<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">Spatialys - Geospatial professional services<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt;-qt-block-indent:0;-qt-user-state:0">http://www.spatialys.com<o:p></o:p></p>
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