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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Daniele,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> For your first question, you should operate with a no_data value outside the range of pixel generated by interpolation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> So you should do a preprocessing to change no_data value outside of this range. Alternatively, if you want to keep 0 as no_data, you should
clamp negative value to 1 (instead of 0).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> For your second question, it’s normal that interpolation does a kind of dissolution of your edge. If you want to keep original no_data, you should
extract a band mask from your input, and integrate it on your result.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Nicolas<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="FR" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">De :</span></b><span lang="FR" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> gdal-dev [mailto:gdal-dev-bounces@lists.osgeo.org]
<b>De la part de</b> Daniele Romagnoli<br>
<b>Envoyé :</b> mercredi 29 mars 2017 10:07<br>
<b>À :</b> gdal-dev@lists.osgeo.org<br>
<b>Objet :</b> [gdal-dev] Interpolation questions<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">While investigating on some rendering issues we have with java JAI based operations involving interpolations, I have found the topic of bicubic interpolation introducing values outside of the input dynamic,
as well as negative values and overshooting.<o:p></o:p></p>
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<p class="MsoNormal">The problem we had was with a grayscale GeoTIFF with noData=0.<br>
For some very dark area (pixel near to zero) the output of the interpolation was a negative value being clamped to zero (which is noData).<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">So it was introducing noData in regions where all pixels were valid.<o:p></o:p></p>
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<p class="MsoNormal">Therefore I did a test with gdal_translate -r cubic and I got same result:<br>
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<img width="462" height="141" id="_x0000_i1025" src="cid:image002.jpg@01D2A894.35516A80"><br>
<o:p></o:p></p>
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<p class="MsoNormal">on the left side: original image<o:p></o:p></p>
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<p class="MsoNormal">on the right side: cubic interpolation <o:p></o:p></p>
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<p class="MsoNormal">weird white shapes you see on the top right are zero/nodata pixels.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I got different result once using cubic spline instead.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">What is the difference between the two algorithms?<o:p></o:p></p>
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<p class="MsoNormal">Second question:<o:p></o:p></p>
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<p class="MsoNormal">I did a gdal_translate -outsize 800% 800% from a small pixellated image.<o:p></o:p></p>
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<p class="MsoNormal">is it expected to have output pixels exceeding the original shape of the valid input pixels?<o:p></o:p></p>
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<p class="MsoNormal">I was thinking the interpolated image pixels should stay within the original pixel edges.<br>
<br>
Let me provide an image to clarify:<br>
<img width="462" height="118" id="_x0000_i1026" src="cid:image004.jpg@01D2A894.35516A80"><br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">Left image: original<br>
Right image: cubic spline.<br>
See how the 2 edges are different, having the cubic spline producing some kind of buffering on the edge (note the white-missing pixels on the borders are nodata).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Please, let me know.<o:p></o:p></p>
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<p class="MsoNormal">Best Regards,<o:p></o:p></p>
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<p class="MsoNormal">Daniele<o:p></o:p></p>
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<p class="MsoNormal"><br>
<br clear="all">
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-- <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.5pt">==</span><o:p></o:p></p>
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==<br>
<br>
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-------------------------------------------------------<o:p></o:p></p>
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