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<p class="MsoNormal"><span style="color:#1F497D">Hi Jan,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">I’m suspecting you have Aperio images (and their quirky not quite integer downsamples). Personally I don’t use
</span><span lang="EN-US">openslide_get_best_level_for_downsample, manually getting the levels with
</span><a href="http://openslide.org/api/openslide_8h.html#a19eedd09aef2bd65db7e3b0a44e54738">openslide_get_level_count</a>
<span lang="EN-US">/</span><span lang="EN-US"> </span><a href="http://openslide.org/api/openslide_8h.html#a60daced5880dc3e4cf428dd81ba67bee">openslide_get_level_downsample</a>
<span lang="EN-US">and finding the closest level (or best level for a particular purpose). Why do I do this when openslide_get_best_level_for_downsample exists?.. I actually have no idea. I’m guessing at some point in the past I had exactly the issue you have
and this was a workaround that worked. openslide_get_best_level_for_downsample definitely rounds down, which is kind of sensible in many use cases .. asking for 4 and getting 4.0001 is actually lower resolution than you wanted .. where do you draw the line?
4.1? 5?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">D.</span><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="color:#1F497D"><o:p> </o:p></span></a></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:EN-GB"> openslide-users
[mailto:openslide-users-bounces+d.r.magee=leeds.ac.uk@lists.andrew.cmu.edu] <b>On Behalf Of
</b>Jan Hudecek via openslide-users<br>
<b>Sent:</b> 13 December 2016 12:40<br>
<b>To:</b> openslide-users@lists.andrew.cmu.edu<br>
<b>Subject:</b> openslide_get_best_level_for_downsample and numeric stability<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I’ve noticed openslide_get_best_level_for_downsample doesn’t always return the best level. I have an image with 4 levels:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">{w:85655 h:42392}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">{w:21413 h:10598}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">{w:5353 h:2649}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">{w:2676 h:1324}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">So openslide_get_level_downsample for second level returns downsample value 4.0000700509036573.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">When I call openslide_get_best_level_for_downsample(handle, 4) it returns 0. When I call it with 4.001 it returns 1 as expected.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I’m not sure what’s best solution. Should get_best_level_for_downsample also “look” upwards – ie. if there is a level with a slightly larger downsample use it? Or should I just add epsilon (how big?) to the downsample
before calling it myself?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I would like to avoid scaling the image 4x as it is a performance hit and completely unnecessary.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:NL">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:NL">Jan Hudecek</span><u><span lang="EN-US" style="font-size:8.0pt;font-family:"Arial","sans-serif";color:blue;mso-fareast-language:NL"><o:p></o:p></span></u></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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