Hi,<br><br>This is now fixed in VTK. Thanks for the help in finding the bug.<br><br>Andy<br><br><div class="gmail_quote">On Thu, Mar 7, 2013 at 10:21 AM, Hong Yi <span dir="ltr"><<a href="mailto:hongyi@renci.org" target="_blank">hongyi@renci.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div link="blue" vlink="purple" lang="EN-US">
<div>
<p class="MsoNormal">I found some discrepancy in velocity gradient result for a test data between passing dim=1 and passing dim=3 when computing derivatives, and after some debugging and tracing, I think I found the root of the discrepancy which looks like
a bug to me in vtkLine::Derivatives() function. Specifically, here is the code of the function:<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">-------------------------------<u></u><u></u></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:blue">void</span><span style="font-size:9.5pt;font-family:Consolas"> vtkLine::Derivatives(<span style="color:blue">int</span> vtkNotUsed(subId),
<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> <span style="color:blue">double</span> vtkNotUsed(pcoords)[3],
<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> <span style="color:blue">double</span> *values,
<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> <span style="color:blue">int</span> dim,
<span style="color:blue">double</span> *derivs)<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">{<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">double</span> x0[3], x1[3], deltaX[3];<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">int</span> i, j;<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"><u></u> <u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">this</span>->Points->GetPoint(0, x0);<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">this</span>->Points->GetPoint(1, x1);<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"><u></u> <u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">for</span> (i=0; i<3; i++)<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> deltaX[i] = x1[i] - x0[i];<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">for</span> (i=0; i<dim; i++) <u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">for</span> (j=0; j<3; j++)<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">if</span> ( deltaX[j] != 0 )<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="background:yellow">derivs[3*i+j] = (values[2*i+1] - values[2*i]) / deltaX[j];</span><u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">else</span><u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> derivs[3*i+j] =0;<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">}<u></u><u></u></span></p>
<p class="MsoNormal">-------------------------------<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">The suspicious line is highlighted above. When dim=1, this code generates correct result. However, when dim=3 for velocity, values[2*i+1]-values[2*i] are not using the correct indices since the values will be like (vx0, vy0, vz0, vx1, vy1,
vz1) and the values[2*i+1]-values[2*i] will be vy0-vx0, vx1-vz0, and vz1-vy1 rather than the expected vx1-vx0, vy1-vy0, and vz1-vz0.
<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">I did not find any bug filed on this, so I am confirming here: is this a bug?<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Thanks,<span class="HOEnZb"><font color="#888888"><u></u><u></u></font></span></p><span class="HOEnZb"><font color="#888888">
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Hong<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
</font></span></div>
</div>
<br>_______________________________________________<br>
Powered by <a href="http://www.kitware.com" target="_blank">www.kitware.com</a><br>
<br>
Visit other Kitware open-source projects at <a href="http://www.kitware.com/opensource/opensource.html" target="_blank">http://www.kitware.com/opensource/opensource.html</a><br>
<br>
Please keep messages on-topic and check the VTK FAQ at: <a href="http://www.vtk.org/Wiki/VTK_FAQ" target="_blank">http://www.vtk.org/Wiki/VTK_FAQ</a><br>
<br>
Follow this link to subscribe/unsubscribe:<br>
<a href="http://www.vtk.org/mailman/listinfo/vtkusers" target="_blank">http://www.vtk.org/mailman/listinfo/vtkusers</a><br>
<br></blockquote></div><br>