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<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:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-------------------------------<o:p></o:p></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),
<o:p></o:p></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],
<o:p></o:p></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,
<o:p></o:p></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)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">{<o:p></o:p></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];<o:p></o:p></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;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"><o:p> </o:p></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);<o:p></o:p></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);<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"><o:p> </o:p></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++)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> deltaX[i] = x1[i] - x0[i];<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<o:p></o:p></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++) <o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<o:p></o:p></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++)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<o:p></o:p></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 )<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="background:yellow;mso-highlight:yellow">derivs[3*i+j] = (values[2*i+1] - values[2*i]) / deltaX[j];</span><o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">
<span style="color:blue">else</span><o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> derivs[3*i+j] =0;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">}<o:p></o:p></span></p>
<p class="MsoNormal">-------------------------------<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></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.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I did not find any bug filed on this, so I am confirming here: is this a bug?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hong<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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