<html><body><P><FONT face="Arial, Helvetica, sans-serif" size=2>Hello vtkUsers,</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>I've been using vtkVolumeRayCastIsosurfaceFunction and vtkContourFilter (as used in the medical examples that come with vtk) to visualize isosurfaces.</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>More specifically, I've been using vtkStructuredPointsReader to read data from a vtk file with the option of visualising an isosurface either with vtkVolumeRayCastIsosurfaceFunction or with vtkContourFilter (with vtkPolyDataNormals and vtkPolyDataMapper).</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>The problem I'm facing is that it appears I get different results when I <BR>visualize the same isosurface with the same data with the two different methods.<BR>I'm aware that vtkVolumeRayCastIsosurfaceFunction is supposed to be slower but <BR>more accurate, but it seems that the results are even topologically different.</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>Since I'm only doing some experiences, I'm using a very simple vtk data file <BR>with only 8 vertices of a single cube (with STRUCTURED_POINTS) and I visualize<BR>an isosurface that intersects that cube. I've been using several examples, but <BR>the ones I see problems were adapted from this IEEE TVCG paper (section 5 <BR>table 2):<BR>"Improving the Robustness and Accuracy of the Marching Cubes Algorithm <BR>for Isosurfacing" from Lopes and Brodlie.</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>I don't have much experience with vtk so I might be doing something wrong... <BR>it's just that I would expect the result of using vtkContourFilter to be faster <BR>but not as high-quality as vtkVolumeRayCastIsosurfaceFunction, but i would not <BR>expect that the isosurfaces resulting from it to be topologically different...</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>Is there something i'm missing/doing wrong or is this something internal to vtk?</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>As an example, when i visualize an isosurface of value 100 with this next file,<BR>I get different isosurfaces.</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2># vtk DataFile Version 1.0<BR>My testing for cubes!!!<BR>ASCII</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>DATASET STRUCTURED_POINTS<BR>DIMENSIONS 2 2 2<BR>ASPECT_RATIO 1 1 1<BR>ORIGIN 0.0 0.0 0.0<BR> <BR>POINT_DATA 8<BR>SCALARS volume_scalars unsigned_char 1<BR>LOOKUP_TABLE default<BR>103<BR>97<BR>94<BR>104<BR>120<BR>98<BR>98<BR>98<BR></FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>thxs in advance,</FONT></P>
<P><FONT face="Arial, Helvetica, sans-serif" size=2>Sérgio</P></FONT><br> <br><hr><a href="http://www.gamedev.net">GameDev.net Email Service</a> - "Plenty of 1's and 0's"<br></body></html>