<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>Bryn, Dan,</div><div> Dan is right, seed values are directly passed on to the two fast marching filters,</div><div>therefore the arrival times of the two fronts will have an offset according to the </div><div>seed value you use. However, the filter than takes the gradient vectors of the</div><div>arrival times of the two fronts (in order to take the dot product between them), </div><div>so any offset brought in by the seed values is effectively canceled out by the </div><div>the gradient operator. Hence, you get no change by changing seed values.</div><div>Hope this clarifies it.</div><div>Best regards</div><div><br></div><div>Luca</div><div><br></div><div><br></div><br><div><div>On Feb 12, 2009, at 11:15 AM, Dan Mueller wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Forward to Insight-users (with attachments removed to avoid bounce).<br><br>---------- Forwarded message ----------<br>From: Dan Mueller <<a href="mailto:dan.muel@gmail.com">dan.muel@gmail.com</a>><br>Date: 2009/2/12<br>Subject: Re: [Insight-users] Seed Value for CollidingFrontsImageFilter<br>To: <a href="mailto:blloyd@vision.ee.ethz.ch">blloyd@vision.ee.ethz.ch</a><br>Cc: <a href="mailto:Insight-users@itk.org">Insight-users@itk.org</a><br><br>Hi Bryn,<br><br>The seed value is used by the underlying<br>FastMarchingUpwindGradientImageFilter. The output of a<br>FastMarchingImageFilter is an arrival function, which specifies at<br>what "time" the expanding front reaches a given point. The seedValue<br>is used as the initial front, so changing this value changes the time<br>the front reached the seedPosition. Changing this value will result in<br>a (slightly) different arrival function (most likely it will be offset<br>by the value you assigned, eg. 500).<br><br>From my understanding, the CollidingFrontsImageFilter computes two<br>arrival functions (for two different initial seeds). From these the<br>gradient is computed, then the dot product, and finally thresholded<br>for values when the dot product > 0. Therefore I suspect that you will<br>not see any difference in the output of the CollidingFrontsImageFilter<br>for different seed values -- the gradient information extracted from<br>the arrival functions will be the same, even they are slightly offset.<br><br>For your reference, I have attached two images showing what happens<br>with different seed values using the FastMarchingImageFilter.<br><br>Hope this helps.<br><br>Regards, Dan<br><br>2009/2/12 Bryn Lloyd <<a href="mailto:blloyd@vision.ee.ethz.ch">blloyd@vision.ee.ethz.ch</a>>:<br><blockquote type="cite">Hi,<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Does the Value at the seed points used in the CollidingFrontsImageFilter<br></blockquote><blockquote type="cite">have any meaning or can/should it be ignored?<br></blockquote><blockquote type="cite"><a href="http://www.itk.org/Doxygen/html/classitk_1_1CollidingFrontsImageFilter.html">http://www.itk.org/Doxygen/html/classitk_1_1CollidingFrontsImageFilter.html</a><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">I have modified the CollidingFrontsImageFilterTest to use different seed<br></blockquote><blockquote type="cite">values, but cannot find any difference in the result.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">In the ITK manual the value is set to zero, but no explanation is given.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">NodeType node;<br></blockquote><blockquote type="cite">const double seedValue = 0.0;<br></blockquote><blockquote type="cite">node.SetValue( seedValue );<br></blockquote><blockquote type="cite">node.SetIndex( seedPosition );<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Thanks<br></blockquote><blockquote type="cite">Bryn<br></blockquote>_____________________________________<br>Powered by <a href="http://www.kitware.com">www.kitware.com</a><br><br>Visit other Kitware open-source projects at<br><a href="http://www.kitware.com/opensource/opensource.html">http://www.kitware.com/opensource/opensource.html</a><br><br>Please keep messages on-topic and check the ITK FAQ at: http://www.itk.org/Wiki/ITK_FAQ<br><br>Follow this link to subscribe/unsubscribe:<br>http://www.itk.org/mailman/listinfo/insight-users<br></div></blockquote></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; 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"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>--</div><div>Luca Antiga, PhD</div><div>Head, Medical Imaging Unit,<br>Biomedical Engineering Department,<br>Mario Negri Institute.<br>mail: Villa Camozzi, 24020, Ranica (BG), Italy<br>phone: +39 035 4535-381<br>email: <a href="mailto:antiga@marionegri.it">antiga@marionegri.it</a><br>web: <a href="http://villacamozzi.marionegri.it/~luca">http://villacamozzi.marionegri.it/~luca</a></div></div></div></div></div></div></span></div></span></div></span> </div><br></body></html>