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POConvex Hull calculation in Point Cloud Library fails in 2 as well as 3 dimensions
 

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  1. POConvex Hull calculation in Point Cloud Library fails in 2 as well as 3 dimensions
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    <p>i'm following the tutorial of the PCL docs for calculating a 2D Convex Hull <a href="http://www.pointclouds.org/documentation/tutorials/convex_hull2d.php" rel="nofollow">see here</a>.</p> <p>I have a cloud and some indices, project them on a plane with given coefficients and then calculate the convex hull. Here's the code:</p> <pre><code>PointCloud&lt;PointXYZ&gt;::Ptr tmpInliers(new PointCloud&lt;PointXYZ&gt;()); ProjectInliers&lt;PointXYZ&gt; proj; proj.setModelType(SACMODEL_PLANE); proj.setInputCloud(someCloud); proj.setIndices(someIndices); proj.setModelCoefficients(someCoefficients); proj.filter(*tmpInliers); PointCloud&lt;PointXYZ&gt;::Ptr hull(new PointCloud&lt;PointXYZ&gt;()); ConvexHull&lt;PointXYZ&gt; chull; chull.setInputCloud(tmpInliers); chull.setComputeAreaVolume(true); chull.setDimension(3); &lt;--- see below chull.reconstruct(*hull); </code></pre> <p>The results i get for the total area and volume are around: </p> <pre><code> Area &amp; Volume of convex hull: 7.8726e-312 2.122e-314 </code></pre> <p>For values of <strong>tmpInliers</strong> ranging around</p> <pre><code>(-0.80562,-0.787018,2.25184) (-0.477351,-0.798953,2.11432) (-0.633823,-0.750283,2.96717) [....] </code></pre> <p>If i change "setDimensions" to "2" i get the following error</p> <pre><code>[pcl::ConvexHull::performReconstrution2D] ERROR: qhull was unable to compute a convex hull for the given point cloud (size of cloud)! </code></pre> <p>In the following example i am constructing a example and fail in every case (setDimension set to 2 or 3) with one of the failures from before (either "qhull was unable..." or a strange result according to values from the ConvexHull.</p> <pre><code>PointCloud&lt;PointXYZ&gt;::Ptr hugeBox(new PointCloud&lt;PointXYZ&gt;()); hugeBox-&gt;push_back(PointXYZ(10, 10, 10)); hugeBox-&gt;push_back(PointXYZ(10, 10, -10)); hugeBox-&gt;push_back(PointXYZ(10, -10, 10)); hugeBox-&gt;push_back(PointXYZ(10, -10, -10)); hugeBox-&gt;push_back(PointXYZ(-10, 10, 10)); hugeBox-&gt;push_back(PointXYZ(-10, 10, -10)); hugeBox-&gt;push_back(PointXYZ(-10, -10, 10)); hugeBox-&gt;push_back(PointXYZ(-10, -10, -10)); // Project inliers onto plane model PointCloud&lt;PointXYZ&gt;::Ptr hugePlane(new PointCloud&lt;PointXYZ&gt;()); ProjectInliers&lt;PointXYZ&gt; proj; proj.setModelType(SACMODEL_PLANE); proj.setInputCloud(hugeBox); proj.setModelCoefficients(coefficients); proj.filter(*hugePlane); // get the convex hull of plane vector&lt;Vertices&gt; polygonsOut; PointCloud&lt;PointXYZ&gt;::Ptr hugeHull(new PointCloud&lt;PointXYZ&gt;()); ConvexHull&lt;PointXYZ&gt; chull; chull.setInputCloud(hugePlane); chull.setDimension(2); chull.reconstruct(*hugeHull, polygonsOut); </code></pre> <p>I'm a little stuck here. Why does it fail if i set it to 2 dimensions? And if i set it to 3 dimensions i occasionaly the following warning:</p> <pre><code>qhull precision warning: The initial hull is narrow (cosine of min. angle is 0.9999999999999991). A coplanar point may lead to a wide facet. </code></pre> <p>I understand that this is the case if i have a planar projection, but how to avoid this? </p>
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