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PCL SACSegmentation - setAxis and setModelType has no effect in Output

asked 2013-04-29 20:41:56 -0500

KarthikMurugan gravatar image

updated 2014-01-28 17:16:23 -0500

ngrennan gravatar image

I want to estimate the ground plane present in the pointcloud and remove it. I used the plane model segmentation tutorial for that.

Instead of removing the ground plane it removes the vertical planes(wall) in the pointcloud.

I changed the axis perpendicular to which the plane is estimated but get the same result..

This is the code :-

  Eigen::Vector3f axis = Eigen::Vector3f(0.0,1.0,0.0);

  pcl::ModelCoefficients::Ptr coefficients (new pcl::ModelCoefficients);
  pcl::PointIndices::Ptr inliers (new pcl::PointIndices);
  // Create the segmentation object
  pcl::SACSegmentation<pcl::PointXYZI> seg;
   seg.setAxis(axis);

  seg.setOptimizeCoefficients (true);

  seg.setModelType (pcl:: SACMODEL_PLANE);
  seg.setMethodType (pcl::SAC_RANSAC);
  seg.setDistanceThreshold (0.5);

  seg.setInputCloud (cloud);
  seg.segment (*inliers, *coefficients);

When I input a stream of pointclouds through ROS, I get Model coefficients as :-

Model coefficients: 0.980676 0.00643671 0.195532 0.127706
Model coefficients: 0.0974007 -0.00814183 0.995212 2.64382
Model coefficients: 0.0235566 -0.997725 -0.0631705 4.7102
Model coefficients: 0.0404465 -0.999113 -0.0117141 4.65797

ie sometimes it removes ground plane and at some other instant it removes walls... the coordinate system in the pointcloud is

x axis -> forward
y axis -> right
z axis -> up

Now since ground plane is along x-z plane perpendicular to y axis I set the Axis as (0.0,1.0,0.0) so that according to PCL api ground_plane perpendicular to y axis be removed... but as i said only at some instants it is removed and at other instants vetical planes (wall) gets removed...

I tried all possibilities of (1,0,0) ,(0,1,0),(0,0,1) but it has NO EFFECT in the output result...The same result I mentioned above is obtained...

Also changed ModelType to SACMODEL_PERPENDICULAR_PLANE , SACMODEL_PARALLEL_PLANE ,still no desired result

Please pour in your suggestions where I am going wrong..

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2 Answers

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answered 2013-04-30 02:46:37 -0500

I think you need to also set the eps_angle parameter.

If you check the source code for sac_model_parallel_plane.hpp, you'll see that the axis is only used in the isModelValid() function. And, even then, only when eps_angle_ > 0. Looking in sac_model_parallel_plane.h, we see that eps_angle_ is initialized to 0. So, unless you explicitly set this parameter, the model will effectively ignore the parallel-to-axis constraint.

Try adding this:

seg.setEpsAngle(  30.0f * (M_PI/180.0f) );
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Comments

Thank you Jeremy.. So is the angle parameter signify angle tolerance of plane with the axis or its normal with the axis??? (as it is in this : http://docs.pointclouds.org/trunk/classpcl_1_1_s_a_c_segmentation.html )

KarthikMurugan gravatar image KarthikMurugan  ( 2013-04-30 18:54:13 -0500 )edit

Read the PCL documentation for setEpsAngle() at the link you provided. It is: "maximum allowed difference between the model NORMAL and the given axis in radians". So, the "axis" parameter is the expected plane normal vector, and "epsAngle" is the allowable variation for a plane to count as "valid"

Jeremy Zoss gravatar image Jeremy Zoss  ( 2013-04-30 19:27:20 -0500 )edit

Thank u Jeremy.... tat worked!!! now only the ground plane gets removed...

but Still at some instants nothing gets removed... do you know wat could be the reason behind it... I tried values for epsAngle from 20 to 45 ....any rectification to be made ??

KarthikMurugan gravatar image KarthikMurugan  ( 2013-05-01 07:04:59 -0500 )edit

Also what is the difference between SACMODEL_PERPENDICULAR_PLANE and SACMODEL_PARALLEL_PLANE... I get the same result for both!!!

KarthikMurugan gravatar image KarthikMurugan  ( 2013-05-01 07:07:43 -0500 )edit

Your questions are fairly specific to PCL, and are not really ROS-related. You should try addressing these questions to the PCL user's list: pcl-users@pointclouds.org.

Jeremy Zoss gravatar image Jeremy Zoss  ( 2013-05-01 07:11:34 -0500 )edit

hi i have the same problem. In my pointcloud, the wall is a dominant plane, everytime i tried to extract the ground (set axis method) it still gives me the wall. How did you solve your problem

dmngu9 gravatar image dmngu9  ( 2015-04-22 19:06:58 -0500 )edit

I also have the same problem. It should return a plane that is within the 'epsAngle' of the specified 'axis' even if one doesn't fit well, right? It seems like what is happening is that there are 0 inliers in the model that is found (which should be an error condition?).

daviddoria gravatar image daviddoria  ( 2015-12-02 16:40:11 -0500 )edit
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answered 2017-05-23 13:47:33 -0500

biglotusturtle gravatar image

I was having this problem as well so if anyone is interested here is how I solved it.

I was also looking for a ground plane in my point cloud and because of my cameras coordinates frame I wanted the X-Z plane.

pcl::SACSegmentation<pcl::PointXYZRGB> seg;

//Set up parameters for our segmentation/ extraction scheme
seg.setOptimizeCoefficients(true);
seg.setModelType(pcl::SACMODEL_PERPENDICULAR_PLANE ); //only want points perpendicular to a given axis
seg.setMaxIterations(500); // this is key (default is 50 and that sucks)
seg.setMethodType(pcl::SAC_RANSAC);
seg.setDistanceThreshold (0.05); // keep points within 0.05 m of the plane

//because we want a specific plane (X-Z Plane) (In camera coordinates the ground plane is perpendicular to the y axis)
Eigen::Vector3f axis = Eigen::Vector3f(0.0,1.0,0.0); //y axis
seg.setAxis(axis);
seg.setEpsAngle(  30.0f * (PI/180.0f) ); // plane can be within 30 degrees of X-Z plane

The key for me ended up being the maxIterations parameter. The default I believe is 50 iterations and it would not find the correct plane with so few. I found that at a minimum I needed setMaxIterations(500) before it found the correct plane.

Hope this helps.

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Asked: 2013-04-29 20:41:56 -0500

Seen: 6,693 times

Last updated: May 23 '17