How to segment/extract a sphere using pcl_ros?

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UPDATE:

After much trial and error, I have given up on trying to get this to work with a chain of nodelets. It is so much easier to do using the C++ examples from http://www.pointclouds.org so I will pursue that approach instead. To extract a sphere, I just modified the cylinder example found at http://www.pointclouds.org/documentat...

[Original question now follows.]

Hello,

I am able to successfully extract planes from a Kinect depth image using Diamondback under Ubuntu 10.04 and using the following launch file:

<launch>
  <node pkg="nodelet" type="nodelet" name="pcl_manager" args="manager" output="screen" />

  <!-- Run a Voxel Grid filter -->
  <node pkg="nodelet" type="nodelet" name="voxel_grid" args="load pcl/VoxelGrid pcl_manager" output="screen">
    <remap from="~input" to="/camera/depth/points" />
    <rosparam>
      filter_field_name: z
      filter_limit_min: 0.5
      filter_limit_max: 1.5
      filter_limit_negative: False
      leaf_size: 0.01
    </rosparam>
  </node>

  <!-- Run a Planar Segmentation filter -->
  <node pkg="nodelet" type="nodelet" name="sac_plane" args="load pcl/SACSegmentation pcl_manager" output="screen">
    <remap from="~input" to="/voxel_grid/output" />
    <rosparam>
      model_type: 0
      distance_threshold: 0.01
    </rosparam>
  </node>

  <!-- Then Extract the Inlier Points -->
  <node pkg="nodelet" type="nodelet" name="extract_plane" args="load pcl/ExtractIndices pcl_manager" output="screen">
    <remap from="~input" to="/voxel_grid/output" />
    <remap from="/extract_plane/indices" to="/sac_plane/inliers" />
  </node>
</launch>

And now I would like to extract a sphere (a 6" ball sitting on a table about 1 meter away from the Kinect) using a similar technique. I was hoping it would be as easy as changing the SAC model_type from 0 to 4 but the following launch file generates the error and warning:

[ERROR] [1299428614.657650314]: [pcl::SampleConsensusModelSphere::optimizeModelCoefficients] Not enough inliers found to support a model (0)! Returning the same coefficients.
[ WARN] [1299428614.657849802]: [/sac_sphere::input_indices_callback] No inliers found!

Here is the launch file I tried for extracting a sphere:

<launch>
  <node pkg="nodelet" type="nodelet" name="pcl_manager" args="manager" output="screen" />

  <!-- Run a Voxel Grid filter -->
  <node pkg="nodelet" type="nodelet" name="voxel_grid" args="load pcl/VoxelGrid pcl_manager" output="screen">
    <remap from="~input" to="/camera/depth/points" />
    <rosparam>
      filter_field_name: z
      filter_limit_min: 0.5
      filter_limit_max: 1.5
      filter_limit_negative: False
      leaf_size: 0.01
    </rosparam>
  </node>

  <!-- Run a Sphere Segmentation filter -->
  <node pkg="nodelet" type="nodelet" name="sac_sphere" args="load pcl/SACSegmentation pcl_manager" output="screen">
    <remap from="~input" to="/voxel_grid/output" />
    <rosparam>
      model_type: 4
      distance_threshold: 0.01
    </rosparam>
  </node>

  <!-- Then Extract the Inlier Points -->
  <node pkg="nodelet" type="nodelet" name="extract_sphere" args="load pcl/ExtractIndices pcl_manager" output="screen">
    <remap from="~input" to="/voxel_grid/output" />
    <remap from="/extract_sphere/indices" to="/sac_sphere/inliers" />
  </node>

</launch>

I am using VGA/30Hz resolution for both RGB and depth. The sphere is a 6" ball sitting on a table about 1 meter away from the Kinect. I have tried different leaf sizes from 0.01 to 0.0001 meters with the same result. I have also tried different distance_thresholds from 1.0 to 0.0001 with the same result. Both the raw depth image and voxel_grid image look very "sphere-like" in RViz. However ... (more)