What is the meaning of contacts returned from GetContact (contact sensor)?

Question

When I use Contact sensor in Gazebo 1.3, I am basically using following code.

  // Get all the contacts.
  msgs::Contacts contacts;
  contacts = this->parentSensor->GetContacts();
  for (int i = 0; i < contacts.contact_size(); ++i)
  {
    for (int j = 0; j < contacts.contact(i).position_size(); ++j)
    {
      std::cout << j << "  Position:"
                << contacts.contact(i).position(j).x() << " "
                << contacts.contact(i).position(j).y() << " "
                << contacts.contact(i).position(j).z() << "\n";
    }
  }

The 'GetContacts' function returns many contact informations (in my case, contact_size was 30~50). And each contact includes a bunch of contact positions. I have looked the position data. All contacts includes quite similar position data.

What is the difference between contacts? Is it a group of contact points by position?

I thought if there is two bodies and two contact points between bodies, the contact_size is 2 and each contact includes a bunch of positions that are located in each contact point. Am I correct?

===== Update 1 ======

I use following two objects

1. PR2's upper arm part

    <link name='l_upper_arm_roll_link'>
      <self_collide>0</self_collide>
      <gravity>0</gravity>
      <pose>0.050000 0.188000 0.790675 0.000000 -0.000000 0.000000</pose>
      <inertial>
        <mass>6.117690</mass>
        <pose>0.210551 0.016309 -0.000561 0.000000 -0.000000 0.000000</pose>
        <inertia>
          <ixx>0.025306</ixx>
          <ixy>-0.003393</ixy>
          <ixz>0.000608</ixz>
          <iyy>0.084737</iyy>
          <iyz>-0.000200</iyz>
          <izz>0.086016</izz>
        </inertia>
      </inertial>
      <collision name='l_upper_arm_roll_link_geom'>
        <geometry>
          <mesh>
            <uri>model://pr2_arm/meshes/shoulder_v0/upper_arm_roll_L.stl</uri>
          </mesh>
        </geometry>
      </collision>
      <visual name='l_upper_arm_roll_link_geom_visual'>
        <geometry>
          <mesh>
            <uri>model://pr2_arm/meshes/shoulder_v0/upper_arm_roll.stl</uri>
          </mesh>
        </geometry>
      </visual>
      <collision name='l_upper_arm_roll_link_geom_l_upper_arm_link'>
        <geometry>
          <mesh>
            <uri>model://pr2_arm/meshes/upper_arm_v0/upper_arm.stl</uri>
          </mesh>
        </geometry>
      </collision>
      <visual name='l_upper_arm_roll_link_geom_l_upper_arm_link_visual'>
        <geometry>
          <mesh>
            <uri>model://pr2_arm/meshes/upper_arm_v0/upper_arm.dae</uri>
          </mesh>
        </geometry>
      </visual>
      <sensor name='l_upper_arm_roll_tactile_sensor' type=contact>
        <contact>
          <collision>l_upper_arm_roll_link_geom_l_upper_arm_link</collision>
          <topic>__default_topic__</topic>
        </contact>
        <plugin name="ros_tactile_plugin" filename="libros_tactile_plugin.so" />
        <always_on>1</always_on>
        <update_rate>100.0</update_rate>
        <visualize>false</visualize>
      </sensor>
      <velocity_decay/>
    </link>
   

2. Desk

    <link name="link">
      <inertial>
        <mass>50.0</mass>
      </inertial>
      <collision name="surface">
        <pose>0 0 0.6 0 0 0</pose>
        <geometry>
          <box>
            <size>1.5 0.8 0.03</size>
          </box>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>500.000000</mu>
              <mu2>500.000000</mu2>
              <slip1>0.000000</slip1>
              <slip2>0.000000</slip2>
            </ode>
          </friction>
          <contact>
            <ode>
              <soft_cfm>0.04</soft_cfm>
              <soft_erp>0.9</soft_erp>
              <kp>50000.0</kp>
              <kd>1.000</kd>
              <max_vel>10.000000</max_vel>
              <min_depth>0.000000</min_depth>
            </ode>
          </contact>
        </surface>
      </collision>
      <visual name="surface">
        <pose>0 0 0.6 0 0 0</pose>
        <geometry>
          <box>
            <size>1.4 0.8 0.04</size>
          </box>
        </geometry>
        <material>
          <script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Wood</name>
          </script>
        </material>
      </visual>
   

As we can see, one is mesh object and another one is a box. Could you kindly answer followings?

1. Each triangle of the mesh gives one contact object like 'contacts.contact(0)' when a box and the triangle collide.
2. One contact object like 'contacts.contact(0)' includes several positions like 'contacts.contact(0).position(~)'. The positions are only limited in the collision area between one triangle and a box.

---

Originally posted by Daehyung Park on Gazebo Answers with karma: 47 on 2013-01-15

Post score: 1

---

Original comments

Comment by cga on 2013-01-15:
Please also respond to question 559, which asks a similar question and wonders what the timing information means. Thanks, Chris

Answer 1

The number of contacts generated depends a lot on the shapes that are colliding. Two triangle meshes may produce a lot of contacts, while a box colliding with a plan will produce fewer.

The contact sensor is designed to report contacts that occur on one or more Collision objects. So, the contacts you get back are all the contacts that have occurred on the specified collision objects.

I would need to see your SDF file to offer more insight.

---

Originally posted by nkoenig with karma: 7676 on 2013-01-15

This answer was ACCEPTED on the original site

Post score: 0

---

Original comments

Comment by Daehyung Park on 2013-01-15:
Thanks!! I got new knowledge about the mesh! I updated the SDF information and additional two questions. Could you kindly answer for it? If my understanding is right, I can start to group and simplify the contact sensor information.