RosConversion

How to make a message ROS convertible?

The goal of this tutorial is to make a message convertible from CS::APEX to ROS format.

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Primer

We first create a new ROS message type to convert to.

Firstly, add the following lines to your package.xml:

<build_depend>csapex_ros</build_depend>
<run_depend>csapex_ros</run_depend>

<build_depend>message_generation</build_depend>
<run_depend>message_runtime</run_depend>

This allows us to generate a new ROS message and imports the later needed csapex_ros package.

Next create a directory msg next to srcand include. Now you can add the ROS message. Create a file msg/TutorialMessage.msg and fill it with the following text:

# Tutorial Message
bool value

Now change the CMakeLists.txt to match the following:

find_package(catkin REQUIRED COMPONENTS
  csapex
  csapex_opencv
  csapex_ros
  message_generation
)

add_message_files(
  FILES
    TutorialMessage.msg
)

generate_messages(
)

catkin_package(
   INCLUDE_DIRS include
   LIBRARIES csapex_tutorial
   CATKIN_DEPENDS message_runtime
)

This will generate a ROS message. You can verify that everything worked by calling

rosmsg show TutorialMessage

which should give you the output

[csapex_tutorial/TutorialMessage]:
bool value

Maintenance

Now we can convert between the cs::APEX version of TutorialMessage and the ROS version.

We achieve this by extending the previously created CorePlugin to register the ROS conversion:

/// PROJECT
#include <csapex/core/core_plugin.h>
#include <csapex/utility/register_apex_plugin.h>
#include <csapex/profiling/timer.h>
#include <csapex_ros/ros_message_conversion.h>
#include <csapex_tutorial/TutorialMessage.h>
#include <csapex_tutorial/tutorial_message.h>

namespace csapex
{

namespace tutorial
{

struct ConvertTutorialMessage
{
    static typename connection_types::TutorialMessage::Ptr ros2apex(const typename csapex_tutorial::TutorialMessage::ConstPtr &ros_msg) {
        typename connection_types::TutorialMessage::Ptr out(new connection_types::TutorialMessage);
        out->value = ros_msg->value;
        return out;
    }
    static typename csapex_tutorial::TutorialMessage::Ptr apex2ros(const typename connection_types::TutorialMessage::ConstPtr& apex_msg) {
        typename csapex_tutorial::TutorialMessage::Ptr out(new csapex_tutorial::TutorialMessage);
        out->value = apex_msg->value;
        return out;
    }
};

class RegisterPlugin : public CorePlugin
{
public:
    RegisterPlugin()
        : timer("Application Duration")
    {}

    virtual void init(CsApexCore& core) override
    {
        timer.restart();

        RosMessageConversion::registerConversion<csapex_tutorial::TutorialMessage, connection_types::TutorialMessage, ConvertTutorialMessage>();
    }

    virtual void shutdown() override
    {
        timer.finish();

        long duration_ms = timer.stopTimeMs() - timer.startTimeMs();

        std::cerr << "the program ran for " << (duration_ms / 1000.) << " seconds" << std::endl;
    }

private:
    csapex::Timer timer;
};

} // namespace tutorial

} // namespace csapex

CSAPEX_REGISTER_CLASS(csapex::tutorial::RegisterPlugin, csapex::CorePlugin)

The struct ConvertTutorialMessage performs the conversion. For more complex messages, this might entail more work, but for this simple example there is only one data member to copy.

The registration is done by calling

RosMessageConversion::registerConversion<csapex_tutorial::TutorialMessage, connection_types::TutorialMessage, ConvertTutorialMessage>();

with the ROS type csapex_tutorial::TutorialMessage, the cs::APEX type connection_types::TutorialMessage and the converter struct ConvertTutorialMessage.

Test

To test the ROS export, first start a roscore. You can now test the ROS export using the example configuration Tutorials / Plug-in Creation / 03 - ROS Export .

Running

rostopic echo /csapex/export -n 1

should print

value: True
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