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Launch file is useful when you want to start/launch many nodes at once without any need to do "rosrun for every node".

In the package, create a folder by the name launch and create a filename.launch file.

It always start with

 <launch>

and if some parameters should be set then

<param name="use_sim_time"  value="true" />

To start a node

<node pkg="name_of_package" type="name_of_packge" name="name_of_executable" 
output="screen"/>

Example

<node pkg="sure_on_cloud" type="estimate_transform_svd" name="estimate_transform_svd" 
output="screen"/>

In the above way, one can call multiple nodes from different packages.

<remap from="/camera/depth/image_rect_raw" to="/camera/depth/image"/>

Nodelets can also be initialized. Sorry I dont know what they are, but they can also be initialized from the example below

  <node pkg="nodelet" type="nodelet" name="standalone_nodelet"  args="manager"/>

 <node pkg="nodelet" type="nodelet" name="point_cloud_xyz"
    args="load depth_image_proc/point_cloud_xyz standalone_nodelet">

Some packages have their topics already set to specific names. They can be remapped to different names.

<remap from="/image_rect" to="/camera/depth/image"/>
<remap from="/points" to="/camera/depth/points"/>
<remap from="/camera_info" to="/camera/depth/camera_info"/>

</node>

</launch>