No name resolution of base topic in image_transport::Subscriber

[yoshito-okada]

image_transport::Subscriber does not resolve the name of base topic although CameraSubscriber, CameraPublisher, and Publisher do. This causes an issue that a base topic remapping does not work when the transport type is not raw. For instance, the following settings subscribe not camera/image_raw/compressed but image_in/compressed.

<remap from="image_in" to="camera/image_raw"/>
<param name="image_transport" value="compressed"/>

[christian-rauch]

I have a similar issue with the SubscriberFilter (https://answers.ros.org/question/290472/topic-remapping-in-synchronised-message_filters/). The full topic name gets compiled before the topic remapping is applied:

image_common/image_transport/include/image_transport/simple_subscriber_plugin.h

Lines 98 to 101 in f9e705a

	virtual std::string getTopicToSubscribe(const std::string& base_topic) const
	{
	return base_topic + "/" + getTransportName();
	}

[bbferka]

I have the same issue on a multi camera system. I though I could simply remap in the launch file from a generic image topic to the actual camera topic and set the image_transport parameter to compressed but it just ended up subscribing to image/compressed and I end up having to remap this latter one instead.

[NicoMandel]

I have the same issue and still no clue how to solve it. Does anyone have a solution yet @christian-rauch ?

[christian-rauch]

The only workaround I found is to manually remap the topic:

ros::NodeHandle n_priv;

image_transport::SubscriberFilter sub_image_depth;

sub_image_depth.subscribe(
    it,
    ros::names::remap("depth/image"), // manually remap topic from 'depth/image' to target
    1,
    image_transport::TransportHints("raw", ros::TransportHints(), n_priv, "depth/image_transport")
);

See https://github.com/christian-rauch/depth_box_filter/blob/master/src/depth_box_filter_node.cpp for the full example.

[NicoMandel]

Thank you for the advice @christian-rauch . I am still a little confused, I have a raspberry running as an onboard computer, with a marker detection package. Can I just apply the fix in this line and then continue with the namespace remapping as is or am I basically hard-coding the names of the image topics now, without a chance to remap?
The line that is throwing the thing off is this one in a launch file, with the output turning into:

Subscriptions: 
 * /ml_detector/image_raw/compressed [unknown type]
 * /webcam/camera_info [sensor_msgs/CameraInfo]

[christian-rauch]

Can I just apply the fix in this line and then continue with the namespace remapping as is or am I basically hard-coding the names of the image topics now, without a chance to remap?

If you apply the remapping via ros::names::remap("image_raw") then you do not have to hardcode anything.

Just take the ROS node at https://github.com/christian-rauch/depth_box_filter as an example. The remapping is done in the launch file and the remap method provides this mapping in the node.

[NicoMandel]

Somehow, after applying the fix, see here
And using a launch file to try with my webcam, see here
with the following lines:

        <remap from="~image_raw" to="$(arg image_topic)"/>
        <remap from="~camera_info" to="$(arg camera_info_topic)"/>
        <param if="$(arg compressed)" name="image_transport" value="compressed"/>

I still get the same issue, with the subscription going to:

Subscriptions: 
 * /ml_detector/image_raw/compressed [unknown type]
 * /webcam/camera_info [sensor_msgs/CameraInfo]

What am I missing here? I recompile and source the workspace...

[christian-rauch]

Use my depth_box_filter as a template to verify that this approach works for you. This example contains the node with the remapping and a launch file, which you can use to reproduce this effect for your node. If the remapping in my nodes does not work, please report an issue report there. Your launch file differs from mine in that you use the private namespace (~).

Otherwise, this issue is not a discussion forum. If you have further troubles with remapping topic names, please ask this specific question at https://answers.ros.org.

[KenYN]

Just a quick note for nodelets: you need to use nodelet::Nodelet::getRemappingArgs() to resolve this issue. EDIT: Here is my workaround:

// Suppose we have a <remap from="image_raw" to="/another/location/with/camera/image"/>
// Also class ThisClass: public nodelet::Nodelet { ... };
std::string inTopic = ros::names::resolve(getNodeHandle().getNamespace(), std::string("image_raw"));
auto const &inRemapped = getRemappingArgs().find(inTopic);
if (inRemapped != getRemappingArgs().end())
{
    inTopic = inRemapped->second.c_str();
}
// Now we can subscribe with the remapped topic
m_image_sub = m_it.subscribe(inTopic, 1, &ThisClass::image_raw_cb, this);