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## note.0= [[image_transport/Tutorials/PublishingImages|Writing a Simple Image Publisher]]
## note.1= [[image_transport/Tutorials/SubscribingToImages|Writing a Simple Image Subscriber]]
## descriptive title for the tutorial
## title = Running the Simple Image Publisher and Subscriber with Different Transports
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## description = This tutorial discusses running the simple image publisher and subscriber using multiple transports.
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## next.0.link= [[image_transport/Tutorials/ManagingPlugins|Managing Transport Plugins]]
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<<IncludeCSTemplate(TutorialCSHeaderTemplate)>>

<<TableOfContents(4)>>

<<Buildsystem()>>

=== Running the Publisher ===
Make sure that a roscore is up and running:

{{{
$ roscore
}}}
In a previous tutorial we made a publisher node called "my_publisher"; now run the node with an image file as the command-line argument:

{{{
$ rosrun image_transport_tutorial my_publisher path/to/some/image.jpg
}}}
To check that your node is running properly, list the topics being published:

{{{
$ rostopic list -v
}}}
Look for `/camera/image` in the output, which should look something like:

{{{
Published topics:
 * /rosout [roslib/Log] 1 publisher
 * /camera/image [sensor_msgs/Image] 1 publisher
 * /rosout_agg [roslib/Log] 1 publisher

Subscribed topics:
 * /rosout [roslib/Log] 1 subscriber
}}}
If you have non-default transport plugins built, you may see additional published topics in the `/camera/image/` namespace.

=== Running the Subscriber ===
In the last tutorial, we made a subscriber node called "my_subscriber"; now run it:

{{{
$ rosrun image_transport_tutorial my_subscriber
}}}
You should see a window pop up with the image you gave to the publisher.

Let's look at the node graph:

{{{
$ rosrun rqt_graph rqt_graph
}}}
{{attachment:transport_graph.png}}

=== Changing the Transport Used ===
Currently our nodes are communicating raw <<MsgLink(sensor_msgs/Image)>> messages, so we are not gaining anything over using `ros::Publisher` and `ros::Subscriber`. Let's change that by introducing a new transport.

The `compressed_image_transport` package provides plugins for the "compressed" transport, which sends images over the wire in either JPEG- or PNG-compressed form. Notice that `compressed_image_transport` is not a dependency of your package; `image_transport` will automatically discover all transport plugins built in your ROS system.

Shut down your publisher node and restart it. If you list the published topics again, you should see a new one:

{{{
 * /camera/image/compressed [sensor_msgs/CompressedImage] 1 publisher
}}}

Now let's start up a new subscriber, this one using compressed transport.  The key is that `image_transport` subscribers check the parameter `~image_transport` for the name of a transport to use in place of "raw". Let's set this parameter and start a subscriber node with name "compressed_listener":

{{{
$ rosparam set /compressed_listener/image_transport compressed
$ rosrun image_transport_tutorial my_subscriber __name:=compressed_listener
}}}

You should see an identical image window pop up. Let's check the node graph again:

{{{
$ rosrun rqt_graph rqt_graph
}}}
{{attachment:transport_graph_with_compressed.png}}

`compressed_listener` is listening to a separate topic carrying JPEG-compressed versions of the same images published on `/camera/image`.

Note that if you just want the compressed image, you can change the parameter globally in-line:
{{{
$ rosrun image_transport_tutorial my_subscriber _image_transport:=compressed
}}}


=== Changing Transport-Specific Behavior ===
For a particular transport, we may want to tweak settings such as compression level, bit rate, etc. Transport plugins can expose such settings through [[rqt_reconfigure]]. For example, `/camera/image/compressed` allows you to change the compression format and quality on the fly; see the [[compressed_image_transport|package documentation]] for full details.

For now let's adjust the JPEG quality. By default, "compressed" transport uses JPEG compression at 80% quality; let's change it to 15%:

{{{
$ rosrun rqt_reconfigure rqt_reconfigure
}}}

Now pick /camera/image/compressed in the drop-down menu and move the jpeg_quality slider down to 15%.  Do you see the compression artifacts in your second view window?

Dynamic Reconfigure has updated the dynamically reconfigurable parameter /camera/image/compressed/jpeg_quality.  You can verify this by running:

{{{
$ rosparam get /camera/image/compressed/jpeg_quality
}}}

This should display 15.

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