GUI support of image_transport compressed datatypes

[jhoare]

We would like to use the compressed image formats so we can more efficiently stream images over a wireless network.

[pjreed]

Mapviz's image plugin already uses ImageTransport under the hood to handle it subscriptions, which means it is possible to make it used a compressed image transport by setting the appropriate parameter. For example, if mapviz is running as a node named mapviz_1474307178780514021, you can run this:

$ rosparam set /mapviz_1474307178780514021/image_transport compressed

Afterward, mapviz will subscribe to the compressed versions of any new image topics that support it. Existing subscriptions will probably need to be re-done, which is easily done by changing the topic name to something else and then changing it back.

It would be nice if this could be configured through the GUI, though.

[jhoare]

Ah okay good to know. I was expecting the interface to be more similar to rqt_image_view where I select the "compressed" image topic to used the compressed transport.

[jhoare]

Note, when using "compressed" image transport and visualizing multiple images I get the following error (but everything seems to be working correctly):

[7fe9d06c3a40] [/commandcenter/mapviz_messier_20127_7398547913354522751/ServiceManager::advertiseService:147]: Tried to advertise a service that is already advertised in this node [/commandcenter/mapviz_messier_20127_7398547913354522751/compressed/set_parameters]

[pjreed]

Hmm, I can confirm that, and it is peculiar. It doesn't seem like that hurts anything, but it's not immediately obvious why ServiceManager::advertiseService would end up getting called as a result of ImageTopic::subscribe... I'll investigate.

[evenator]

ImageTransport::subscribe is a little...magical. For the compressed topic, it creates a service for communicating the compression information from the publisher to the subscriber. Other image transport plugins have other problems--Theora cannot handle multiple subscribers or resubscribing. Those are the only two I've used, so I can't comment further, but suffice to say, ImageTransport can be a little fragile when you're using it for anything besides single publisher->single subscriber as in a processing pipeline.