Does anyone familiar with the details of roscpp or imae_transport know, specifically in the context of no subscribers, (a) how much overhead is there to publish something? and (b) does that overhead scale with the size of the message?

Specifically I am interested in the case of images. Consider three cases: 

1) a node publishes an image, and it is shown in image_view
2) a node publishes an image, but it is not shown in image_view or anywhere else
3) a node does not publish an image

Clearly there will be a lot more computation associated with (1) than (3), but what about (2) -- is it closer to (1) or (3) or somewhere close to the middle?

--Nick