sharedmem_image_transport is a plugin for transporting images via shared memory instead of network loopback, when running on a single machine.
This package behaves as a normal image_transport plugin (see compressed_image_transport or theora_image_transport). However, it requires to have the sharedmem_manager node running. The sharedmem_manager creates the shared memory segment in which all images are stores. The segment size is defined by the private parameter segment_size (in bytes).
Obviously, this transport will only work between process on the same machine, and possibly on the same user.