A message filter is something that a message enters and may or may not be spewed out of at a later point in time. The correct answer (a) image and message "filters".
Message filters is a roscpp and rospy utility library. It aggregates widely used message "filtering" methods into a unified domain. A message filter is something that a message enters and may or may not be spewed out of at a later point in time.
The time synchronizer, for example, accepts messages of various sorts from many sources and outputs them only if each of those sources has received a message with the same timestamp.
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