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Sensor data gives you two things: Where an obstacle is and where an obstacle isn't. Imagine you have a single laser that starts at point A encounters an obstacle at point B.

The marking operation is putting a lethal value in the costmap at point B.

The clearing operation is taking all the points between A and B and marking them as free space.

The process of finding the line is often referred to as ray tracing, and is explained here: http://en.wikipedia.org/wiki/Bresenham's_line_algorithm