Conversion between world and occupancy grid msg

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Hi all,

I have a map of the environment my robot has to move in. The map is stored in the standard way as an occupancy grid map and served via map_server service. I want to implement a PRM on top of this map so I need to access the value of the occupancy grid from the world coordinate. To be more precise, given a point in the map frame (say [x0,y0]), I need to access the value of the occupancy grid of [x0,y0], that is, I need a conversion from the world coordinate to the index of the vector that represents the occupancy grid.

Do I have to implement everything from scratch or there are already some api that perform this conversion?

Regards