[ros-users] costmap grids

[Eitan Marder-Eppstein]

Nitin,

On Mon, Aug 9, 2010 at 11:09 AM, nitinDhiman <nitinkdhiman at gmail.com> wrote:

>
> Thanks Eitan,
> It did help.
>
> I want to combine an occupancy grid, generated by non-ROS algorithm setup,
> with Costmap2D grid in some another node and later use this new grid for
> path planning purpose. I agree on point that if vision sensor can also
> publish point cloud then Costmap2D can take over from there. But it may be
> good to have sth like this where point cloud publication is not possible.
>

In that case you could write code to take the GridCell messages being
published over ROS by the costmap (see
http://www.ros.org/wiki/costmap_2d#Published_Topics) and combine them with
your custom grid in some other node. You can get the resolution of the cells
by looking at the cell_width and cell_height fields of the
nav_msgs/GridCells message.

>
> About occupancy grid in ROS. It is in for of cell and each cell have
> resolution as meter/cell. I have impression that ROS always expect this
> resolution to be small. Do we deal with cases where multiple laser scans
> are
> passing through a cell. Then how it is dealt? For example, five laser beams
> pass through a cell and 2 represent obstruction while 3 do not. How this is
> modelled in Costmap2D?
>

The costmap lacks a probabilistic model so it will just take the latest
observation. We did this for efficiency reasons, and haven't found it to be
a huge issue... though you could write a version of the costmap that deals
with things differently.


>
> About path planning for rectangular robots, I am hoping to get an field A*
> based implementation in place.  I hope it will suffice.  I will also have a
> look at SBPL planner. Has someone tried using CHOMP?
>

I don't think that anyone has used CHOMP for the base, though it has been
used for our robot's arms.


>
> About move_base having one node only. Am I correct if I say CostMap
> generation using CostMap2DROS, global and local path planning are happening
> on this move_base node? If yes, Isn't it is loaded. One would probably like
> to take out say path planning as different node to other processor ! Or
> Advantage of having all of them in one place is "there are no network
> delays" ?
>

You're right that a lot of things are happening in the move_base node and
that its primarily done to avoid network delays. Major operations within the
node are threaded, so it can take advantage of something like a multi-core
machine, but you can't take the components contained in the move_base node
and distribute them across multiple machines. In the future, I'd like to
rewrite much of move_base using a nodelet based approach that will allow you
to configure navigation components to run in the same process or across the
network, but I'm not sure exactly when I'll get around to that.

Hope all is well,

Eitan


>
> thank you
> nitin
>
>
>
>
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