[PlayerStage-Gazebo] Bodies placed by center of mass not center of geometry

[Kevin B. <ba...@gm...>]

Hello,

I played with Gazebo 0.7 some time back and recall that when creating
models everything was based off of some point. I used my vehicle's
center of rotation and this made for a relatively logical layout.

Now I'm trying to create a similar model in Gazebo 0.8/SVN and running
into some trouble. It seems that when creating one body with multiple
geometries that the (0, 0, 0) point must be the center of geometry,
rather than some arbitrary point (such as center of rotation), like
you could do in code for Gazebo 0.7. This is still manageable and
somewhat logical, though it can be annoying to dimension a vehicle
based off the center of geometry.

But then came time to place the wheels. It seems that when placing a
body relative to another body, the XYZ coordinates must be in relate
to the center of _mass_ of the bodies. For simple objects this okay,
as it will be the same as the center of geometry. But if you have a
vehicle with an uneven weight distribution, not only would you have to
find the center of mass to determine the wheel location (very awkward,
but doable) but you also would need to modify the wheel locations if
you change the weight distribution, which does not make sense.

Is this the setup merely because of ease of implementation, or is
there reason behind it? I'm completely new to the Gazebo 0.8 codebase,
but I'm willing to spend some time trying to make it a bit more
logical to create models if a patch changing this would be accepted.

Thanks for the information,
-Kevin

For those interested a simple model to test this with can be found at
the link below. Run it and note the wheel locations (Turn on Bounding
Box). Then change the mass of box1 and box2 and notice the wheels new
location.

http://es.udmercy.edu/~barryke/boxtest.model