Generate gazebo models about icub-models-generator

[traversaro]

In the current generation process in place, it should be trivial to generate a Gazebo model for any specific iCub instance (then exposing just some "major" models for simplifying the life to our users is doable as well).

We discuss a bit on how to implement this with @nunoguedelha today, updating the issue to avoid to forget.
We would like to use the structure that we set up in https://github.com/robotology-playground/icub-models . Currently in that repo the URDF can be used by both ROS and YARP software with no problem. Using them directly in gazebo is a bit problematic, so the options are:

• Automatically generate a separate Gazebo model directory, copying at build/install time the meshes, and add that one to GAZEBO_MODEL_PATH,
• hack a bit the system, and keep the existing structure without copying meshes in a different position, so that models can also be used directly from the source directory.

[traversaro]

Relatively to the second option, let's take note of somethings.
A Gazebo model is identified by a first-level directory (containing a model.config) inside one of the directories specified in the GAZEBO_MODEL_PATH environmental variables [1].
This means that the iCub directory in https://github.com/robotology-playground/icub-models can't be put directly in GAZEBO_MODEL_PATH, because the urdf models are inside the robots/iCub*** directory (and this is a constraint of the YARP system to handle robot-specific configuration files).
A possible solution to this is to add iCub/robots to the GAZEBO_MODEL_PATH.
Once a model is correctly found by Gazebo, the meshes and other assets (such as .ini YARP configuration files) need to be found by Gazebo. Gazebo uses a URI scheme, and automatically convert any package:// scheme to model://. The nice thing is that the model:// identify a file with its location in any of the directories contained in GAZEBO_MODEL_PATH, not only the directory for which the original model was loaded!
So, to load assets that are contained in the iCub/meshes directory (and eventually iCub/gazebo etc, etc) it is sufficient to add the appropriate directory to GAZEBO_MODEL_PATH env variable.

[1] : https://bitbucket.org/osrf/gazebo/src/b09f8d3594774551e76bd53789eb72b575fb2775/gazebo/gui/InsertModelWidget.cc?at=default&fileviewer=file-view-default#InsertModelWidget.cc-331

[traversaro]

Related: https://bitbucket.org/osrf/gazebo/pull-requests/2479 .
By the way, I think that the ideal way to solve this kind of problems would be if URDF/SDF had a relative path URI scheme, to specify the location of assets relatively to the URDF/SDF file.

[nunoguedelha]

This issue originated an Epic (#52) that is now tracking all the deriving issues and PRs for implementing the model generation process described here.

Closing this issue.