Add exportFirstBaseLinkAdditionalFrameAsFakeURDFBase option to ModelExporter

[traversaro]

Even if the fast majority of URDF models embed a "fake" base link in the form of a parent URDF link with no inertia to workaround ros/kdl_parser#27 , some models existing in the wild, such as the ROS-Industrial KUKA robots (see for example https://github.com/ros-industrial/kuka_experimental/blob/indigo-devel/kuka_kr16_support/urdf/kr16_2.urdf) actually have a root URDF link with a mass and inertia, and just add additional frames as child frames, i.e. :

  <link name="base_link">
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="2"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>
    </inertial>
    <!-- ... -->
  </link>
  <!-- ROS base_link to KUKA $ROBROOT coordinate system transform -->
  <link name="base"/>
  <joint name="base_link-base" type="fixed">
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <parent link="base_link"/>
    <child link="base"/>
  </joint>

to generate URDF models in this form, it is necessary to add an explicit option to the URDF ModelExporter, to decide how to generate the URDF "fake link" corresponding to the first additional frame of the base link.

[traversaro]

I am not documenting this in the changelog as it is an additional option of the ModelExporter class, that it is first introduced in v0.12, and for which there is a changelog entry.

[prashanthr05]

Model-kdl-urdf-parser relationship based question.
I do not quite understand what might happen if the base link has more than one additional frames and you do not want the first additional link as the fake base link?

I see here https://github.com/traversaro/idyntree/blob/b780bd770a2a2960833cea8abf159a3e576f73c0/src/model_io/urdf/src/URDFModelExport.cpp#L350
we are handling only one additional frame scenario.

Maybe we should create an issue for this to be handled later if it's not necessary now.

And by this feature in the current PR, we handle additionally handle a case if we do not want the first additional frame to be default fake-base.

[traversaro]

I do not quite understand what might happen if the base link has more than one additional frames and you do not want the first additional link as the fake base link?

In that case, baseFakeLinkFrameIndex remains set to FRAME_INVALID_INDEX, and all the additional frames of the base link get exported as child fake links in https://github.com/traversaro/idyntree/blob/b780bd770a2a2960833cea8abf159a3e576f73c0/src/model_io/urdf/src/URDFModelExport.cpp#L394 .

[prashanthr05]

I see.

In that case, baseFakeLinkFrameIndex remains set to FRAME_INVALID_INDEX, and all the additional frames of the base link get exported as child fake links

Do you think this goes in the documentation?

[prashanthr05]

I approved the PR anyways because it was just my personal doubt. Maybe I was a bit confused with the Model and ModelExporter differences ;D

[traversaro]

I see.

In that case, baseFakeLinkFrameIndex remains set to FRAME_INVALID_INDEX, and all the additional frames of the base link get exported as child fake links

Do you think this goes in the documentation?

Added in 1db7d57 .