/sdf/world/physics/@type attribute specification contains Gazebo Classic-specific details.

[traversaro]

While reviewing https://github.com/robotology/gym-ignition/pull/336/files, I noticed that ignored is not an allowed value for the /sdf/world/physics/@type attribute according to the SDF spec: http://sdformat.org/spec?ver=1.8&elem=physics#physics_type .

However, the problem seems to be that the /sdf/world/physics/@type attribute specification contains Gazebo Classic-specific aspects.

Environment

Issue related to SDF specitication, version 1.8

Description

• Expected behavior: The SDFormat specification, being used in different simulator, should not contain Gazebo Classic information that do not apply to other simulators (such as Ignition Gazebo)
• Actual behavior: The list of supported physics engine in the SDF format specification is the one of Gazebo Classic.

Steps to reproduce

Read http://sdformat.org/spec?ver=1.8&elem=physics#physics_type .

This is related to #508, but it is specific about the /sdf/world/physics/@type attribute while that issue is more related to the /sdf/world/physics/ode, /sdf/world/physics/bullet elements.

[traversaro]

fyi @xela-95 , I wanted to open an issue for that but I found this one.