Add units and docs for Joint3D/2D stiffness and damping #96523
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aaronfranke
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I updated the PR to include an overhaul to the documentation of these values. I was planning on just adding the units to the documentation, but the descriptions were really bad and often plain wrong. I'm adding the bug label because I believe the old documentation was so bad that this qualifies as a bug fix. On PR #94862, @akien-mga mentioned that we should only use the Newton-based units for vehicle wheel suspension. Should we do the same here? Note that for 2D, we can't use Newtons, because Godot's 2D doesn't use meters. |
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| </member> | ||
| <member name="angular_limit_y/restitution" type="float" setter="set_param_y" getter="get_param_y" default="0.0"> | ||
| The amount of rotational restitution across the Y axis. The lower, the more restitution occurs. | ||
| The amount of rotational restitution across the Y axis. The lower, the more restitution occurs. Restitution is a dimensionless value. |
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Unsure if lack of units is worth specifying, given every other description is now very explicit about it.
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I think it is worth specifying, just to be extra clear.
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Similar to what #94862 does for VehicleWheel3D, this PR adds units for the Joint2D/3D-derived nodes.
General info on stiffness and damping in metric:
Stiffness is measured in different units depending on the type of stiffness. For linear stiffness, it is measured in Newtons per meter, or kg/s² in SI base units. For angular stiffness, it is measured in Newton-meters per radian, or kg⋅m²/s²/rad in SI base units. This difference is because linear stiffness involves force divided by distance (N / m), while angular stiffness involves torque divided by angle (N⋅m / rad).
Damping is measured in different units depending on the type of damping. For linear damping, it is measured in Newton-seconds per meter, or kg/s in SI base units. For angular damping, it is measured in Newton-meter-seconds per radian, or kg⋅m²/s/rad in SI base units. This difference is because linear damping involves impulse divided by distance (N⋅s / m), while angular damping involves torque-impulse divided by angle (N⋅m⋅s / rad).
Godot-specific info:
The "softness" property in 3D is incorrectly named, it's actually stiffness. We should rename this in the future.
The "softness" property in PinJoint2D is actually softness, inverse stiffness, so it's s²/kg instead of kg/s². This may be worth replacing in the future for consistency with other joints using stiffness. The conversion is
1.0 / softness == stiffnessand1.0 / stiffness == softness.The "relaxation" property on HingeJoint3D and ConeTwistJoint3D is inverse damping, so it's rad⋅s/kg/m² or rad/N/m/s. This may be worth replacing in the future for consistency with other joints using damping. The conversion is
1.0 / relaxation == dampingand1.0 / damping == relaxation.