Update PinJoint2D API with angle limits and motor speed

[Ughuuu]

Closes: godotengine/godot-proposals#7646

[Ughuuu]

Took the motor part from here: https://github.com/slembcke/Chipmunk2D/blob/master/src/cpSimpleMotor.c
The joint limits part from here: https://github.com/slembcke/Chipmunk2D/blob/master/src/cpRotaryLimitJoint.c#L25

As I understood, the whole physics api now uses parts from chipmunk physics engine, which seems to be right.

Still need to write impl for calculating initial angle between objects. Right now the limits are for angle difference, but it works.

Screen.Recording.2023-09-13.at.19.55.13.mov

Also, one small issue is the angular limits seem to be kinda soft, not 100% sure why.

Let me know what you think.

[AThousandShips]

We should add a reference to COPYRIGHT.TXT to cover that since it was forgotten when that file was added, something like:

Files: ./servers/physics_2d/godot_joints_2d.cpp
Comment: Chipmunk2D Joint Constraints
Copyright: Copyright (c) 2007 Scott Lembcke
License: MIT

And add it just before the line Files: ./servers/physics_3d/collision_solver_3d_sat.cpp with one line of space between the entries

[Ughuuu]

Fixed the above issue, implemented feedback, review again and it's ready for merging I think, from my side at least. Works also same with implementation in box2d, well, almost (most notable difference is in box2d limits are harder, while with this implementation it seems limits are a bit softer).

[rburing]

Node properties are more commonly called *_enabled than enable_*, which I think make sense because enable_* sounds more like a function. So for the PinJoint2D node I propose having a motor_enabled property with setter set_motor_enabled and getter is_motor_enabled, and an angular_limit_enabled property with setter set_angular_limit_enabled and getter is_angular_limit_enabled. I also propose renaming the motor/target_velocity property to motor_target_velocity, and similarly replacing angular_limit/lower by angular_limit_lower and angular_limit/upper by angular_limit_upper. This makes the properties easier to write in scripts, and we can still make two groups in the inspector using the common prefixes, similar to how it is done in RigidBody2D here:

godot/scene/2d/physics_body_2d.cpp

Lines 1038 to 1041 in 5f1e56f

	ADD_GROUP("Angular", "angular_");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "angular_velocity", PROPERTY_HINT_NONE, U"radians,suffix:\u00B0/s"), "set_angular_velocity", "get_angular_velocity");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "angular_damp_mode", PROPERTY_HINT_ENUM, "Combine,Replace"), "set_angular_damp_mode", "get_angular_damp_mode");
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "angular_damp", PROPERTY_HINT_RANGE, "-1,100,0.001,or_greater"), "set_angular_damp", "get_angular_damp");

[Ughuuu]

Thanks for the idea, very good naming scheme. I like it better. Updated. Also, angular_limit sounds a little bit weird. Should it be angular_limits?

[akien-mga]

Thanks!

[Ughuuu]

Thank you!

[hackenshaw]

Why is it a soft limit? Is that intentional? When I think of limits I expect it to be a hard limit (like a hinge)

[Ughuuu]

The code was taken from another physics engine that was initially used in Godot Physics 2d, chipmunk physics engine. And there it is implemented like that. I am not really an expert in 2d physics so working with an already implemented solution was easier.

I have added though in this PR also the licensing and the link where it was taken from.

If you want hard limits, in box2d for eg. they have hard limits. I guess it depends on the physics engine and how it's implement? If not we can think of adding another parameter that specifies how hard/soft are the limits?

[hackenshaw]

This is definitely a good step. Thanks for your contribution. I'm also not a physics expert but I needed it for a client project. I'll take a look how it's implemented in box2d. Physics is always super finicky

[Ughuuu]

In the box2d one the angle limits are not yet implemented as this was just merged.
In any case, here is the box2d repo: https://github.com/appsinacup/godot-box2d

[mitsuhashish]

If you want hard limits, in box2d for eg. they have hard limits. I guess it depends on the physics engine and how it's implement? If not we can think of adding another parameter that specifies how hard/soft are the limits?

I think adding another parameter for the stiffness of the limits would help to provide both soft and hard limits

[cody82]

With which torque does it drive the joint?

[Ughuuu]

For the angle limit or for the motor? Also the code is taken from the chipmunk engine, as can be seen, so I'm assuming it's similar to that.

[cody82]

I meant for the motor. Looks like it can apply unlimited torque.

[Ughuuu]

I think so. If you want a different case I can make a proposal and add a maxTorque property maybe?

[cody82]

Right. But actually the motor does not work for me at all. It just does nothing. I have a wheel which is connected to another body but nothing happens if I set motor_enabled to true and motor_target_velocity to 100. What could be wrong?

[Ughuuu]

Here is project for other people to test:

Screen.Recording.2023-09-18.at.22.23.26.mov

JointLimits.zip

Can you try this project that I tested this PR with and see if that works for you and try to work from that?

[Ughuuu]

Also I'm not sure if this kind of help support discussion should happen here, as this PR is closed already. If you need anything, open a new godot proposal or ask on discord or on chat or on forum better. This way it won't notify the main devs also.