Question about symmetry in assemblies using RigidJoint

[roman-dvorak]

Hi,

I am encountering an issue when assembling three objects using RigidJoint. I've noticed that the connections between the objects are not symmetrical, although I expected the outcomes to be consistent on both sides of each joint.

Here is the code I'm using:

class AlProfile(BasePartObject):
    def __init__(
        self,
        length: float,
        rotation: RotationLike = (0, 0, 0),
        align: Union[None, Align, tuple[Align, Align]] = Align.CENTER,
        mode: Mode = Mode.ADD,      
    ):
        context: BuildPart = BuildPart._get_context()

        with BuildPart() as profile:
            cs = cross_section_30_30().sketch
            extrude(cs, amount=length)
        
        super().__init__(
            part=profile.part, rotation=rotation, align=tuplify(align, 3), mode=mode
        )


        ra = RigidJoint("end_a", self, Location(Plane(self.faces().sort_by(Axis.Z)[-1])*Rot(X=180) ) )
        rb = RigidJoint("end_b", self, Location(Plane(self.faces().sort_by(Axis.Z)[0]) ))
        
        LinearJoint("side_a", self, axis= Axis((15, 0, -length/2), (0, 0, length)) , linear_range = (0, length) )
        LinearJoint("side_b", self, axis= Axis((0, 15, -length/2), (0, 0, length)) , linear_range = (0, length) )
        LinearJoint("side_c", self, axis= Axis((-15, 0, -length/2), (0, 0, length)) , linear_range = (0, length) )
        LinearJoint("side_d", self, axis= Axis((0, -15, -length/2), (0, 0, length)) , linear_range = (0, length) )

        self.color = "silver"
        self.label = f"AlProfile 30x30x{length}"

ap = AlProfile(100)
ap_b = Pos((50, 0, 0)) * AlProfile(50)
ap_b.color = "gold"
ap_c = Pos((-50, 0, 0)) * AlProfile(10)
ap_c.color = "blue"

show([ap, ap_b, ap_c], render_joints=True, reset_camera=Camera.KEEP)

Below are images illustrating the two situations:

Situation 1 (correct)

ap.joints["end_b"].connect_to(ap_b.joints["end_a"])
ap.joints["end_a"].connect_to(ap_c.joints["end_b"])

assembly = Compound(label="assembly", children=[ap, ap_b, ap_c])
show(assembly, render_joints=True, reset_camera=Camera.KEEP)

Situation 2 (wrong)

ap_b.joints["end_a"].connect_to(ap.joints["end_b"])
ap_c.joints["end_b"].connect_to(ap.joints["end_a"])

assembly = Compound(label="assembly", children=[ap, ap_b, ap_c])
show(assembly, render_joints=True, reset_camera=Camera.KEEP)

I would like to inquire whether this is a known bug, or if such asymmetry is an intentional feature. I have not found any information about this in the documentation or in issues. Could someone please explain if there is something I might be missing, or how I should proceed?

Thank you for any help or advice.

[gumyr]

A key aspect of connect_to is that it works as the_fixed_joint.connect_to(the_joint_that_will_move). Here is an example:

from build123d import *
from ocp_vscode import *
from bd_warehouse.open_builds import VSlotLinearRail

rail1 = VSlotLinearRail(10 * CM)
RigidJoint("0", rail1, rail1.faces().sort_by(Axis.Z)[0].location)
RigidJoint("1", rail1, rail1.faces().sort_by(Axis.Z)[-1].location)
slide_axis = Axis(
    Edge.make_mid_way(
        *rail1.edges().filter_by(Axis.Z).group_by(Axis.X)[-1].sort_by(Axis.Y)[1:3]
    )
)
LinearJoint("slide", rail1, slide_axis, (0, 10 * CM))
rail1.color = Color(0x929966)

rail2 = VSlotLinearRail(2 * CM)
RigidJoint("0", rail2, rail2.faces().sort_by(Axis.Z)[0].location)
RigidJoint("1", rail2, rail2.faces().sort_by(Axis.Z)[-1].location)
rail2.color = Color(0x669299)

rail3 = VSlotLinearRail(6 * CM)
RigidJoint("0", rail3, rail3.faces().sort_by(Axis.Z)[0].location)
RigidJoint("1", rail3, rail3.faces().sort_by(Axis.Z)[-1].location)
rail3.color = Color(0x996692)

rail4 = VSlotLinearRail(8 * CM)
RigidJoint("0", rail4, Rot(0, -90, 0) * rail4.faces().sort_by(Axis.Z)[0].location)
RigidJoint("1", rail4, rail4.faces().sort_by(Axis.Z)[-1].location)
rail4.color = Color(0xC0C0C0)

rail1.joints["1"].connect_to(rail2.joints["0"])
rail2.joints["1"].connect_to(rail3.joints["0"])
rail1.joints["slide"].connect_to(rail4.joints["0"], position=6 * CM)

show(rail1, rail2, rail3, rail4, render_joints=True)

There is a description this in the docs here (https://build123d.readthedocs.io/en/latest/joints.html#rigid-joint) "Once a joint is bound to a part this way, the connect_to() method can be used to repositioning another part relative to self which stay fixed - as follows:"

Does this help?