Create panel > Helpers > reactor > Point-Point
Menu bar > reactor > Create Object > Point-Point Constraint
reactor toolbar > Create Point-Point Constraint button
The Point-Point (point-to-point) constraint lets you attach two objects together, or an object to a point in world space. It forces its objects to try to share a common point in space. The objects can rotate freely relative to each other, but always have the attachment point in common. When you set up the constraint, the point is defined in the object space of each object involved. During the simulation the constraint tries to apply forces to the objects so that the two pivot points defined by the two objects match.
reactor also provides two variations on the Point-Point constraint: Limited and Stiff Spring. With a Limited Point-Point constraint, you can specify limits for how much the child object can rotate relative to the parent. A Stiff Spring constraint is similar to a standard Point-Point constraint with one important exception: It holds the constrained bodies apart at a specified distance, as if they were attached at each end of an invisible rod. The Stiff Spring variant requires a point in the body space of each of two bodies, and a constant distance by which reactor must keep these two points separated. Each body is free to rotate around its point.
The constraint is simulated when it has the correct number of rigid bodies attached and is included in a valid Constraint Solver. When not selected, an invalid Point-Point constraint is red in the viewport.
To attach objects to the Point-Point constraint:
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On the constraint’s Properties rollout, click the Child pick button, and then select the object to use as the child object in one of the viewports. By default, this attaches the constraint to the body at the body’s pivot point.
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If you do not want to assign a second body to the constraint, turn off the Parent check box if necessary.
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If the constraint is to be two-bodied, make sure Parent is on and then use the Parent pick button to set the parent for the constraint. By default, the attachment point for the parent is also aligned with the child body's pivot point, which means that the bodies will maintain their current positions relative to each other in the simulation.
To change the attachment positions for the bodies:
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In the modifier stack open the constraint’s sub-object list.
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Access the sub-object level corresponding to the object in whose space you want to move the attachment point: Parent Space or Child Space. If the constraint is single-bodied, Parent Space allows you to move the world attachment position.
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The corresponding point becomes active in the viewports and can be moved using the Move tool.
The attachment point maintains its position relative to its corresponding object. For example, if you move the child object, its space, and hence its attachment point, moves with it. The attachment point doesn't have to actually be on either object.
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To reset the attachment point to the child object’s pivot, reattach the objects to the constraint, or click the Align Spaces To > Child Body button.
To limit the rotation between the attached objects:
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In the Constraint Type group, choose Limited.
By default, the child body can rotate a total of 90 degrees around each axis relative to the parent during the simulation: 45 degrees clockwise and 45 degrees counterclockwise.
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You can then choose which limits to display around the Parent Space constraint axis by turning on each one's check box.
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If you then change the limits, the limit display updates to reflect your changes to help you choose appropriate values. Upon simulation you can see that the rotation between the objects has been limited.
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You can also change the orientation of the axis to be limited by rotating the Child Space and Parent Space sub-objects.
To create a Stiff Spring constraint:
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With a Point-Point constraint selected, open the Properties rollout.
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You can now specify a Length value for the stiff spring. The default is 0.0: the same as a Point-Point constraint.
During the simulation, reactor works to maintain this distance between the constrained objects' attachment points. To use the current distance between the attachment points as the length, click the Set button.
Parent Space—At this sub-object level, you can select and move the constraint attachment point in the parent body's local space. When you assign a parent object to the constraint, the Parent Space attachment point is set to the child object’s pivot point. If you alter the position of the Parent Space attachment point, it maintains its new position relative to the parent during the simulation. If the constraint is single-bodied, the Parent Space's position represents the point in the world where the constraint is attached.
Child Space—At this sub-object level, you can select and move the constraint attachment point in the child body's local space. When you assign a child body to the constraint, the Child Space attachment point is set to the child object’s pivot point. If you alter the position of the Child Space attachment point, it maintains its new position relative to the child during the simulation.
Parent—When on, the constraint has two bodies and you can designate an object to be the parent body. When off, the constraint is single-bodied and the point occupied by the Parent Space sub-object is the parent.
You assign the parent object by clicking this button and then selecting a rigid body from any viewport. Thereafter the button displays the name of the parent body.
Child—Displays the name of the second rigid body attached to the constraint. You assign the child object by clicking this button and then selecting a rigid body from any viewport.
Align Spaces To—Use these options to align the bodies' local constraint spaces. You can find out more about each option in Working With Constraint Spaces.
Lock Relative Transform—When on, the relative transform between the child and parent constraint spaces is locked: If you move either space in the viewport, the other space moves and rotates along with it, and vice-versa.
Strength/Tau—Govern the impulses applied to the constraint's bodies in order to maintain the constraint, and so how strongly the constraint works to restrict their movement. You can find out more about these parameters in Strength and Tau.
Limited (Limited constraints only)—Lets you specify how much the child object can rotate relative to the parent around the x, y, and z axes - for each axis there is a minimum (counterclockwise) and maximum (clockwise) limit. You can use the check boxes to switch on or off the limit displays in the viewport.
Length (Stiff Springs only)—The distance that reactor should maintain between the constraint attachment points.
Set—Sets Length to the current distance between the attachment points.
Breakable—When on, the constraint is breakable. If its breakable limits are exceeded during simulation, it ceases to exert impulses on the attached objects. You can find out more about breakable constraints in Breakable Constraints.