This tutorial shows you how to set up advanced physics constraints to limit the movement of a physicalized entity using three dimensional objects as surface constraints, and the use of spline paths.

Constraining Movement Within a 3D shape

3D Entity Constraint Setup

1. Add a Physics Area component

2. Set the Area component area Type to Box, Sphere, Capsule, Cylinder, or Geometry. This shape will function as the actual surface constraint.

   If you enable proxies (via the ViewModes toolbar → Overlay Collision Proxies button, see image below), the Area component will show the selected shape type in red, and this exact volume will become the space over whose surface your constrained entity can move.

   
   Overlay Collision Proxies button

   
   Red physics proxy

   To learn how to display the ViewModes toolbar, see this page.

   Use the Size, Radius and Height properties to control its size. For more complex constraints, pick Geometry and load a closed CGF mesh. Keep in mind that with the Geometry type, the proxy mesh of the CGF will be used as a constraint, not the visible mesh.

   Smooth surfaces will work best.
3. Create a physicalized object (Mesh + RigidBody components, with a mass or density value).
4. Add a plane constraint component to the object. The orientation doesn't matter. Point and line constraints will not work for 3D physics area components.

5. Use Properties → Entity Links to link the physicalized object to the area shape you added.

   Make sure you close the Pick Object tool after you select the area shape.
6. Copy the Name of the entity link and paste it into the Helper Link Name property of the Plane Constraint component that is attached to the physicalized object. The entity is now constrained to the area shape.
7. Make sure your physicalized entity is positioned within the red physics area you have, with its pivot as close as possible to a surface plane, otherwise it will snap into the nearest part of the constraint area on game start, possibly colliding with other entities.

Movement will be constrained to the edges - the outer surface - of a volumetric physics area component (shown in red when proxies are shown. This could work for something like an insect that should only crawl along a surface and doesn't fly, or a circus motorcycle that should only travel along the inner surface of spherical cage.

Note that you can also add a Physics Area component to an Area box, sphere, solid, path, or even a clip volume. (You can't add components to water volumes.) The geometry of the area shape itself (shown in blue) will be ignored by the constraint, so whether you use an area shape or an empty entity makes no difference, as the constraint shape itself comes from the Physics Area component. I.e., the only reason to use an area or clip volume would be to serve a function other than to constrain movement. The area shape is effectively just a container, but its shape is ignored by any moving objects constrained to it. Remember that the actual geometry that is used as the surface constraint can only be seen if physics proxies are made visible, and is displayed in red.

In the example image here, the red sphere is generated by the physics area component, and will be used as the surface constraint; the blue box is generated by the area box, and will have no effect as a constraint.

Red sphere generated by Physics Area component

Moving Objects Along a Spline Path Constraint

This kind of path constraint can be used for cases like allowing a submarine or whale to follow a specific path through water, or a bird or plane to follow a specific, invisible path through air. (For objects that follow geometry, like a bobsled track, or terrain, it probably makes more sense to just use physics and allow gravity or a physics impulse to move the object.)

Adding the Path Constraint

1. Use the Create Object tool to add an Area → Shape entity. Draw the desired path. You'll note that at this point, your shape is composed only of straight lines. Later, this path will be interpolated to a spline path automatically.
   It's recommended to draw the simplest possible path, double clicking on the last point to exit the Create Object mode. Then you can use the Edit Shape button on the Properties panel's Shape properties to move the existing points, Ctrl-Click on the path to add new points, double-clicking to remove existing points.

   You must have a minimum of three points.
2. The path can be open (a line) or a closed loop (like a roller coaster or race track).

3. Add a Physics → Area component. Set the area component area Type to Path(Spline). The area component will show the path in red, and you'll see the allowed movement area drawn as a tunnel:
   
   Spline path
   Use the radius property to control the tunnel's diameter.

   The radius of the tunnel should be bigger than the physics proxy of the object passing through it.

You'll see that the original straight line path has now been interpolated to a spline path. You may wish to edit the original area shape while keeping proxies visible so you can adjust the path of your spline.

Adding a Constrained Movable Entity

1. Use the Create Object tool to create a physicalized entity (Mesh + RigidBody components). Assign it a Mass.

2. Add a Physics → Line Constraint component to the physicalized entity.

   While the line constraint can be along any axis, keep in mind that the object will be rotated on game start so the line constraint is parallel to the spline path, so if your movable entity has a recognizable front and rear, be sure to make the line constraint and the entity itself parallel to the spline path nearest the object, facing forward in the direction of flow.

   A spline is a generalization of a line constraint, which allows the constrained object to move along the spline path. No other constraint type will work.

3. With the physicalized entity selected, in the Properties → Entity Links panel, click on Link Tools → Pick and link the entity to the area shape.

   Make sure you close the Pick Object tool after you select the area shape.
4. Copy the name of the entity link between the physicalized entity and the area shape.

5. Paste the link name into the line constraint component's Attachment Parameters → Helper Link Name property. This constrains the object's movement to the area spline path.

   In order for your entity to move, the Radius of the of the physics area component's Shape → Radius properties must be large enough to accommodate your object. A too-small radius will "squeeze" the object and impede its movement.
6. To test the constraint, enable Physics/AI (Ctrl + P) and use the Pull Physics tool to drag your object along the area path.

Moving the Entity Along the Path Automatically

While you can use any force to move your object along the path constraint, you can also use the Physics → Area component's Flow value to push entities along the spline path. If travel is not merely in one direction, but the path twists in various directions or is a closed loop, disable the Uniform property in order to keep the force behind the object no matter which way it faces. Otherwise, a positive Flow value on the Y axis, for example, will only push the object in a positive direction on the Y axis, and will push the object backwards if the direction of the path rotates 180 degrees.

Controlling Twist

If your moving entity has an obvious top/bottom, you may notice that it twists in "barrel rolls" as it moves – twisting around the line constraint's axis. To control this:

1. Disable Lock Rotation on the Line Constraint component.
2. When an object is constrained to a spline path using a Line Constraint component, the Minimum Limit and Maximum Limit on the line constraint no longer affect how far the object can slide along the line; they control how far the object can rotate around the axis of the line constraint. These units are specified in degrees. Experiment with a range of values to get the desired amount of rotation.
3. For further control, add a Point Constraint component to the moving object using the Z axis (or whatever axis is perpendicular to the line constraint).
4. Enable Free Position.
5. Erase the default Helper Link Name on the Point Constraint component.
6. To see the point constraint's axis, you can move it slightly up or down so it doesn't overlap the line constraint. Set the Minimum Limit to -360 and Maximum Limit to 360.
7. The Maximum YZ Angle value controls how much the object can "barrel roll," but also how far the entity can twist around its own Z axis, so this value has to be large enough so it can navigate the turns in your spline path. Test, adjust and use the minimum possible value to avoid the "barrel roll" effect.

Video Tutorial