Joints

Dropping objects on the floor is an easy task. But what if you want to simulate the physics of objects that are jointed together in a hierarchy? Rodeo understands most every joint that you can establish for an object in a hierarchy within the Joint Editor of Animator. Please refer to the Animator documentation how to edit the joint parameters of an object.

In the Joint tab of Rodeo you can verify the current joints of your simulation objects. The checkboxes show you which axes are locked for the currently selected object. If an object is linked to a parent object and has at least one axis constrained it will have a green "J" as an icon in the object list to the right. A gray "J" means that the object is either on the root level of the hierarchy or its linkage is set to "free" and has no limits.

Note: you can't edit the locked axes here. Please edit the joint setup in the Joint Editor of Animator.

Limited joints

Rodeo supports limited joints. These are joints that are only allowed to rotate within a defined range or only allowed to move in defined ranges. Position and rotation limits can be set in the Joint Editor of Animator. The rotational limits of a joint will be drawn in Rodeos OpenGL window as incomplete circles that show the limits of the rotation. See also the image to the right.

Limited joints and locked axes

Rodeo can solve up to two free axes with limited rotations. A joint with three free axes that are limited will yield in unstable results. Rodeo compensates for this with the definition of a twist axis.

Twist Axis

In case of a joint with more than 1 degree of rotational freedom you need to tell Rodeo how to interpret the rotational joint limits. Think of your arm for example. You can twist the arm around its own axis - the twist axis. And you can swing your arm up and down within certain limits and fore and back within certain limits. In Animator you are able to prescribe those limits for each rotation, but you need to tell Rodeo which of the axes is the twist axis. The Twist Axis is drawn as a white arrow in the OpenGL window in the Joint tab. See also the image to the right.

Handling of Twist Axis for different joint setups

If one axis is free and limited and two are locked there is no need for a twist axis.

If two axes are free and limited and one is locked Rodeo will set the locked axis as twist axis.

If three axes are free and limited the user must define the twist axis with the pulldown menu.


Examine limited joints with the Ragdoll project.