Adding a secondary motion factor to a rig using constraints

By Michael Malinowski

   This tutorial will go through the process of setting up secondary motion and skin movement on characters using constraints as apposed to the use of softbodies etc.
I will start from the very beginning, so we can see the problems and benefits. So to begin with lets create a cube, duplicate it, move it and scale it, then repeat the process so you create something similar to the following image.

 
Now enable constrain compensation, and choose constrain pose whilst having cube2 selected, then select cube1, set the blend to 0.5 ( this is the strength of the constraint), now choose cube3, select the constraint and select cube2 ( set this blend to 0.3) and so one, so cube2 is constrained to cube1, cube3 is constrained to cube2, and cube4 is constrained to cube3.
Now if you rotate cube1 you will notice that the constrained cubes all follow behind in a delayed motion, so already were beginning to make use of secondary motion. However, we can quickly spot a problem with this setup.


 

What if we wanted to set this secondary motion on the arms of a dinosaur, so that when it walks its arms bounce up and down automatically, we could do this using the above setup, however we would have no control over the arms as they are now constrained to another object which gives us little scope for animation
Luckily for us the solution is equally as easy to setup, take a look at the image to the left, this is a basic setup whereby there is no secondary motion in place for the arms, and the arms are currently children of the upper spine.

 

   So lets being the fun part, we need to create another object, I am using an implicit sphere, although it really doesn't matter what time of object you use for this. We will then position constrain the new object to the upper spine, making sure you have compensation enabled, set the blend to whatever value you feel fit, for this I will set it to 0.5 then set it to a custom slider later so the values can be easily changed.
 

Now if the dinosaur moves you will notice that the object we just constrained moves with an offset. All we need to do now, is make the new constrained object a child of the spine so its all part of the same hierarchy, then cut the parenting between the spine and the arms, and instead make the arms a child of the constrained object.
Now if you move the entire rig you will see the secondary motion apparent on the arms as these are children of the constrained object, because of this they can still be seperatly keyframed as they are in no way constrained.
This same method can be used anywhere throughout a rig, it can be useful for things such as arms, tails, heads, even skin.

  In this quick example video file you can see the dinosaur walking. (excuse the rough animation, I only spent quite literally 60 seconds setting the keyframes) but you can see the amount of extra movement you get, all of which is entirely editable, and all the arms, head and tail are still keyframable giving the animator just as much control as he/she would have without the secondary motion. From all that motion, only 3 objects were keyframed! The rest was all automated.
With this in mind you can begin to think of the possibilities this gives!

click the image on the left to view the file.

  

if you have encountered any problems with this tutorial then please email . hejherbert@hotmail.com and i will try to resolve it. This was created using Softimage|XSI 3.0, but should run down to 2.0.

hope this helps.

Mike Malinowski

   

 

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