Rigging props and assets are a lot of fun because usually they are not complex as hero characters and are also great to experiment on. I generally approach them the same way I rig characters and the questions I ask myself before starting the rigging process are:
what is the prop/asset supposed to be doing?
in how many shots is the asset seen or how much screen time does it have?
The answer to the first question simply tells us what must be provided in the rig. The answer to the second question tells us how detailed and complex those controls must be. In other words if the prop/asset is in every shot it better be easy to animate and control the rig. If it is in only one shot it makes no sense to spend more time on the rig than animating it.
In “B is for Ben” the spaceship is the most important element besides Ben. But for the most part it is not really animated, so I concentrated more on the functionality rather than optimizing it.
Analyzing the ship
The main checklist for the rig was quite simple:
it needed to have retractable legs with fine controls
it had to have squash&stretch quality for all elements (body, wings, engine…)
the cabin glass needed to open and close as well as ‘leg doors’
I build the spaceship leg rig the same way I would build a leg rig for a character. It has an IK system with stretch controls, twisting, soft ik and all other controls that are usually found in leg rigs. The foot has a few controls required to fine tune the pose but doesn’t have any reverse foot or similar system in it simply because it is not required. In the film the foot only needs to allow the ship to rest on a flat surface and rotate when flying, thus only simple FK controls are used in the foot itself.
Squash & Stretch
To achieve squash & stretch I used a lot of bend deformers that deform the whole ship at the same time and some local bend deformers for the wings, engines and smaller elements to allow overlapping of deformation when animating.
Squashing and stretching the body of the ship causes the legs to become misplaced or out of position when deformation occurs. This is because legs are part of an IK system and thus should not have bend deformers applied to them (which would also cause the feet to not rest on the ground). To correct this, the beginning or the hip of each leg is attached to a position on the ship geometry no matter how it is deformed and basically always sticks to this point no matter how the ship geometry is deformed. This way the leg is always attached correctly to the ship and the IK system keeps the foot on the ground.
For the smaller elements such as the cabin glass and leg doors I mostly use simple transform SOP tools in Houdini. They are simple to use and get the job done quickly. Order of deformation is the only thing to keep in mind, so that smaller elements are deformed first and larger deformation like the whole ship squash & stretch are done later, otherwise the smaller deformation will not work correctly.