[Tutorials] Car Rigging XSI.pdf

Q & A

Q&A | Rigging a car

SOLUTIONS / FIXES / ADVICE

● In reality, a car – even a toy car – does not

simply move forwards: it also rocks up and

down on its suspension as it travels over

bumps in the road. In XSI , a simple primitive

rig can be used to simulate this motion

QU ES TIO N O F

T HE MO NTH

S ubmitte d by D avid B urns,

via email

SOFTIMAGE | XSI

“How do you set up a model

car for realistic animation?”

This issue’s answer is supplied by Ola Madsen, who works

as 3D artist for Digital Context in Sweden, animating

everything from medical treatments to children’s toys

“B eneath the technical wizardry, the way in which

perfectly ﬂ at, the components of the car fail to react to this vertical

motion, adding metaphorical as well as literal bumps to the workﬂ ow.

But instead of going through the lengthy process of animating

these different parts manually, we can make use of XSI’s dynamics

engine. By adding Rigid Body Dynamics (RBD) to the animation rig,

we can recreate the same essential behaviour as a real car.

FAC TFILE

FOR

Softimage | XSI

DIFFICULTY

Elementary to

Intermediate

TIME TAKEN

One hour

ON THE CD

• Full-size

screenshots

• Start and ﬁ nished

XSI scene ﬁ les

ALSO REQUIRED

N/A

a car works is fundamentally very simple.

Energy generated by the engine is transferred

to the wheels, which in turn, forces them to

rotate. Due to the friction between the tyres and the ground, this

rotation then drives the entire car forwards.

However, when rigging a model of a car for animation, 3D artists

traditionally approach the problem the other way around. It would

be far too complicated to derive the motion of the entire car from

the rotation of the wheels; instead, hierarchies and/or constraints

are used for the overall motion, while expressions are used to make

the wheels rotate accordingly. But while this approach gets the job

done, it isn’t particularly intuitive. If the road surface is anything but

MAKING MOTORS

In this tutorial, we’ll be illustrating this technique on the toy car

above. Working with simpliﬁ ed geometry enables you to interact

intuitively with the components of the scene without losing the

accuracy of the simulation, so we’ll be using an animation rig made

up of simple primitives to simulate the workings of its suspension.

Creating a separate primitive rig eliminates any uncalled-for

calculation and enables you to adjust elements such as the body or

wheels later in production, more or less on the ﬂ y. In this way, one

underlying rig can be used for many different cars.”

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Rigging a car | Q&A

STAGE ONE | Creating the basic rig

If you don’t have a virtual car of your own, start by

opening car.scn from this issue’s CD. While the scene

may appear to be empty at ﬁ rst, opening an Explorer

view shows that it contains all the essential components of a

toy car, although they’re currently hidden. We’ll get to these

later in the tutorial, so just leave them as they are for now.

The ﬁ rst component to create for the rig is the

chassis or the stand-in for the body. As the exact

shape of this object will be irrelevant to the

eventual animation, there’s no reason to use anything fancier

than standard primitives. So, from the Get > Primitive >

Polygon Mesh menu, choose Cube, leave the Length set to 8

and name it ‘rig_chassis’.

While the shape of the chassis might be accurate

enough, we still want its overall width and length

to match up with the high-resolution version of the

car. Scale the cube down to 0.5 on the Y axis and up to 1.7 on

the Z axis. Translate the cube upwards about 4 units along

the Y axis so the chassis is slightly above the supposed

ground plane.

Although we’ve simpliﬁ ed the structure of the car

quite drastically, there’s really no reason for the

driving experience to become unpleasant as a result.

So with your driver’s comfort in mind, we’re going to add

suspension to the car. In order for this to function properly,

we’ll need to create two new objects for each wheel.

As these objects will be used merely to simulate the

suspension effect for the wheels, their actual shape

and size really won’t matter. So from the Primitive >

Polygon Mesh menu, create a new Cube and set the Length

to 1. In the Top viewport, position the Cube just to the right

of the chassis and roughly where the wheels are intended to

sit on the Z axis.

Next, in the Right viewport, move the Cube upwards

so it’s slightly below the top of the chassis. With the

cube still selected, press [Ctrl]+[Alt]+[D] to create a

duplicate and translate it downwards so it aligns with the

bottom of the chassis object. The added [Alt] key in the

shortcut ensures that the new copy stays at the same

position as its original, as opposed to just using [Ctrl]+[D].

Select both suspension cubes, press [Ctrl]+[Alt]+[D]

again and move the duplicates back along the Z axis

to the rear of the car chassis (where the wheels will

be positioned). Next, select all four cubes and duplicate them.

Now, simply add a minus in front of the value in the X axis

transformation box (the SRT Text Box in the Transform panel)

to reposition them on the opposite side of the car.

From the Primitive > Polygon Mesh menu, create a

Cylinder and set the Radius to 2 and the Height to

1.5. While the level of subdivision won’t make any

difference to the accuracy of the simulation (since we won’t

be using the actual geometry for the calculation), it will give

a better visual appearance. So, increase the U Subdivisions to

15 or so and name it ‘rig_wheel’.

Rotate the wheel 90 degrees along the Z axis and

align it to any of the lower suspension cubes. Next,

create three duplicates and align one at each of the

remaining suspension cubes. Go over the scene and make

sure none of the objects are interpenetrating, as this will

create very unpredictable results if we’re to simulate

collisions for them.

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Q&A | Rigging a car

STAGE TWO | Activating RBD for the rig

It’s important to recognise some of the differences

involved when animating with RBD compared to a

more traditional line of attack. When using RBD, all

the objects are being calculated in global space rather than

local. As a result, all objects in the rig should be located

directly under the scene root (or at least on the same level)

and not be relying on standard hierarchies or joint relations.

With the components of our rig completed, it’s time

to start adding their respective RBD properties. So,

press [4] to switch to the Simulate panel. The ﬁ rst

thing we’ll need to do is to turn all the objects into active

rigid body objects. To do so, select all the objects (there

should be 13 in total) and from the Simulate > Create > Rigid

Body menu choose Active Rigid Body.

As you won’t be using any actual collisions on our

objects (apart from our wheels, which we’ll come to

in a moment) you may as well turn off their

activeness. So in the Rigid Body Properties Editor, change the

Collision Activeness to Muted and make sure the Collision

type is set to Bounding Box. You can leave the other

parameters as they are, for now.

STAGE THREE | Rigid Constraints

To stick the different bits and pieces together we’ll

need to use Rigid Constraints rather than one of the

usual Constraints. Since all the objects in the rig

naturally should stick together in the end, we’ll need to use

three different types of constraints in order for the

components to work in the preferred manner.

The only parts we really want to stick to the chassis

are the upper suspension cubes. Start by selecting

one of them and from the Create > Rigid Body >

Rigid Constraint menu, choose Fixed. Pick the chassis object

and leave the parameters in the PPG as they are. Repeat for

the other three upper suspension cubes.

To create the suspension effect, select one of the

upper suspension cubes and from the Create > Rigid

Body > Rigid Constraint menu and choose Slider.

Pick the lower cube related to the one you’ve got selected.

We’ll edit the parameters for all the sliders at once, so leave

them for now. Repeat for the three remaining pairs of cubes.

STAGE FOUR | Creating the suspension

Select all four Slider Constraint objects and press

[Enter] to display their PPG. The Spring (Kp)

parameter determines how fast each linkage will

contract. However, we want the very opposite effect to take

place, so to make it expand instead we’ll need to use negative

values. Enter about -150 as the Spring (Kp).

The default dampening effect is a bit too low, so

increase the Dampening (Kd) to about 40. The Rest

Length (R) is the Sliders’ preferred length and

should be set to about -2.5. Which values to use for the

different parameters is really a matter of taste, so ultimately

you should tweak them until they suit your speciﬁ c needs.

Next, we’ll need to limit the minimum and maximum

length of each Slider, so switch to the Limits tab.

The Minimum Extension determines the minimum

length the Slider is allowed to reach during the simulation,

whereas Maximum Extension sets the maximum. Activate

both and set the Minimum to 1 and the Maximum to 2.5.

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Rigging a car | Q&A

STAGE FIVE | Setting up the wheels

Select all four wheels, right click on the Selection

button in the Selection panel and choose Simulation

Properties. Select the Rigid Body Properties in the

PPG and change the Collision Activeness to Active. As the

wheels are round, we also need to change the Collision type

to Bounding Sphere. And ﬁ nally, increase the Friction to 1.

Select one of the wheel objects again and from the

Create > Rigid Body > Rigid Constraint menu, choose

Hinge and pick the corresponding lower suspension

cube. The Hinge Constraint limits the movement of the wheel

to revolving around a preferred axis, which is just what we’re

looking for. Repeat for the other three wheels.

Select the two rear Hinge Constraint objects and

press [Enter] to display their PPG. Switch to the

Motor tab and click the Motor Active checkbox. The

Maximum Velocity determines the rate of the rotation,

whereas the Torque determines the maximum amount of

force to be transferred. Set both the Velocity and the Torque

to around 800 or so.

STAGE SIX | The road ahead

If at any time you need to reposition or reorient any

of the car rig’s components, it’s vital that you

remember to update their initial state. If you miss

this step, they’ll simply return to their previous initial state

as soon as you run the simulation. To set their new state,

select all the objects and from the Modify > Rigid Body menu,

choose Set Initial State.

Press [8] to open the Scene Explorer, select the

ground object and press [H] to unhide it. We don’t

want this object to be affected by any forces in the

scene, but we do want it to be included in the simulation.

From the Create > Rigid Body menu, choose Passive Rigid

Body. In the PPG, change the Collision type to Actual Shape

as we want the car to follow the actual shape of the ground.

The last thing we need to do is to add an actual

force to the scene, so from the Get > Force menu,

choose Gravity and you’re done. To get a more

accurate result, click the Explore button in the Select panel

and choose Environments. Expand the tree Environment >

Operators and pick the Dynamics Operator. In the PPG,

change the Sub steps under Simulation Accuracy to about 10.

Now unhide the high resolution version of the car body and

the wheel (of which you obviously will need another three

duplicates) and use a standard Constrain > Pose to constrain

them to their respective counterpart in the RBD rig. Note that

you should use cnscomp (constraint compensation) for the

body, due to the scaling of the chassis. Your toy car is now

rigged and ready for animation.

●

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