This tutorial is an add on to the original Scene object interaction tutorial I wrote for Softimage 3D some time back. The concept is the same, though the workflow is now a bit different. If you'd like to read the older version of this tutorial, which covers more of the theory, click here to link. Otherwise, you can follow along with this tutorial by clicking here to download the sample scene. The scene has been imported and modified from Softimage 3D, and is animated up until frame 118.

To allow for the interaction between the character and the bottle, we will use a position constraint and an orientation constraint. No constraints have been added to this scene yet, other than those that control the character rig.

If you scrub through the animation, notice that at frame 98, the Twiggy character first makes contact with the bottle of beer.

The entire character is keyed at this point (see the Automatic keying tutorial for info on quick character keyframing in XSI) This frame will become the starting point for the lesson. Multi select the bottle as well as Twiggy's Right hand constraint box. In your camera view show the centers of both selected objects.

It is important to note how far apart these two centers reside from one another. In our case, we need the bottles center to line up exactly over top of the right hand constraint boxes center. One possible way, would be to move the bottles center to line up with the center of the latter mentioned hand box, but a quicker and easier method is available.

Get a null from the primitives menu cell. In an explorer view rename the null, calling it BOTTLE_CONSTRAINT_NULL. Click on constraint>position When your mouse cursor displays the 'pick' option, select the hand box. Close the pop up menu that appears. Your null should have jumped to the center of the hand box. With the null still selected, click on constraint>orientation and select the hand box once again. This time the nulls center flips around so that its X axis aligns itself to the hand boxes X axis. Make this null, the parent of the bottle of beer by dragging and dropping the beer bottle on top of the null in an explorer view.

Now, whenever you move or rotate the Characters hand constraint box, the beer bottle will follow along. Unfortunately, the bottle follows the hand box from the very start of the animation to the very end. What we need to do is disable the constraints effect from frame 1 to frame 97, and then have the constraint become active at frame 98 when the characters hand has wrapped itself around the beer bottle. (pic5)

In an explorer, make sure the scope is set to All Except Parameters. Select the BOTTLE_CONSTRAINT_NULL and expand it by clicking on the + sign. Open up the kinematics node, and finally open up the constraints folder. Within the folder you should see the nodes for position and orientation constraints that were set earlier. Click on the gradient image to the left of the position constraint node. A pop up window containing the position contraints property page appears.

Notice the check mark beside the position constraint box. The constraint is a boolean function, a command that returns a true / false value. When the box is checked, XSI returns a true value for the position constraint meaning that it is active at the given frame. All we need to do is to keyframe the true / false values of the function at selected frames to activate and deactivate the constraint. We will replicate this same process for the orientation constraint. Key the position constraint on frame 98 by clicking with your left mouse button on the green chip to the left of the word 'active'. The chip turns red with a spline curve inside the box. The script log will show you that the constraint is returning a true value.

Set your time slider to frame 97 and deactivate the constraint by unchecking the check mark to the right of the word 'active. Set a key for the deactivated null by clicking the green chip once again. In two frames the bottle null is transformed into a slave object via the activation of the position constraint. Set your time slider back to frame 98, and repeat the above steps, but this time key the true / false values of the orientation constraint. The last step, I usually take to ensure that my constraints do not pop when transitioning from a constrained state to an unconstrained state is to key the slave object in place the frame before it becomes constrained. In this case, I would select the bottle null as a branch, and key its local position and orientation. This ensures a smooth transition, because it locks the bottle in 3D space at the time of constraint activation. In Softimage, keyframe animation is always overidden by constraint data.

From this point on, you can keep on posing and keying your character as he picks up the bottle, takes a long swig and then slams the bottle back down onto the table. The constraint will remain active until you have the character put the bottle down. In the sample scene, the character puts the bottle back on the table at frame 160. To remove the constraint by frame 161, you would simply do the reverse of the above steps. Drag the time slider to the frame prior to the slave object being deactivated. Select the slave object (the bottle null as a branch) and key its local position and orientation to prevent popping when you deactivate the constraint. Open up the position constraint property editor, and key the constraint as being active at frame 160, then advance the slider to frame 161, and key the constraint in a deactive state. Repeat this process for the orientation constraint. At frame 161, you are now free to animate the characters arms dropping down by his sides as he feels the effects of his chilled brew. The bottle will remain on the table. The Softimage 3D version of this tutorial contains a couple of additional constraint setups involving the activation and deactivation of constraints, for the interaction between the bottle and the characters lips.

Scene object interaction in Softimage XSI was written by Adam Sale. Adam is a Technical Director and co-founder of Joncrow Entertainment. He can be reached at adam@joncrow.com