Animating a mechanic billboard in Softimage XSI

While the mechanical billboard is outdated at best, your virtual version is just about to start its glory days.

Start by opening the Billboard.scn scene from this issues CD. The scene contains a bunch of trilons or triangular prisms, 300 to be precise. Using a couple of thousands would obviously be more striking but for the sake of demonstration the number is keep low. While any of the prisms could be used to animate their rotation, it’s a good idea to separate the animation parameters from the actual billboard. Start by selecting the null object named Prisms_Rotation. Press [C] to activate the rotation tool and press [K] to set a keyframe at frame 1. Go to frame 16, rotate the Prisms_Rotation 120 degrees on the Y-axis and set a new keyframe. With the rotation still at 120, set another keyframe at frame 80. Press [0] (zero) to open the Animation Editor and select the function curve for the Y rotation (roty). From the Curves menu choose relative Cycle. To change the pace of your mechanic billboard, just move one of the 3 keyframes.

Press [2] to switch to the Animate toolbar. From the Create > Parameter menu choose New Custom Parameter Set. Enter Billboard as the name and click OK. Press [8] to open an Explorer, select the Billboard parameter set and press [Shift] + [P] to add a new custom parameter. Name the new Parameter Rotation_Delay. The parameter will be used to control the offset of the prisms. Changing the slider will automatically change the delay. The offset is evaluated in seconds; consequently you’re going to use very small values. Change the Maximum Value Range to 0.01 and click OK to close the PPG.

Open an Explorer and select the Prism_No1. Scroll down to Prism_No300 and shift select it to select all the objects in order from 1 to 300. Press [Alt] + [4] to open the Script Editor. Set your script language to Jscript (File > Preferences) and enter the text beneath.
Press the Run button in Script Editor to execute the script. Open the Bilboard PPG and playback the animation while changing the Time_Delay slider.

Using a simple loop in Jscript adds the expression with the delayed rotation in seconds rather than the frustrating hour required if done manually.

The project files used in this tutorial can be found at: http://www.redi-vivus.com/Caffeineabuse/Billboard.zip

Quick tip
The order in which the objects are selected also controls the extent of the offset, which in turn is set by Rotation_Delay slider. So, select the object in the order you want them to turn.