August 2008

Create Duplicates from Animation

Scattering or placing a large number of objects by hand is not only a tedious task but also a quite tricky one, at least if you’re trying to maintain perfect spacing, angles, etc between all the objects. Fortunately enough, there are several ways to automate the process, though some of them might not be self-evident for the newcomer. A very handy feature in XSI that is easily overlooked is the ability to create duplicates or instances based on an objects animation.

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Sucked Trough a Straw

Step 1

The scene
Start by opening the scene Garden_hose.scn. The scene contains a garden hose, a curve which the hose is extruded along (and can be used to animate the hose) and a low resolution melon which will deform the hose. If you need the melon to be visible outside the hose as well, simply parent it under the low-res version.

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Tail Motion

Step 1
Hierarchies
The truth is that it will be a whole lot harder setting up the simulation than doing it by hand. While the motion may appear complex due to all the action going on at the same time, it’s in fact really easy to recreate in XSI. From the Create > Skeleton menu chose 2D Chain and draw a chain with 7 bones or so in the Front viewport. Now, select all the bones, press [C] to activate the Rotate Tool and press the Add button in the Transform Panel.

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Exploding Wall

While constructing scenes certainly is rewarding, it just won’t come close to the fun of blowing it all up. If you don’t’ have a brick wall at hand you can use the brick_wall.scn scene included on this issue’s CD. Select the ground_object and press [4] to switch to the Simulate Toolbar. While the ground object should be included in the simulation we don’t want it to be affected by the gravity, so from the Create > Rigid Body menu chose Passive Rigid Body. Scroll down to the Rigid Body Properties section and set the Elasticity to about 0.2 and both the Static and Dynamic Friction to about 0.75. The complete opposite is true for the bricks, so select all of them (66 in total) and from the Create > Rigid Body menu chose Active Rigid Body. Click the Lock icon in the Rigid Body PPG to lock it in place as you’ll return to it in a second. From the Modify > Rigid Body menu chose Edit Simulation Properties… In the PPG, change the Play Mode to Standard and check the Caching Checkbox. You’ll probably want to increase the Accuracy later on, but leave it quite low for now due to the otherwise decreased performance.

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Propeller Rotation

Dive right into the solution by opening the scene submarine.scn form the CD. The easiest way to achieve a smooth and fluid motion for your submarine is to animate it along curve (path). In addition to controlling the sub’s actual position, this approach will enable you to automate the rotation of the propeller based on the speed of the sub.

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Render Models in Separate Passes

Keeping all the objects within the same scene ensures that any changes made to the lightning, camera position, shared materials, etc always will be up to date for all objects, and perhaps more importantly, at all times. However, once your scene reaches a certain number of objects it will most certainly become clear that the tedious task of rendering each as a separate image by hand just won’t be a viable solution. At this stage you’ll be left with two natural choices; either get an intern to do the job for you or if that’s not an option (or if you happen to be that intern) it’s about time you start looking at Script Editor – in two senses that is.

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Procedural Asteroid

Step 1
The geometry
From the Get>Primitive>Polygon Mesh menu pick Icosahedron. Increase the Geodesic Frequency to 2. From the Modify>Deform menu pick Shape Jitter. In the Amplitude section of the PPG, change the X to about 4 and the Y and Z to about 3. (You can use the Seed slider to give each asteroid a unique shape). With the object still selected, chose Create>Poly.Mesh>Subdivision and set the Subdivision level to 3 to smoothen the asteroid.

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Animating a Rolling Tire

There is a number of ways to make the tire rotate properly based on its movement. On the following two pages we will give you an idea about how to approach the problem, and show you two different ways to solve it. Regardless of which of these techniques you end up using you will need to know the circumference of the tire. So the first thing we will need to do is to dig up the old trigonometry books we all loved so much at school.

After flipping through the book we know that the relationship between the circumference of a circle and its radius is the number π (pi). This means that once we know the radius, we can easily calculate the length the tire has to travel to make a full rotation by setting up the following equation, where C is circumference and r is the radius: C= 2π*r

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Modeling a Tire

The first thing to do is to get hold of good reference material; such can easily be obtained by searching Google or any of the manufacturer’s WebPages. Once settle on what type of tread pattern to create, either load it as a Rotoscope image in XSI or use it as it is.

Start by creating a Grid and lower the Y Length to 1.5. Increase the U Subdivisions to 19 or so and the V to 2. Select the middle V edge row and, in the top viewport, move it downwards so its just above the end of the grid. Move each of the U edge rows to form the proper gaps between the treads (additional screenshots can be found on the cover CD). Next move the polygons so you get sort of a V-shaped pattern. Select all the polygons that are to form the height of the tread, press [CTRL + D] to duplicate them and move them slightly upwards. With the polygons still selected, from the Modify>Poly.Mesh menu choose Bevel Components.

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Adding Random Motion

The first thing to be considered is the structure of the animation. If we apply the judder directly to any of the axis of the object, it will be difficult to control or make changes to the general animation later on in the production. So the idea is to split the jiggling and overall motion into separate components, so they can be controlled independently from one and other. Granted that at a first glance, it might not appear as elegant as with the use of expressions but once you’ve set it up, you’ll se just how much more straightforward and intuitive this approach will be.

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