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Figure 40-9: Depending on the mass property, objects sink or float.

Calculating and Previewing a Simulation

Although you’ve already had some experience with the Preview window in the examples, more controls are available than just playing the animation. To preview the simulation, select reactor Preview Animation or click the Preview Animation button on the reactor toolbar. This opens the Havok window like the one shown in Figure 40-10. This window lets you play with your simulation. The Simulation Play/Pause menu (keyboard shortcut, P) executes the simulation. Dragging with the left mouse button rotates the scene, and you can zoom in and out with the scroll wheel.

Caution	The Preview window runs only if the OpenGL or the Direct3D display drivers are used. The
	window uses OpenGL by default, or you can set it to use DirectX with the DirectX option in
	the Display rollout. The display driver that is being used is displayed in the title bar of the
	Preview window.

Using the Preview window

The fun part of the preview window is that you can interact with the objects. Right-clicking (when the simulation is playing) and dragging on the object moves it. If you find a position that you want to capture for Max, you can use the MAX Update Max menu command to set the starting positions of the objects in Max.

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Figure 40-10: The Preview window is a fun place to play with a simulation.

If you want to reset the animation to its starting positions, you can use the Simulation Reset menu command. The Display menu includes several options that you can turn on to change the display. Objects can be seen as Faces or Wireframes. If you have a particularly complex scene to preview, then wireframes may be easy to work with. You can also display the Simulation Edges (which shows the collision boundaries of the objects), Grids, the Origin, and a Flashlight to add light to the scene. The Display menu also includes a Camera Settings option that you can use to set the near and far Clipping Planes and the Preview window’s Field of View.

The Performance menu includes options for setting the frames per second and the number of substeps used to compute the simulation. For most animations, the default of 10 substeps is sufficient, but if you want Max to spend more time computing an accurate solution, you can try a higher substep value.

Caution

Don’t use a high substep value with water.

Creating animation keys

To compute the animation keys for the simulation, select the reactor Create Animation menu command or press the Create Animation button in the reactor toolbar. The progress is displayed at the bottom of the Max interface. You can cancel the simulation at any time with the Esc key.

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Analyzing the scene

After the Preview window is opened, a warning dialog box appears if the scene has any errors or warnings that could cause trouble with the simulation. It also warns of unrealistic data, such as property settings that are too high or too low.

Tip The one warning that isn’t included in the warning dialog box is if objects have no Mass value. If objects in your simulation are just sitting there, then make sure that they have a Mass value.

If you want to check your scene without opening the Preview window, you can use the reactor Utilities Analyze World menu command. This command checks your scene for any trouble and presents problems in a dialog box. If no problems are found, then a separate dialog box tells you that no problems were found.

Tutorial: Dropping a plate of donuts

All the great books have an element of tragedy, so consider a policeman carrying a dozen donuts on a plate when he stumbles and drops the plate. Donuts everywhere, how tragic! This animation sequence would be difficult or at least time-consuming, but not with reactor.

To use reactor to animate a falling plate of donuts, follow these steps:

1.Open the Falling plate of donuts.max file from the Chap 40 directory on the CD-ROM. This file includes a simple plate of donuts created from primitives.

2.Select the reactor Create Objects Rigid Body Collection menu command, and click in the Front viewport. In the RB Collection Properties rollout, click the Pick button and select the plate object. Click the Pick button again, and select the Box object that represents the floor.

3.Select the Torus object and choose reactor Apply Modifier Soft Body Modifier (or click the Soft Body Modifier button in the reactor toolbar).

4.Then select reactor Create Objects Soft Body Collection, and click again in the Front viewport. In the SB Collection Properties rollout, click the Add button and select all the Torus objects again.

5.Select reactor Open Property Editor, and select the Box object; in the Properties rollout, enable the Unyielding option. Select the plate object, and make its Mass value 5.0. Then select all the donuts, and make their Mass value 0.25. Enable the Mesh Convex Hull option.

6.The last step is to execute the simulation. This is done with the reactor Create Animation menu command. It takes some time to compute a solution for this example. When it completes, press the Play Animation button (or press the / key) to see the results.

Figure 40-11 shows the upturned plate of donuts.

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Figure 40-11: Animating these falling donuts, simulated as soft body objects, was easy with reactor.

Constraining Objects

Constraints are ways to limit the amount of motion that an object can do. Using constraints can help control objects in the scene as they interact with other objects. Perhaps the simplest constraint is to enable the Unyielding option in the Property Editor. This option makes a rigid body that won’t move and is a good option for the ground plane.

Other constraints are found in the reactor Create Objects menu and consist of Constraint Solver, Rag Doll Constraint, Hinge Constraint, Point-Point Constraint, Prismatic Constraint, Car-Wheel Constraint, and Point-Path Constraint, as listed in Table 40-4.

Table 40-4: reactor Constraints

Toolbar Button Name	Description
Constraint Solver	Contains all active constraints used in the scene
Rag Doll Constraint	Causes a model to act as a human figure
Hinge Constraint	Allows angular rotation like a hinge

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Toolbar Button Name	Description
Point-Point Constraint	Links two points together; good for rope ends
Prismatic Constraint	Used to constrain the motion of two rigid bodies to a single
	axis with no rotation
Car-Wheel Constraint	Causes a car to move linearly as a wheel object is rotated
Point-Path Constraint	Limits a point to move only along a path

After a constraint object is added to the scene, you can select the objects that will be included as child and parent objects using the buttons in the Properties rollout. The Properties rollout also includes buttons to align the constraint to the Child Body, Parent Body, Child Space or Parent Space. For each constraint, you can set the Strength and Tau of the connection. This determines how strong the link is and how easily broken. In addition to the Strength and Tau values, you can set Limits and allow the hinge to be Breakable under a defined Linear or Angular force value.

When a constraint’s Child is first selected, the Constraint’s icon is positioned at the pivot point of the child object. If you look in the Modifier Stack for the Constraint object, you’ll find subobject modes for Child Space and Parent Space. If you select these subobject modes, you can change the position of the constraint’s child and parent objects.

	Using a Constraint Solver
	In order to use most constraints, you need to add a Constraint Solver to the scene. Then you
	can use the Modify panel to add Constraints to the list to be solved. The Constraint Solver
	needs to know about any Rigid Body Collections that are attached to any Constraints in the
	scene. To identify all the Constraints that are part of the Constraint Solver, click the Highlight
	button.
Tip	If the simulation includes any constraints that the Constraint Solver doesn’t know about, the
	Constraint Solver icon appears red in the viewports.

Rag Doll constraint

The Rag Doll constraint defines all the joint limits common in a human figure. It can be used to animate a lifeless body colliding with various rigid body objects. Using the Rag Doll constraint, you can manually define how the body joints can twist, rotate, and move.

These joints are fairly common for human bodies. Discreet has created a script to create a human body proxy that creates a rag doll with the correct constraints already defined. The script is named rctRagdollScript.ms. It can be found in the scripts directory where 3dsmax is installed.

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You can execute this script by opening the Utilities panel, clicking the MAXScript button, and clicking the Run Script button. This opens a file dialog box. Locate the script, click Open, and the script runs. Running this script opens the Rag Doll dialog box, shown in Figure 40-12. Using this dialog box, you can provide a Name for the rag doll and set its Height and the number of Vertebra. The Create Humanoid button makes the rag doll appear in the viewports.

Once positioned, you can press the Constrain Humanoid button in the Constrain Humanoid rollout. This adds all the necessary constrains to the rag doll.

Figure 40-12: Fully constrained humanoid figures can be created using the rctRagdollScript.ms script.

The Point-to-Point constraint lets you attach two objects together by a common point. The attach point is the pivot point of the child and parent objects. It can be used to animate a lifeless body colliding with various rigid body objects. Using the Rag Doll constraint, you can manually define how the body joints can twist, rotate, and move.

Tutorial: Swinging into a wall

Playing with the rag doll object is just plain fun. Remember that in the Preview window you can use the right-click button to throw the doll around. For this example, we use a couple of Point-to-Point Constraints along with a Rag Doll constraint to create a simple scene where the rag doll swings from a rope into a brick wall.

To animate a rag doll swinging on a rope, follow these steps:

1.Open the Swinging into a wall.max file from the Chap 40 directory on the CD-ROM.

This file includes a simple scene consisting of several Boxes, a Cylinder for the rope, and a brick wall.

2.Open the Utilities panel and click on the MAXScript button. Click the Run Script button in the MAXScript rollout. In the file dialog box, locate the Scripts directory where Max is installed. Select the rctRagdollScript.ms file and click the Open button.

3.In the Rag Doll panel that appears, open the Create Humanoid rollout and click the Create Humanoid button. Then move the Rag Doll panel to the side, but don’t close it. Select the rag doll that appears at the origin and move it so one of its hands is positioned close to the end of the Cylinder object.

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4.In the Rag Doll panel, open the Constrain Humanoid rollout and click the Constrain Humanoid button. This automatically adds all the needed constraints for the rag doll. Then close the Rag Doll panel.

5.Select the Cylinder object and choose reactor Create Object Point-Point Constraint and click in the Left viewport close to the rag doll. In the Properties rollout, click the Child button and select the hand object that is close to the Cylinder. Then enable the Parent option, click the Parent object, and select the Cylinder. Open the Modify panel, select the Parent Space subobject mode, and move the gizmo in the Front viewport to be on the Cylinder. Do the same for the Child Space subobject. This defines where the two objects will be attached.

6.Repeat Step 5 to create a Point-to-Point Constraint where the Cylinder touches the roof object with the Cylinder as the Child object and the roof as the Parent object.

7.Press the H key to open the Select Objects dialog box and select the RagdollRBCollection object. Then click the Add button in the RB Collection Properties rollout. Click the All button in the Select Rigid Bodies dialog box and click Select. Repeat this step for the RagdollCSolver icon to add the two new Point-to-Point Constraints.

8.Select all three Box objects in the scene and choose reactor Open Property Editor. In the Physical Properties rollout, enable the Unyielding option. Then select the Cylinder object and set its Mass value to 0.5.

9.Select reactor Preview Animation to see the resulting preview. Then select reactor Create Animation to create the animation keys.

Figure 40-13 shows the swinging rag doll.

Figure 40-13: Using constraints gives you control over the animation motion.