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We did mention in our opening chapter that one common use for dynamic rigid body simulations was taking prefractured geometry and simulating the collapse of solid objects, such as buildings, bridges, and the like. This effect requires the simulation of geometry that comes in all shapes and sizes. In order to make use of mParticles to produce such an effect then, we would need to have the option of using irregular-shaped geometry in a simulation--not being stuck with standard primitive shapes only.
To show that this is indeed possible, we're going to make use of the prefractured castle turrets that we have set up in our start scene. To save ourselves a little bit of time here, if I just hit the 6 key to pull up Particle view, you can see we have already set up MassFX Flow, which of course also creates a MassFX World helper in our scene set at the world origin. We have made a single change to the flow. In this instance we have replaced the Birth Grid operator with a Birth Group. However, if we were to hit the Play button in our animation controls at this moment in time, we wouldn't find any particles being generated.
This is because we haven't actually specified any particle objects for the Birth Group operator to work with. To set this up, we need to have our prefractured geometry selected. Now, because I know we have all of that geometry on its own layer, let's open up the 3ds Max Layer Manager, select the Fracture_Turrets layer, and then click the Select Highlighted Objects and Layers button. Then of course we can close the Layer Manager dialog. Jumping back into Particle view, we need to select the Birth Group operator, and in the Practical Objects field we can click the Selected button to add all of the currently selected geometry.
What we get initially doesn't look quite right. This is because we still have a shape operator in our flow. As this is clearly not going to be required, we can just delete it. Let's close Particle view for a moment and just take a closer look at what we now have in our scene. If I just select a piece of the segmented geometry and then hit the W key to enable the Move tool, you can see, as I moved the geometry off to one side, that we now have plain gray particles sitting in place, essentially re-creating the form of our castle turrets.
I am just going to use Ctrl+Z to undo that move. This means we can clean things up a little in our viewport by reopening our Layer Manager and hiding our Fracture_Turrets layer. You may be thinking that what we have here seems like a bit of a backward step, as we are left with plain gray turrets whereas before we had some nice brightly colored materials applied. What we are seeing is actually a graphical display error. The particles should appear with the materials visibly in place and indeed, for you on your machine, they may have done just that.
If not, all we need to do is open up Particle view again, click on the Birth Group operator, and scroll down to the Acquire Shapes section. Now, if we just twiddle our Sub-Material ID Offset spinner up and then back down, that should pop our textures back into view. If not, again, don't worry, because if you were to take a render, you would definitely see that your video materials are in place. Now even though we have our particles looking okay in the viewport and we know that if we take a render they will look okay, if I just select our MassFX Shape operator, over in the parameters, you can see that we still have our collision shape for the particles set to Box.
In fact, if I just set the Display As option to Shaded, what we have here would obviously simulate in a very unrealistic manner. To fix this, let's set the Collide As option to Convex Hull, which clearly gives us a collision mesh that much more closely matches our turret geometry. If we want, we can of course turn our shading enough now by setting Display As to None. There is one extremely important distinction between mParticles and the MassFX dynamic simulation that we need to highlight or reiterate at this point.
mParticles are running inside their own MassFX world. This means they are not controlled by, nor do they interact with, the standard MassFX dynamic system. To run an mParticle simulation then, we need to use 3ds Max's animation controls, are opposed to the Start Bolton on the MassFX toolbar. Let's do just that. As we can see, our mParticles definitely simulate, but there is a distinct lack of interaction between our mParticles and the stand geometry.
To change this behavior, all we need to do is add a specific modifier to the object we want our mParticles to interact with. Let's select the stand then and from the modifier list over in the Command panel, we can add a PFlow Collision Shape World space modifier. As this is by default set to use Geometry as the collision shape, we can just click the Activate button. Our final interaction step is to add a MassFX collision test to our flow. So from the depot, let's left-click and drag one in, making sure we set it below the MassFX world operator.
All we need to do now is add our standard geometry into the deflectors list. So with the operator selected, let's click the By List button and then select the stand Ggeometry from the dialog. Now we can close Particle view and again run our simulation. We now have a dynamic rigid body particle simulation that uses scene geometry as particles. Whilst this may not seem to be incredibly exciting, nor indeed a simulation effect that the MassFX system itself couldn't create, the truth is we can now use all the power of the particle flow system to affect our simulation in any way we want.
This gives us the ability to create simulation effects that are just not possible with the standard MassFX dynamics system.
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