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Explaining the difference between Rigid and Soft Bodies

From: CINEMA 4D Essentials 9: Particles and Dynamics

Video: Explaining the difference between Rigid and Soft Bodies

In the previous movie we took a look at the basics of the Dynamics Engine and created something called a Rigid Body. The Dynamics Engine also allows for something called Soft Bodies. And Soft Bodies are, well, they're soft. It's like the difference between a block of sugar and a Jell-o cube. They're both sweet and delicious but they behave in very different ways. Let's create a very basic dynamic relationship, I'll add a cube to the scene, and let's also add a Plane. And take the cube and raise it up on the Y, let's back up just a bit, and now I'm going to right-click on the Plane and make that under the Simulation Tags, a Collider Body, and then right-click on the cube and go to Simulation Tags and make that a Rigid Body. And now when we rewind back to zero and hit Play, we have our dynamic relationship.

Explaining the difference between Rigid and Soft Bodies

In the previous movie we took a look at the basics of the Dynamics Engine and created something called a Rigid Body. The Dynamics Engine also allows for something called Soft Bodies. And Soft Bodies are, well, they're soft. It's like the difference between a block of sugar and a Jell-o cube. They're both sweet and delicious but they behave in very different ways. Let's create a very basic dynamic relationship, I'll add a cube to the scene, and let's also add a Plane. And take the cube and raise it up on the Y, let's back up just a bit, and now I'm going to right-click on the Plane and make that under the Simulation Tags, a Collider Body, and then right-click on the cube and go to Simulation Tags and make that a Rigid Body. And now when we rewind back to zero and hit Play, we have our dynamic relationship.

Now let's Delete the tag on the cube and right-click on the cube, and go to Simulation Tags and add in a Soft Body Tag. Now let's Rewind back to zero and hit Play. Well, it looks like nothing happened. The reason that it looks like nothing happened is because of the way that the Soft Bodies work. What the Soft Bodies do is they look at the points that make up the object and they try to create little virtual springs between those points. If you remember from our previous module on deformers, I made a very important statement, and that is a single polygon or a single polygon edge cannot be bent.

The polygons can be twisted but the edges cannot be bent themselves, their edges are always straight lines. Well, that's what's happening in the case of this cube. It's not deforming into a Soft Body because it only has one edge per side. So let's go in to the Cube Settings, and go to the Object Properties, and change the Object Properties Segments from one to, say, five. Now let's rewind back to zero and hit Play. You can see that our cube is flexible. Let's rewind back to zero and rotate the cube, I'll hit R on the keyboard and rotate the cube around.

Let's rotate it so that the point is down. There we go. Now rewind back to zero and hit Play, you can see that the cube is very flexible, let's Rewind to back zero. Under the Cube Properties, if we change this number of segments from 5 x 5 x 5, let's crank it up to, say, 10 x 10 x 10 and rewind back to zero and hit Play again. And you can see that now not only is the cube very flexible, but when it hits the ground, it doesn't spring back. That's because there are so many springs in there that the springs all collapse on one another.

The other thing you may have noticed is that the Playback was very sluggish. That's because there are so many more springs in the cube that there's a lot more information for it to process. Let's take that back down five and take a look at the Properties in the tag. The only difference between the Rigid Body Tag and the Soft Body Tag is this pull-down under the Soft Body options. If I turn this to Off, and I'll rewind back to zero and hit Play, you can see our cube is now back to being a Rigid Body.

If I Rewind back to zero and turn that back on to Made of Polygons/Lines, then you see that our cube is squishy again. Now the Made of Clones Option is for using with MoGraph Cloner Objects, and for now though, we'll leave it on Made of Polygons/Lines. Underneath the Soft Body Pull Down are all the options related to the springs, and these basically affect the stiffness of the object and how the object behaves when it bounces and collides. The spring settings control the actual stiffness of the springs.

If I crank up the structural value, let's make it up to something like 5,000, really high. Rewind back to zero and hit Play. You see the object hits the ground and then it doesn't bounce back anymore that's because the springs are so stiff that they can't return any energy, Let's hit Stop. Now I'm going to rewind back to zero, and let's change the structural value from 5,000 back to 100 and take a look at the Stiffness Option. And out of the Stiffness Option, that relates to, again, how the springs behave. By default, it's set to zero, but let's change the value from zero to something like 10, and then we'll hit Play again.

You'll see that our object is now very springy, it's much more stiff than it was before. If we scroll down and look at the Pressure option, that allows you to actually introduce air pressure inside of the object, so we can actually inflate it. If we rewind back to zero, now I'll increase the pressure, let's make it something like, say, 50. Now it looks like nothing happened, but when I hit Play, you're going to see the object instantly inflate and then fall down. That really feels like that object has a bunch of air inside of it.

The Soft Body Option can be a really great Tool for making jiggly things, or objects that need to collapse when they bounce. The key is to tweak your Spring Values to get the behavior that you want. Once you've got those Spring Values set correctly, that will determine whether your object behaves like Jell-o that's been sitting out in the counter for a while or Jell-o that just came out of the fridge.

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CINEMA 4D Essentials 9: Particles and Dynamics

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Rob Garrott
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