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How collisions are calculated

From: Creating Simulations in MassFX and 3ds Max

Video: How collisions are calculated

In a typical 3d scene we refer to the objects we view and manipulate in the viewport as pieces of geometry, or meshes. In a dynamic simulation, however, we need to understand that there is a distinction made between what we see in the viewport, which you refer to as graphical mesh, and what we simulate or calculate collisions with, which is referred to as a physical mesh or a physical shape. Very rarely are the two identical in makeup. In fact, for the sake of speed, we will oftentimes prefer them not to be.

How collisions are calculated

In a typical 3d scene we refer to the objects we view and manipulate in the viewport as pieces of geometry, or meshes. In a dynamic simulation, however, we need to understand that there is a distinction made between what we see in the viewport, which you refer to as graphical mesh, and what we simulate or calculate collisions with, which is referred to as a physical mesh or a physical shape. Very rarely are the two identical in makeup. In fact, for the sake of speed, we will oftentimes prefer them not to be.

This physical shape or mesh is a nonrendering representation of the graphical mesh and is created when we set a piece of geometry in a scene to be either a rigid or soft body object. The physical mesh is what is used as a collision object. We cannot stress that very important point enough. It is this that will collide or interact with other objects in the simulation and thereby create the simulated motion that we are looking for. The fact that these physical meshes have to be specifically added or attached to objects in the scene means that, well, only elements that we deem required are actually added into the simulation, of course ensuring that the number of calculations required to produce the simulated effect are kept to a minimum.

Now, to optimize a graphical mesh, or what we think of as a 3D model--maybe for something like a real-time game engine or possibly just to get faster render time from our scene--we would edit the number of vertices and polygons making up the geometry. Most likely, we would reduce the overall count wherever possible. Well, in a similar way, when it comes to a dynamic simulation, we really want--in fact need--to control the amount of information in our physical meshes. The lighter the physical mesh, in terms of vertices and faces, the easer the collision detection process for the simulation engine and so the faster our simulation will calculate.

Typically speaking, the amount of information needed in a mesh for accurate dynamic simulation purposes really is far less than that needed for an accurate or pleasing graphical representation. For example, to render a perfectly smooth sphere we may need to have somewhere in the region of two hundred vertices on our object before we would stop seeing any surface faceting. But to create a reasonably accurate simulation using the same sphere, we would really only need our collision mesh to have something like thirty-six vertices, which is a considerable reduction and would make all the difference in the world to our simulation speed.

Typically, simulation software will have a number of simplified primitive shapes available for use as physical meshes or shapes. In MassFX, for instance, we have primitives such as Sphere, Box, and Capsule. Whenever possible, we will want to use these shapes simply because they are typically both smoother and faster inside the simulation than a polygonal representation such as a concave or a convex hole. To use the example found in the 3ds Max help file, a beachball using a faceted convex hole for its physical shape will roll unevenly on the ground, resulting of course in an unrealistic-looking simulation.

Using the built-in sphere physical shape to represent the ball, however, would let it roll smoothly across the ground and would yield much-improved collision detection performance. Of course primitive shapes we only be useful in a certain, and possibly limited, number of situations, so we may find that we need to use more complex collision shapes such as convex and concave holes. In fact, under certain circumstances we may even need to generate a physical mesh from the actual graphical mesh we have in the scene.

Typically though, this only works for the static or passive rigid body type, and of course it has the potential to impact performance quite significantly, particularly if our mesh is a complex one. The more complex our physical or a collision mesh becomes, the higher the number of calculations required to track its interaction with other physical shapes in the scene. This in turn will most likely result in our simulations becoming slower and slower as collisions increase in complexity and/or frequency. Of course, now that we understand how our simulation tools are calculating collisions--that is, these physical representations are being used--we are in a much better position to be able to tweak and fine-tune the setup so as to get the best possible performance from a given simulation.

Keeping an eye on which physical shapes we're using and where possible, using the simple shapes available, we'll go a long way towards keeping our dynamic simulations as speedy and interactive as possible.

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This video is part of

Image for Creating Simulations in MassFX and 3ds Max
Creating Simulations in MassFX and 3ds Max

51 video lessons · 2513 viewers

Brian Bradley
Author

 
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  1. 3m 27s
    1. Welcome
      58s
    2. Working with the exercise files
      46s
    3. Setting up the 3ds Max project structure
      1m 43s
  2. 39m 20s
    1. Why simulate and not animate?
      3m 38s
    2. A look at gravity and drag
      3m 55s
    3. Understanding volume, mass, and density
      3m 45s
    4. What are Newton's laws of motion?
      3m 20s
    5. Finding believable frames per second and substeps
      3m 5s
    6. Understanding the difference between rigid and soft bodies
      3m 28s
    7. More about rigid body types
      3m 32s
    8. How collisions are calculated
      4m 35s
    9. Learning the difference between concave and convex meshes
      6m 24s
    10. What is a constraint and how do we use it?
      3m 38s
  3. 24m 20s
    1. A look at the MassFX and the 3ds Max user interfaces
      5m 52s
    2. Exploring the MassFX workflow
      5m 14s
    3. Discovering ground collision and gravity
      4m 49s
    4. Adjusting substeps and solver iterations
      3m 43s
    5. Using the Multi-Editor and the MassFX Visualizer
      4m 42s
  4. 44m 11s
    1. Breaking down the shot
      4m 51s
    2. Setting up the launchers
      3m 59s
    3. Setting up the drop system
      4m 30s
    4. Prepping the cans
      3m 33s
    5. Refining the simulation on the launchers
      5m 9s
    6. Refining the simulation on the colliders
      6m 5s
    7. Baking out the simulation for rendering
      5m 37s
    8. Reviewing the simulation with an animation sequence
      5m 3s
    9. Adding an animation override
      5m 24s
  5. 33m 32s
    1. Adding a rigid constraint and creating breakability
      8m 3s
    2. Creating a moving target with the Slide constraint
      4m 47s
    3. Creating springy targets with the Hinge constraint
      5m 59s
    4. Spinning targets using the Twist constraint
      4m 57s
    5. Creating crazy targets with the Ball & Socket constraint
      4m 58s
    6. Constructing a MassFX Ragdoll
      4m 48s
  6. 36m 51s
    1. Applying the mCloth modifier and pinning the hammock
      5m 55s
    2. Setting up the hammock's physical properties
      5m 39s
    3. Working with the mCloth interaction controls
      6m 14s
    4. Attaching the hammock to animated objects
      4m 5s
    5. Putting a rip in mCloth
      6m 14s
    6. Using mCloth to create a rope object
      4m 53s
    7. Creating a soft body object
      3m 51s
  7. 14m 47s
    1. Adding forces to a simulation
      5m 27s
    2. Putting forces to practical use
      5m 33s
    3. Using forces with mCloth
      3m 47s
  8. 35m 27s
    1. Walking through mParticles
      4m 38s
    2. Using fracture geometry
      6m 0s
    3. Creating breakable glue: Part 1
      4m 19s
    4. Creating breakable glue: Part 2
      5m 19s
    5. Creating a gloopy fluid: Part 1
      4m 14s
    6. Creating a gloopy fluid: Part 2
      4m 41s
    7. Adding forces to mParticles
      6m 16s
  9. 1m 5s
    1. What's next?
      1m 5s

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