Game Physics

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Game Physics

• What is meant by Physics in games (old)?
• Collision Detection (testing)
• But want the physical effect of hitting an
  immovable object.
• Also needed to provide the input (forces) for
  physical simulations (equations) of moveable or
  deformable objects.
• More intelligent (automated) interpolation
• Particle systems Artistically defined
  generation with Newtonian-based controls.

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Game Physics

• What is meant by Physics in games (new)?
• Fluid dynamics
• Deformable models
• Ragdoll (non-scripted) articulation
• (More) Accurate simulation

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Issues in Game Physics

• Do I need to buy a separate processor?
• What if the calculation takes too long?
• How can I cheat? What can I cheat on?
• How often do I need to update?

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Discussion items

• Dead-reckoning
• Emergent Behavior
• Collision Detection
• Newtonian Physics and gravity
• Forward Kinematics
• Inverse Kinematics
• Free-form deformation
• Fluid Flow

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Collision Detection

• Complicated for two reasons
• 1. Geometry is typically very complex,
  potentially requiring expensive testing
• 2. Naïve solution is O(n2) time complexity, since
  every object can potentially collide with every
  other object

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Collision Detection

• Two basic techniques
• 1. Overlap testing
• Detects whether a collision has already occurred
• 2. Intersection testing
• Predicts whether a collision will occur in the
  future

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Overlap Testing

• Facts
• Most common technique used in games
• Exhibits more error than intersection testing
• Concept
• For every simulation step, test every pair of
  objects to see if they overlap
• Easy for simple volumes like spheres, harder for
  polygonal models

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Overlap TestingUseful Results

• Useful results of detected collision
• Time collision took place
• Collision normal vector

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Overlap TestingCollision Time

• Collision time calculated by moving object back
  in time until right before collision
• Bisection is an effective technique

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Overlap TestingLimitations

• Fails with objects that move too fast
• Unlikely to catch time slice during overlap
• Possible solutions
• Design constraint on speed of objects
• Reduce simulation step size

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Intersection Testing

• Predict future collisions
• When predicted
• Move simulation to time of collision
• Resolve collision
• Simulate remaining time step

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Intersection TestingSwept Geometry

• Extrude geometry in direction of movement
• Swept sphere turns into a capsule shape

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Intersection TestingSphere-Sphere Collision
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Intersection TestingSphere-Sphere Collision

• Smallest distance ever separating two spheres
• If
• there is a collision

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Intersection TestingLimitations

• Issue with networked games
• Future predictions rely on exact state of world
  at present time
• Due to packet latency, current state not always
  coherent
• Assumes constant velocity and zero acceleration
  over simulation step
• Has implications for physics model and choice of
  integrator

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Dealing with Complexity

• Two issues
• 1. Complex geometry must be simplified
• 2. Reduce number of object pair tests

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Collision ResolutionExamples

• Two billiard balls strike
• Calculate ball positions at time of impact
• Impart new velocities on balls
• Play clinking sound effect
• Rocket slams into wall
• Rocket disappears
• Explosion spawned and explosion sound effect
• Wall charred and area damage inflicted on nearby
  characters
• Character walks through wall
• Magical sound effect triggered
• No trajectories or velocities affected

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• Real-time Graphics Methods
• Real-time graphics is all about hackery
• Its impossible to get a realtime, high-fidelity
  global illumination rendering
• The questions you should ask
• What effects do I want in my game?
• What corners am I willing to cut?

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• Billboards
• A billboard is a flat object that faces
  something
• There are lots of different billboarding methods,
  but well stick with the easiest, most used one
• Take a quad and slap a texture on it. Now we want
  it to face the camera. How do we do that?
• Bread and butter of older 3d games and still used
  extensively today
• Monsters (think Doom)
• Items
• Impostors (LOD)
• Text
• Faked smoke, fire, explosions, particle effects,
  halos, etc.
• ing lens flares
• Bad news Little to no shading

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• Light Mapping
• Wed like soft lighting and soft shadows but the
  fixed function pipeline wont let us have our
  way. Plus, real lighting is slow once we involve
  multiple lights.
• Hmm
• Most of our world geometry is static
• We can blend multiple textures together in
  multiple passes (multitexturing)
• Radiosity is good at diffuse
• Radiosity is view independent

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Light Mapping

• Lets precompute the global illumination (sans
  specular) using radiosity, store it in a light
  map, and blend that with the detail texture
• Thats the gist of it. Implementing it can be
  tricky.
• Dont need to use radiosity either
• Fun Fact id software used tohave a SGI Origin
  2000 (16 x 180mhz, 1.2GB RAM) to crunch maps.
  They sold it on ebay in 2000.
• Note probably should be called dark mapping

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• No Light Mapping
• Quake 3 Arena nvnews.net

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• Light Mapping!
• Quake 3 Arena nvnews.net

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• Low-Poly Modeling
• Trying to make effective, complex models that a
  game engine can handle is extremely difficult
• For the longest time games were just forced to
  have blocky looking models
• Thanks to normal mapping, now we can make really
  low poly models look incredibly complex

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Low-Poly Modeling

• This requires that the artist make a normal map
• She can draw the normal map manually
• Or create two versions of the model, one complex
  and one simplified, and use an automated tool to
  generate the normal map that represents the
  differences of the two models

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• Keyframed Animation
• Same idea as keyframes in traditional animation
• A keyframe represents position and orientation at
  a point in time
• Break the model into chunks (head, torso, etc.)
  and each chunk has vectors/matrices associated
  with it
• Keyframes specify the values of those matrices at
  specific points in time and their geometry
• Interpolate position and rotation (quaternions
  again) between those time periods.
• Quake 3 is keyframed and uses 15 keyframes/sec
• Uses a lot of memory because you need to store
  the geometry at each keyframe, but keyframing is
  fast

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• Skeletal Animation
• The model has a skeleton and the skeleton is
  animated, not the model
• Every vertex on the skin is connected to a bone
  so when the bone moves so does the skin. This is
  done with hierarchy and matrix transforms (sound
  familiar?)
• This involves transforming vertices so we can do
  this in a vertex shader (blend registers)
• Skeleton allows for realistic physics simulation
  on the model
• Keyframing is almost dead. Doom III, Half-Life 2,
  and other new games all use skeletal animation.
• The memory cost of keyframing was so high that
  Quake 3 was almost changed to skeletal animation
  at the last minute
• Related facial animation

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• Motion Capture
• Elegant hack
• If youve seen decent human animation in a game,
  its mocap. Video games are the biggest market
  for motion capture and now the movie industry is
  getting into it.
• Motion capture actor is actually a legitimate
  profession now
• Tom Hanks, that guy who did Gollum in LOTR
• Note Former MLB pitcher
• John Rocker

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• Material Libraries
• Technical Directors and Artists at a game company
  now have the luxury of creating material
  libraries whole sets of shaders and associated
  texture maps that can be used to enrich in-game
  models
• 3D movie production, especially at places like
  Pixar, always work with material libraries,
  allowing people to focus solely on shader
  production
• This frees the 3D and 2D artists to focus on art,
  not technology

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• Doom III

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• Halo 2

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• Black and White 2