Dipartimento di Tecnologie dell Informazione Università di MIlano. Physics Engines LOGO

Transcription

1 Dipartimento di Tecnologie dell Informazione Università di MIlano Physics Engines LOGO

2 Computer game physics Computer animation physics or game physics involves the introduction of the laws of physics into a simulation or game engine, particularly in 3D computer graphics, for the purpose of making the effects appear more real to the observer. There are several elements that form components of simulation physics: Physics engine is program code that is used to simulate Newtonian physics within the environment. Collision detection is used to solve the problem of determining when any two or more physical objects in the environment cross each other's path.

3 Definition A physics engine is a computer program that simulates Newtonian physics models, using variables such as mass, velocity, friction and wind resistance. It can simulate and predict effects under different conditions that would approximate what happens in real life or in a fantasy world.

4 There are generally two types of physics engines, real-time and high precision.

5 High precision vs. Real-Time High precision physics engines require more processing power to calculate very precise physics and are usually used by scientists and computer animated movies. In video games, or other forms of interactive computing, the physics engine will have to simplify its calculations and lower their accuracy so that they can be performed in time for the game to respond at an appropriate rate for gameplay. This is referred to as real-time physics.

6 Rigid body vs soft body and fluids Physics based character animation in the past only used rigid body dynamics because they are faster and easier to calculate, but modern games and movies are starting to use soft body physics now that it is possible. Soft body physics are also used for particle effects, liquids and cloth. Some form of limited Fluid dynamics simulation is sometimes provided to simulate water and other liquids as well as the flow of fire and explosions through the air.

7 Physics Processing Unit (PPU) February 2006 saw the release of the first physics processing unit from Ageia, which functions like a graphics card, taking the weight of physics processing off the CPU. The unit was most effective in accelerating particle systems however the performance improvement for rigid bodies was noticeable.

8 General Purpose processing on Graphics Processing Unit (GPGPU) GPGPU is another promising approach for real time physics engines, including rigid body dynamics. ATI and Nvidia provide rigid body dynamics on their latest graphics card. ATI claims X1900 XT should deliver 9 X the performance of an Ageia PhysX card

9 Particle systems The term particle system refers to a computer graphics technique to simulate certain fuzzy phenomena, which are otherwise very hard to reproduce with conventional rendering techniques. Examples of such phenomena which are commonly done with particle systems include fire, explosions, smoke, flowing water, sparks, falling leaves, clouds, fog, snow, dust, meteor tails, or abstract visual effects like glowy trails, etc.

10 A Fire particle system

11 A galaxy particle system

12 An abstract trail of particles

13 Rigid Body In physics, a rigid body is an idealization of a solid body of finite size in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it. In classical mechanics a rigid body is usually considered as a continuous mass distribution

14 Soft body dynamics is an area of physics simulation software that focuses on accurate simulation of a flexible object. That is, the object is deformable, meaning that the relative positions of points of the objects can change. There are many dynamic forces that have direct influence on an objects behavior (friction, gravity, collisions, springs, wind, etc..), and using soft body dynamics creates a believable illusion of the object. Soft bodies include: clothes, hair, sand, and water.

15 Typical implementation Typically the particle system's position in 3D space or motion therein is controlled by what is called the emitter. The emitter is mainly characterized by a set of particle behavior parameters and a position in 3D space. The particle behavior parameters might include spawning rate (how many particles are generated per unit of time), particle initial velocity vector (i.e. which way will it go when it is emitted), particle life (how much time does each individual particle live before being extinguished), particle color and variations throughout its life, and many more.

16 A typical particle system update loop (which is performed for each frame of animation) can be separated into two distinct stages, the parameter update/simulation stage and the rendering stage.

17 Simulation stage During the simulation stage, the amount of new particles that must be created is calculated based on spawning rates and interval between updates, and each of them is spawned in a specific position in 3D space based on emitter position and spawning area specified. Also each of the particle's parameters, like velocity, color, life, etc, are initialized based on the emitter's parameters. Then all the existing particles are checked to see if they have exceeded their lifetime, in which case they are removed from the simulation. Otherwise their position and other characteristics are modified based on some sort of physical simulation, which can be as simple as adding the velocity to the current position, and maybe accounting for friction by modulating the velocity, or as complicated as performing physically-accurate trajectory calculations taking into account external forces.

18 Also it is common to perform some sort of collision checking with specified 3D objects in order to make the particles bounce off obstacles in the environment. However particle-particle collisions are rarely used, as they are computationally expensive and not really useful for most of the simulations.

19 Particle system references Particle Systems: A Technique for Modeling a Class of Fuzzy Objects William T. Reeves (ACM Transactions on Graphics, April 1983) The ocean spray in your face. Jeff Lander (Game Developer, July 1998) Building an Advanced Particle System John van der Burg (Gamasutra, June 2000) Particle Engine Using Triangle Strips Jeff Molofee (NeHe) Building a Million-Particle System Lutz Latta (Gamasutra, July 2004)

20 Ragdoll Physics Early video games, going as far back as computer graphics themselves, used hand-created death sequences for characters. This had the advantage of low CPU usage, as all that was needed to animate a "dying character" was choose from a set number of pre-drawn animations. As computers increased in power, it became possible to do limited real-time physical simulations. A ragdoll is therefore a collection of multiple rigid bodies (each of which is ordinarily tied to a bone in the graphics engine's skeletal animation system) tied together by a system of constraints that restrict how the bones may move relative to each other.

21 The term ragdoll comes from the fact that the articulated systems, due to the limits of the solvers used, tend to have little or zero joint/skeletal muscle stiffness, leading to a character collapsing much like a toy rag doll, often into an improbable and comedic position. Its modern use goes beyond death sequences. There are games about fighting where the player controls one part of the body of the fighter and the rest moves along.

22 Due to the computationally expensive nature of performing simulations, most games using ragdolls use very simple approximations of characters: Extremity bones such as fingers often go unsimulated. Simple joints are used instead of actual constraints imposed by a true skeleton. (For example, human knee joints are often modelled as a rigid hinge even though an actual human knee allows some rotation.) Simplified collision hulls are used to detect contact with other rigid bodies rather than detecting collision with the mesh.

23 The chief advantage ragdolls offer over traditional animations is that they allow much more correct interaction with the surrounding environment. Where it would be absurd to try and hand-craft custom animations for all conceivable circumstances, ragdolls fill in and generate a mostly-correct interpretation of events on the fly.

24 Examples Toribash Ragdoll Masters, a fighting game with ragdolls (shareware download) Ski stunt simulator (java) Falling woman (flash) President Bush as a ragdoll (flash) Thomas Jakobsen's article about the integration of Verlet Physics Stair Dismount and Truck Dismount Ragdoll-Throw-Around (created with the Flade physics engine for flash) Ragdoll (deviantart.com)

25 Reference to Real-time physics engines Open source: Open Dynamics Engine Bullet OPAL Closed source/limited free distribution: Newton Game Dynamics Tokamak physics engine Commercial: Havok PhysX (formerly NovodeX and incorporaring Meqon) nv Physics SDK Endorphin, an animation tool that combines physics and AI with genetic algorithms

26 Reference to High precision physics engines Mechanica from Parametric Technologies Corporation. Working Model, from Design Simulation Technologies. (2D only). Falling Bodies, from Animats. (Ragdoll physics).