iiD v1.0 User Manual
Contents
1. listed in arbitrary order e Body compositing Bodies can hold multiple shapes Shapes within a single body do not collide with one another Useful for creating compositions of shapes e Raycast queries upon the world e Dynamic AABB tree Broadphase e Prismatic motor and limit e Soft revolute joint e Post projection positional correction e Object slicing and fracturing e Buoyancy physics for water simulation Contact r dot gaul at digipen dot edu2. can be quite useful To do so is similar to querying an AABB except instead of providing an AABB a single point is provided Here is a code example bool myCallBack iiD Body body Do something with body ia Return true to search for more hits Return false to end query return false Vec2 myPoint 0 0f 0 0f scene QueryPoint myCallBack myPoint Know when Bodies Collide A certain callback is provided to the scene and will be called whenever two rigid bodies collide with one another Use the SetContactCallback function on the scene class in order to set this callback Ghosts Sometimes it is useful to have a rigid body that does not resolve collisions but only reports them To create a body like this simply set the is ghost bool within the BodyDef Joints A joint creates some rules on how two specific bodies will interact with one another Joints are useful for creating interesting levels and game mechanics Joints can be used to create rag dolls bridges breakable compositions of bodies motors and more By default bodies connected by two joints do not collide however this can be changed by setting a bool within the JointDef of a joint type Soft Joints Some joints can be soft A soft joint will not be completely rigid and will give way to strong forces Soft joints are great for creating springs or squishy things A soft joint has two parameters that need to be p
3. bject is referred to as a body or rigid body A rigid body contains a shape either Polygon or Circle and never deforms Restitution How bouncy a rigid body is OBB Oriented bounding box Pretty much just a box that can rotate AABB Axis aligned bounding box This means a box aligned with the x and y axes and cannot ever rotate Feedback and Reporting Bugs I m very interested in hearing feedback from any DigiPen team that uses the physics engine also need to know about any potential bugs Please contact me at r dot gaul at digipen dot edu Distance Units The iiD engine works with an arbitrary distance unit The standard distance of a medium sized object has a width of one recommend zooming in if your simulation feels slow and floaty Creating a Scene All rigid bodies in a simulation belong to a Scene The Scene class is one of the only parts of the iiD engine the user interacts with To create a Scene simply create one like so Vec2 gravity 0 9 8f const float dt 1 0f 60 0 Renderer renderer i1iD Scene scene gravity dt amp renderer 10 Rendering iiD The Renderer is a small interface used for debug drawing All rendering for iiD must be done externally to iiD Simulating Rigid Bodies Once a scene has been constructed all that is left to do is to add some rigid bodies to it To create a rigid body the scene expects a BodyDef and Shape to be provided Here is an example 11D Polygon p
4. eometry and are much more inexpensive to simulate than dynamic bodies To create a static body set the material type to Static orsetthe is static Boolean in the BodyDef to true Static bodies have a density of zero Creating a Polygon Creating a polygon is easy A polygon requires a list of vertices in world coordinates Here is an example of creating a triangle 11D Polygon poly 11D BodyDef def Vec2 v Veezt 1 0 ED Veo2 1 0 00E Vec2 0O 0f 1 poly Set v 3 deft Set S 9f 0056 U ks iiD Body triangle scene CreateBody poly def Of The vertices do not need to be dynamically allocated Creating a Circle Creating a circle is very simple 11D BodyDef def iiD Circle circle 5 i1iD Body myCircle scene CreateBody circle def Creating an OBB An easy to use function called Set ASBox was created to aid in setting up OBBs Call this function from the Polygon itself before creating a rigid body SetAsBox excepts half width and height values Demo The iiD demo has many demo simulations These simulations provide examples on how to use the iiD engine and suggest everyone to refer to the demos regularly as you learn to use the iiD library VD Physics Engine by Rand Here is a short video of the demo on youtube Shapes There are two types of shapes in iiD polygons and circles All polygons MUST be convex Please see the Simulating Rigid Bod
5. events and collisions with callbacks e Islanding simulation and sleeping e Warm starting e Generic 2D polygon and sphere collision detection e Efficient data oriented implementation e Math is built in Dependencies iiD itself has no dependencies The test demo uses GLUT and Windows h in order to run the demo Compiler Dependencies iiD was written in a portable way allowing it to compile on both G and Visual Studio All other major compilers should be supported as well Support for any compiling issues will be provided to all game teams interested in using the iiD library Glossary LCP Linear complementary problem Solving constraints can be thought of as an LCP See LCP Wikipedia page for more details Joint Aconstraint between 2 rigid bodies Joints make 2 rigid bodies interact with each other in interesting ways and are very useful for gameplay mechanics Warm Starting Optimization involving storing old LCP solutions and using them to kick start new solutions Sleeping When a rigid body is not moving around it does not need to be solved Sleeping objects will never move until something awake comes into contact with it This is an optimization Constraint Arule that forces a rigid body to behave in a certain way Constraints are solved with sequential impulses Sequential Impulses Amethod of using impulses to solve constraints within a physics engine Originally proposed by Erin Catto Rigid Body A physics o
6. ies section on how to create these shapes Each rigid body must contain one shape in order to be simulated A future version of the iiD library will allow rigid bodies to contain more than one shape in order to support convex and interesting compositions Point to Shape Test All shapes implement an interface that allows the user to see if a point collides with a certain shape shape gt TestPoint Vec2 1 0f 2 0f Scene Queries Raycasting Currently no form of raycasting is implemented in iiD Raycasting is essential for lots of gameplay types and so raycasting will be implemented in the near future AABB Query It is often times useful to test an AABB with a scene to see if any rigid bodies lay within its boundaries To do this examine the following code bool myCallBack iiD Body body Do something with body Return true to search for more hits Return false to end query return false scene QueryAABB myCallBack myAABB As you can see your callback should be very efficient as it will be called for each object that is found to be intersecting with the provided AABB Returning true from a callback will continue the query within the scene and look for more intersections Returning false will end the query right then and there This allows the user to fine tune and optimize their own queries Point Query Checking to see if any rigid bodies in a scene overlap a point in world space
7. iiD v1 0 User Manual All content 2013 DigiPen USA Corporation all rights reserved Contents leg geil Leg Le E Re EE Reuler EE Compiler DEPENCGENCIES ssis siese eneak EEO oss sep Seege ENEE Sege dE Ee Ee EE Feedback and Reporting Bugs RIDE Creating a SCONeG geed eege Ee EE ee Rendering MR E Simulating Rigid Rle L E ENEE Density ANd BA f EE A Eeer Static BOGGS ss gr ere d s Zeedee deeg Ra E REEE EE EE Ea EEA Ae Creating a Polygoon anea e ae a a E a EA a aiaa EE EE lo Creating ati OBB EE RAV Cel d BABB al POAC QC EE Know when Bodies Collide siisii aii ape aiia R EI EE TC SORE JOINTS E leg EE ele 8 elteren Eege geed ee ee eege 8 Weld JOINE EE 9 SEET EE 9 Bretagne Beatie tae dea ee ene SAS 9 MOUSE JOIN saa IEEE dree eege 10 ie LTE 10 Feature TO DO DISE ie Eege eegne eege A ee Eed 10 CONCACE E 11 Introduction iiD is a 2D rigid body simulation library written entirely in portable C All files are appended with iiD as to make the library easy to integrate into existing projects All of the library is within the iiD namespace Features iiD has a few interesting features that are quite desirable for DigiPen game projects e LCP solver with Sequential Impulses o Highly stable efficient and deterministic results e Many types of joints for interesting rigid body interactions e Very simple to use API e Library is not thread safe but can easily be placed onto an isolated thread e Simple communication of
8. ive motion must be along an axis of which is relative to the two bodies You can think of this as the bodies must travel along a line This is create for creating sliding doors and other mechanical systems Prismatic joint constraining a box along a line Revolute motors power a crankshaft Here is example code for setting up a prismatic joint i1iD PrismaticJointDef pdef pdef collisionEnabled true pdef Initialize bl plat Vec2 0 0f 0 0f Vec2 0 0f 1 0f scene CreateJoint pdef Mouse Joint The mouse joint is actually a soft revolute joint made specifically to be used with the mouse for grabbing and moving rigid bodies around The mouse joint is harder to setup than the rest of the joint types so recommend carefully examining the mouse related code in main cpp of the demo to learn more about this joint Mouse joint pulling a rigid body to the left Limitations There are a few limitations to be aware of when using iiD in order to have a stable simulation e Fast moving objects can tunnel teleport through each other Limit the maximum velocity of all objects or create level geometry with thicker walls to prevent this Also try to avoid tiny or very skinny rigid bodies e Very heavy objects on top of very light objects can cause sinking e NA2 broadphase currently e No raycasting currently Feature To Do List Here is a short list of features to be added in the near future to the iiD library
9. laced into the JointDef of the joint type One parameter is called the frequency The frequencyHz of a joint represents how soft or squishy the joint is The other parameter is the damping ratio The damping ratio lowers the velocity of the bodies involved in the joint This is useful for controlling the stability and behavior of the joint JointDef All joints just as rigid bodies require a definition to be created To create a joint a JointDef must be first provided Each different type of joint has a different JointDef Distance Joint The distance joint ensures that two points on two rigid bodies maintain a constant distance from another You can imagine a massless rigid rod connecting the two points together Example of a soft distance joint To create a distance joint examine the following example code 11D DistanceJointDef jointDef jointDef Initialize bodyl body2 anchorWorldA anchorWorldB jointDef frequencyHz 1 5f jointDef dampingRatio 0 05f scene CreateJoint jointDef See the DistanceJoints function inmain cpp of the demo for more example code Ropes The distance joint can be configured to act as a rope Ropes will not be solved unless the distance between the two bodies is large enough Setting up a distance joint as a rope is simple 11D DistanceJointDef jointDef jointDef Initialize bodyA bodyB anchorWorldA anchorWorldB jointDef rope true jointDef length 15 0f jointDef frequencyH
10. oly 11D BodyDef def def material Set iiD Material Wood poly SetAsBox 1 0f 1 0f i1iD Body myBody scene CreateBody poly def The BodyDe f is a small struct that contains information about how to create a rigid body Things like position the material the body is made of restitution and friction orientation and more are set here The example code above will default the position 0 0 and create a rigid body with a polygon shape as an oriented bounding box OBB After bodies have been created the scene needs to update them To update the scene call the Step function scene Step Materials A rigid body has a friction coefficient density and a restitution coefficient Both of these together determine how bouncy and sticky when sliding a rigid body behaves It can be annoying to come up with arbitrary coefficients so a naming convention was put together iiD comes packaged with some types of materials like Wood Metal and Pillow To set a material type see the code example from the previous section Alternatively the friction and restitution can be manually set to custom values Density and Mass Mass is calculated on a per shape basis Mass can be manually set but does not need to be Instead it is recommended to set the density of shapes through the material within the BodyDef Static Bodies A rigid body can be static Static bodies never move ever Static bodies usually represent level g
11. z 0 5f jointDef dampingRatio 0 1f scene CreateJoint jointDef Revolute Joint The revolute joint attaches two points one on each body together The bodies can only rotate around the point together but the point does not separate Revolute joint connecting two rods together Creating a revolute joint involves setting an anchor point specified in world space 1iD RevoluteJointDef rdef rdef Initialize chassis frontWheel Vec2 1 5f 1 0f rdef enableMotor true rdef motorSpeed 5 0f rdef maxTorque 2 0f scene CreateJoint rdef The revolute joint can also limit the angles in which the bodies can rotate Additionally the revolute joint can be setup to simulate a rotational motor Weld Joint The weld joint connects two rigid bodies together by a single point The two connected bodies must move with constant relative position and orientation Weld joints are cool but be warned constraints are not 100 rigid and weld joints will bend under high stress Weld joint connecting two rectangles together Here is an example of creating a weld joint 1iD WeldJointDef wdef wdeft Initialize bl b2 Vee2 1 0f 6 5 J3 scene CreateJoint wdef Angle Joint The angle joint constraints the angle to a constant between two rigid bodies This joint was created mostly for debugging purposes though can be useful on occasion Prismatic Joint The prismatic joint constraints two rigid bodies so their relat
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